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Circuitbenders Forum => Circuitbending discussion => Synths & Samplers => Topic started by: Circuitbenders on January 23, 2007, 07:39:02 PM

Title: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on January 23, 2007, 07:39:02 PM
Seeing as manuals for the Fat Freebass or MAM MB33 seem to absurdly rare heres a scan of the dip switch settings for the Midi channel from a german manual.

(http://www.circuitbenders.co.uk/IMAGES/freebassmidi.jpg)
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: Signal:Noise on January 26, 2007, 10:03:15 AM
How hard are the freebass' to mod? I've been eyeing one up for ages for all my acid need.
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: Circuitbenders on January 26, 2007, 03:07:56 PM
With analogues once you have modded one its fairly easy to work out how to mod another machine in a similar way although understanding the basics of analogue synth circuits is a massive bonus when avoiding killing things. The freebass is a fairly easy machine to work with but the main advice i can give  is to not mess with the IC's or any of the power supply area on the left under any circumstances. They don't take kindly to it.

Interesting point about the freebass. The filter is actually designed to go into self oscillation but because its meant to be a 303 clone and the 303 filter doesn't self oscillate theres a resistor behind the resonance pot that retards the filter resonance. Bridge that and you've got a resonance boost.

If anyone knows where to get schematics for the freebass i would be very interested in getting hold of a copy.
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: jonesie196 on March 04, 2007, 12:54:39 PM
ive just found my old freebase without any powersupply! can anyone spred some light to what kinda V adapter i will need to get this beauty working again?
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: Circuitbenders on March 04, 2007, 01:59:08 PM
freebass' need 12V AC/AC, and make sure it is AC/AC and not AC/DC. It will work on AC/DC but its only a matter of time until the midi dies and it won't start up properly.
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: jonesie196 on March 04, 2007, 02:02:49 PM
cheers for that! ive had a search and cant seem to find one anywhere!

can only find AC/DC!

anychance you would know where id be able to get one?

cheers again, muchly appreciated
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: Circuitbenders on March 04, 2007, 03:26:57 PM
http://www.rapidonline.com/productinfo.aspx?tier1=Electrical+%26+Power&tier2=Power+Supplies&tier3=Plug+In+PSUs&tier4=AC%2fAC+and+AC%2fDC+plug++top+linear+PSU&moduleno=71060

'AC/AC UNREG PLUGTOP PSU 12V AC500MA RC' just under £5

I'm sure i read somewhere that it sounds slightly more chunky if you use a 15v supply but as i don't have 15v AC/AC handy i couldn't tell you.

Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: jonesie196 on March 04, 2007, 05:57:40 PM
just placed my order for one! will be nice to get the beast back working!

cheers for all the help  :D
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: adam on April 25, 2007, 02:48:03 PM
Which resistor is the one for the resonance boost? Thinking about trying this and just about to start repairing my FB!

Thanks in advance.
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: Circuitbenders on April 25, 2007, 08:11:11 PM
Theres 3 resistors lined up horizontally behind the resonance pot. You want to bridge the 3rd one back with a switch. If i recall correctly its just to the right of one of those 2 pin jumper test points.
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: adam on May 03, 2007, 12:09:49 PM
thanks for your help!

Any idea where i can get a replacement resonance pot from? can't seem to find any 47k dual pots to fit anywhere..

for anyone else who's not seen the inside of the FB, the pot can be seen here http://www.mfrathome.plus.com/fb383internal/P1010996.JPG  its the one that doesn't look like the others   ;)

I can get a replacement, but its not an exact fit and i'd prefer not to have to mod a pot to fit.. thanks!

 
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: Circuitbenders on May 22, 2007, 05:49:23 PM
Maqplins do those pots, i think they are called 'miniature potentionmeters' or something. You can also find them at rapid electronics with serrated shafts.
http://www.rapidonline.com/productinfo.aspx?kw=pot&tier1=Electronic+Components&tier2=Resistors+%26+Potentiometer&tier3=Control+Potentiometers&tier4=16mm+Commercial+potentiometers&moduleno=65222

Can anyone with a silver MB33 tell me what voltage it operates on? Both the freebass and the MAM MB33 with the brown-ish front and orange lettering work on 12V AC but i just got hold of a silver MB33 from france and it says 15V DC on the back.

Are they all like this or is it just in certain countries?
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: spk on February 28, 2008, 12:17:27 PM
 i realized why my fb's midi part dead when i found this thread,

i picked mine from ebay with 15 V DC (19 infact!!!) PSU. Never even thought it may be not original.
there was problems with delay start after poweriing it up.

signal stopped during play.
got this thing for only few  months . often used it.
could you help me with that telling wich parts exactly are burned or any other options..?

thanks!

P.S.

EPROM cover was not in it's place, when i opened the housing. May it be that it's ontents erased?
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: Circuitbenders on February 28, 2008, 07:26:06 PM
Its not actually a problem with the power supplies as it turns out it doesn't actually matter if you use 12v AC or 15v DC. Once its gone through the circuitry and voltage regulators in the power supply area of the board its all the same anyway.

We get at least two or three emails every month saying exactly what you have just said and i'm afraid to say that at the moment i don't actually know how to fix it. This appears to be a very common fault with all versions of the freebass. I have had 2 freebasses and an MB33 that displayed the same symptoms i.e you turn it on, the midi LED comes on and it makes a noise like its going to do the autotune start up it normally does but the midi LED never goes off, the autotune doesn't happen and the VCO doesn't respond to any midi messages.

The problem isn't with the firmware EPROM as i swapped the OS chip with one from a working Freebass and the MB33 still didn't work. I've been through the circuit looking for dead components a few times and have come to the conclusion that its most probably something corrupted in the big Philips PCF80C31BH-3 12P microcontroller chip near the power input, but have yet to get around to trying to replace it as its a bit of a monster.

A while back someone got in touch with me saying the same thing and said that they had somehow got in contact with an original MAM engineer who claimed they could fix this problem but wanted to charge more than a secondhand freebass to do so!

If we could actually work out whats wrong with these things we could save a whole lot of machines from the scrap heap.
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: catweazle on February 28, 2008, 08:32:10 PM
I have a MB33 (which is the same as the Freebass I think).

Better use DC adapter, since there is only a single diode as polarity protection and NO rectifier.
Not good to use AC power, the whole circuit oscillates with 50Hz since only one halfwave is rectified thru the diode.
Could possibly kill the polarised electrolytic capacitors. I don't know the microcontroller (8051 derivate) exactly
but I think it is a bit more sensitive for overvoltage and short circuits than a PIC for example.
Maybe that's the reason why the autotune could not get a stable tune (if the ground potential oscillates)

Some ideas:
- the 2 voltage regulators mounted on cooling plate. (7812 and 7805)
  measure the voltage on the right pin if you look onto it (7812=12V, 7805=5V)
  The MB33 needs 5Volts for logic (microcontroller,EPROM...) and 12Volt for the analog part (filter etc).
- that big capacitor next to the power connector.

The EPROM is UV sensitive, content will be erased if radiated with strong UV light. Better cover that
glas window with some tape. But while the case is not opened no UV light will come inside.
I don't think the content of your EPROM is erased or altered.

Update:
- in the MB33 a 100nF cap is soldered across the autotune button
- the input power can go up to 35V, the voltage regulators hold that - but get warm/hot
- optocuppler CNY17 (6 pin IC next to the MIDI connector) can be demaged, holding the
  serial data stream (MIDI data) to the microcontroller on a high level
  which results in a midi command 0xff which is the reset commando (System Reset (Sys Realtime))
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: Circuitbenders on February 28, 2008, 08:47:22 PM
hmmm, every freebass i've ever seen is labelled 12V AC, the dark face MB33 i had was labelled 12V AC but the silver face MB33 is labelled 15V DC. They are all the same machine although the early models has a single sided light brown board where as the later ones have a better quality double sided green board. The early ones seem to die more often than the later ones.

I don't think its the voltage regulators as i've tested them on a dead one against a working machine and they output the same. I'll check out the capacitor but i don't think thats going to be it.

On a working machine when you start it up you get that single low twanging tone and then you hear a kind of octave of notes sweep down as it performs what i assume is the autotune. On a dead machine you get the single low note but you never hear the notes sweep down, as if the startup crashes as it tried to do the autotune and can't go any further.  That sounds like a firmware problem to me but you think it might just not be able to get a stable enough tuning?
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: catweazle on February 28, 2008, 09:17:07 PM
I'm not shure it is a "can't not tune right" problem (only a idea). I had two MB33 (silver) ones none
had this problems. So pictures of the circuitboards could help to find out differences between
older revisions or freebass <-> MB33 differences.
It could really be a firmware error, then we can try to find out how the autotune thing work
and hack it that way it is always in tune (emulate a "in tune" state).
Maybe the microcontroller sweeps down the frequency of the oscilator (VCO)
and compare it wtih a internal generated tone (i don't know how it really works).
Analysing the signals on the microcontroller pins while in autotune mode could get some help
 (comparing working and dead machines). If someone has a oscilloscope or logic-analyzer.
Maybe a pin changes its voltage level if the thing is tuned, so setting the pin to that level will do the job.

Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: catweazle on February 28, 2008, 09:30:13 PM
here is a easy to build logic-analyzer for the PC parallel port:
http://www.xs4all.nl/~jwasys/old/diy2.html
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: spk on February 29, 2008, 07:04:43 AM
thank you guys for such an immediate response and in-depth discussion !
altought i am not so good in digital electronics.
just want to characterize my failure more detailed:
when i am powering the fb,   i can hear octave slide, filters etc are working, when moving pots.
but when it ends, midi LED is going off and no response either.
i had some thoughts that it is opto-isolator faulty (it would be not so hard replacing)
but if it's that philips chip, that seems it's dead end...:/

what do you think about replacing entire midi <-> cv decoder?
which is possible to order as a component
http://highlyliquid.com/kits/mpa/. there is only two parameteres controlled by midi anyway.
thanks
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: catweazle on February 29, 2008, 02:13:04 PM
Yes, midi to CV/GATE will work.
Gate signal
Pin 4 on the microcontroller, cut trace at pin 4 and
apply +5V for Note On and  0V(GND) for NoteOff

CV signal:
Have a look at the DAC (digital analog converter) 20pin IC,
http://www.national.com/mpf/DA/DAC0832.html
this DAC chip converts the digital signal from the microcontroller into a analog current
not voltage, this is a bit tricky. But I can measure a control voltage ~0..5V at pin 2 at the
14 pin TLC274 IC, cut trace at pin 11 on the DAC0832.

I only have MB33 (silver version), so I hope it's the same ICs inside FB etc.
good luck
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: Circuitbenders on February 29, 2008, 03:03:25 PM
The first Freebass i had that died like this, i just found the filter enevelope trigger point, added a 5v trigger input and made it into a rackmount filter unit with the filter FM mods.

I find it weird that i get so many emails from people with dead Freebass' and MB33's and yet this forum is the only place on the net that i've ever seen any discussion about the problem. You'd think that if its that common there would be loads of people discussing it and a solution would be found.

Or maybe its down to a single cheap component thats only just starting to go wrong in every machine..........
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: catweazle on February 29, 2008, 03:15:50 PM
... maybe MAM would make money repairing it ;) ...
kind of a predetermined breaking point
but without a schematic it is a bit difficult to find the failure.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on February 29, 2008, 06:16:40 PM
Replaced the CNY17 & the cap by the power input.

No difference.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: catweazle on February 29, 2008, 08:43:45 PM
To see if the microcontroller is working you can do the following,
there is a 20pin IC 74HC373 (adress latch for eprom),
check if the DAC0832 does something,
measure pin2 at the TLC274 opamp.
look at the pins on these ICs  if they are toggling around
with a "soundcard oscilloscope" for example: http://www.zeitnitz.de/Christian/Scope/Scope_en.html
     
   testlead
        <=------
                  |       100nF
     1M pot |   |<----| |------=> to soundcard audio in
                  |
                _|_ gnd

But if the MIDI LED goes on, the microcontroller is probably working because the MIDI LED is controlled
by microcontroller pin14 via a transistor (siiting beside the LEDs).

damn, I don't have a dead one, somebody send me a dead one
low priced for a poor student for educational purpose only ;)
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: spk on March 06, 2008, 11:03:57 AM
yes, it is strange indeed that its the only place about fb - mb problems..
maybe the faultiness is not so common, or the instrument itself not so popular..
anyway thanx for being one and only!

and one more question.
could you give me the insert point for CV and gate feed?
if i would like to use it with external MIDI/CV gadget ( often used to control 303/808/909)
as my midi led is not glowing when sending data i am pretty shure it is decoder's fault.

example of external midi/cv : http://cgi.ebay.nl/ws/eBayISAPI.dll?ViewItem&rd=1&item=130203115482&ssPageName=STRK:MEWA:IT&ih=003#ebayphotohosting

thnx
spk
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on March 06, 2008, 01:27:27 PM
example of external midi/cv : http://cgi.ebay.nl/ws/eBayISAPI.dll?ViewItem&rd=1&item=130203115482&ssPageName=STRK:MEWA:IT&ih=003#ebayphotohosting

Thats a MIDI to DIN sync converter, you're not going to get very far with that as its for syncing old kit that has an onboard sequencer.

damn, I don't have a dead one, somebody send me a dead one
low priced for a poor student for educational purpose only ;)

I've got a dead silver face MB33 i can donate to the cause. Its got holes drilled in it as i had mods installed before it died and theres some bodging i've had to do to replace parts as the solder pads peel off the second you desolder anything on the early versions, but its no use to me at the moment as it is and if we can work out what the problem is a whole load of machines can be saved from the scrap heap. I hope so anyway as i've just got another early version  Freebass that i'm scared to actually work on as i can just tell its going to die  :-\

You live in germany don't you catweazle? PM me and we can try and sort something out
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: catweazle on March 06, 2008, 11:09:16 PM
@spk:
see post #19, or anybody got more information for cv/gate insert points?

Yes, you'll need a CV/Gate converter. The highly liquid kit mpa would work.
CV should be 0..5Volt  and Gate 0/5V  (Roland compatible, Korg is inverted)

cool idea to build a sequencer into the "dead" mb33.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: squidfanny on March 14, 2008, 10:44:58 AM
I'm just wondering if it is possible to add a couple of CV inputs on my freebass?

It would be quite useful to me if I couple automate the filter and the the envelope from my computer...

I know it's a long shot, but would it be possible to automate the filter from the MIDI input?

cheers.............:-)
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on March 19, 2008, 01:53:51 PM
If someone donates me a knackered Freebass then I'll try and work out what fails.  It can't be *that* difficult.  I'm happy to cover postage (to G61 postcode area), or collect within about 50 miles of Glasgow.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on March 19, 2008, 03:26:23 PM
I'm just wondering if it is possible to add a couple of CV inputs on my freebass?

It would be quite useful to me if I couple automate the filter and the the envelope from my computer...

I know it's a long shot, but would it be possible to automate the filter from the MIDI input?

A filter CV input isn't that hard, the only problem is that the filter is closed at 3.3V so a 0-5V input doesn't really work straight out. You can get around that with a voltage divider and chosing the right place to inject the CV. Next time i have a Freebass open i'll post a photo.

You'd need an external midi-cv convertor with an aux output though as in order to automate the filter from the midii input you'd probably need to re-write the firmware.

If someone donates me a knackered Freebass then I'll try and work out what fails.  It can't be *that* difficult.  I'm happy to cover postage (to G61 postcode area), or collect within about 50 miles of Glasgow.

I've just sent my dead MB33 off to catweazle so with any luck we'll have a solution to this one soon.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: spk on April 10, 2008, 08:40:18 AM
crusty paul u have the PM!
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: DOXXOD on April 28, 2008, 05:55:56 PM
Hello chaps and chapesses, i'm a forum newbie here. Just got hold of a FAT Freebass off a colleage, and after replacing a few doddgy pots am ready to do some bending. Already had a poke around with my wet finger tips n found some ineresting points, one that seems to by pass the decay n turn the tones almost SIDchip style. Will be posting some details as soon as i have some pots n switches hooked up. Any one got any pointers of what area to start looking for the FM mod or the VCO/VCF input switch mods?

Any help would be much appreciated, i dont want to blow this little beast!

Thanks in advance for any replys

Matt.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on April 29, 2008, 07:32:06 PM
bugger, sorry people, ii completely forgot about this, everything has been a bit hectic over the past couple of weeks.

I've just shipped off the only MB33 i had already modded so i can't put up any photos but i'll dig out my notes asap and see what i can do.

From memory, the switch that cuts the VCO from the input when you plug a jack into the VCF input is the middle switching connector on the VCF input jack. If you cut the switching lever off you can connect a switch across the gap to turn the VCO on and off. I usually use a DPDT switch wired so that when its in the ON position the connection is made across the gap but when its OFF the switching lever side is connected to ground.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on April 29, 2008, 07:34:50 PM
Already had a poke around with my wet finger tips n found some ineresting points,

Not something i would advise on a Freebass. You may have one of the versions that isn't prone to dying or you may just be very lucky, but thats asking for it.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: bliprock on July 17, 2008, 04:25:44 AM
 :D yeha thanks circuit benders for the only place i can find my mb33 dipswitch settings. i copied it and went straight to my mb33 and found that it seems upside down. No drama cane farmer though... just read it upside down........i wonder if they put the dip switch in upside down????? or is it just different on the mb33 mk2..anyways keep up the good work and thanks again....
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: superset on July 24, 2008, 04:11:27 AM
Hello all… this is my first post but hopefully not my last.

Like most of you I have a non-functional FB383.  I bought it off of ebay for $10 and figured I’d be able at least, to utilize the filter section and maybe wire up the oscillators for external control.

I’ve documented my work so far which includes  three mods your all familiar with… the Resonance Boost mod, the filter input switch mod, and a Filter trigger mod which triggers the filter section allowing it to be used with external signals.

But what I think will be most helpful to all is a detailed circuit diagram which outlines signal flow through all components of the FB383 and MB33.  I’ve also detailed ground, the supply rail, the 5VDC lines and jumpers throughout.  Take note that the resistors with one black line are zero ohm resistors and are used liberally as jumpers.  You’ll also see overlays on most of the ICs so you can quickly identify the multiplexers, op amps, D/A converters, etc.

What I’m working on now is a schematic of the oscillator section for you all to look at, f with, and bend to your delight.  So far I’m having luck using pin 10 of the TLC2741N quad op amp as a CV input but the circuit begins to oscillate out of control… which some of  you may like, but a bit more stability would be useful in revamping these machines.  I’ll post more as I have something functional.

Anyway… you can find all the pics and documentation to the mods on the Flickr Collection here:

http://www.flickr.com/photos/97633426@N00/collections/72157606341720299/

and the high res circuit diagram is here:

http://www.supersetsounds.com/images/FB383/freebass_signal_flow_final.jpg
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on February 03, 2009, 02:28:22 AM
I'd completely missed this post! Did you ever get any further with this? I could still really do with a schematic for the freebass / MB33.

Nice job on the filter input trigger pics, at least some Freebasses will probably be salvaged to some extent with that information. Good job on the board layout as well, although It should probably be noted that later models have a far better quality double sided green board
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: gimedici on February 26, 2009, 09:05:20 AM
Hi, I have the same problem with my MB33, midi light stays always on and nothing happen. So I bought a second one to try to repair the first; but the second after some tests died as the first.
But I found that during self tuning, sawtooth wave starts at 550Hz roughly and frequency is lowered to 17-19 Hz, then midi light goes off and MB33 is working. In the same time there is a variable tension at pins 1,2, 14,15 of IC 4053. This voltage starts with 5V roughly at power up, and decays to 0.2-0.3 volts during self tuning; when reaches this value midi light goes off and MB33 is working.
But i was not able to understand how this tension controls the 555 oscillator frequency, and where it comes from.
After some tests (voltage measuring, wave finding and measuring) the second MB 33 died, so now I'm desperate.
Could anyone help me please?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on February 26, 2009, 10:11:53 AM
Bizarrely i've rescued three Freebass / MB33's simply buy replacing the DIP switches for the midi channel selection so you might want to try that first.

I'm starting to think there might be various causes for these faults. Stangely i've had two Frebasses sent to me that the owners claimed had exactly the fault you described, but when they arrived i couldn't find anything wrong with them. This would suggest to me that its either something mechanical such as the DIP switches or a dry joint which has been jogged back into place in transit, or its something to do with the power supplies people are using. I'm thinking the first option might be more likely.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on February 26, 2009, 11:05:23 AM
or its something to do with the power supplies people are using.

Now it's funny you should say that.  Are the power supplies the original ones?  Did they come with switch-mode power supplies or linear ones?

I had a lot of problems a couple of years ago with little Alvarion wireless bridges that would keep rebooting every ten seconds.  You'd power them up, and it would do the boot countdown on the status LEDs, then start all over again.  Turned out that the switch-mode power supplies they were supplied with partially failed after a while, and sagged badly when the radio was turned on - the last thing the unit did after booting!  This would trigger a reset.

If the PSU isn't up to the job of running the Freebass then it might have all sorts of weird problems, like never coming out of reset.  Have you 'scoped the supply rails?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on March 01, 2009, 01:51:09 PM
All the PSU's i've ever got with freebasses or MB33's are linear, either 12V AC/AC or more rarely 15V AC/DC
Title: Re: FAT Freebass / MAM MB33 Problems
Post by: gimedici on March 01, 2009, 11:43:28 PM
My PSU are linear too, if you mean that they produce a sinus wave 50Hz at deisred voltage (12V).

Now it's happening something new, but random (or I didn't understand why); sometimes the MB33 2 goes out from the standby condition and CPU and MIDI works because you can play and hear it, but oscillator is out of tune and doesn't articulate the note (only one note). It's not CV sensitive.
This happens when I go testing around the LM723 and the two suplly transistors (BD137 for +12v & BD138 for -12V). I don't see anithing wrong scoping and testing anyway.

The most times  nothing happens and the MB33 remains with midi light on; but i noted that the 555 oscillator doesn't emit the sawtooth wave, that usually emits; better,  it emits the sawtooth in the first seconds after powerup, then tries to tune (lowering pitch), and then dies.

I thint that something goes wrong whith CV, but I can't demonstrate.

Is there someone in internet or in the world could help us?
I hope that some MAM engineer tells us something........Please!
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: gimedici on March 06, 2009, 11:07:49 PM
I FOUND AND SOLVED THE PROBLEM ;D

I'm tallking about MAM MB 33 2 (which transmits potentiometer movements as control change MIDI messages)

The problem was a faulty 4051 mux/demux.
In the board we have two of these ICs, one to multiplex the 6 signals coming from potentiometers (env pwm, cutoff, reso, env mod, accent, decay, distortion) before converting it in digital (thru the ADC0804) and sending it to the micorcontroller (8051), the other to demultiplex 8 signals coming out form the 8051 and translated in analog thru the TLC7524 digital to analog converters. These 8 signals are the previous 6 plus two new used to drive the pitch CV.

During statrtup the 8051 lowers the pitch loweing these two voltages, and monitors the pitch through the square wave produced by the LM393.  (this square wave is obtained from the 555 oscillator sawtooth wave compared with a reference voltage in the LM 393 that is a dual voltage comparator; one side for our ears the other for the microprocessor pin 2)

When the 4051 demultiplexer (near MIDI connectors side) is broken, the CPU can not perform the tuning at powerup, so stays forever with MIDI light on doing nothing.

So, do replace the 4051 (both is better), and your MAM MB 33 will work again (in the most cases)
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on March 07, 2009, 01:23:54 AM
The problem was a faulty 4051 mux/demux.

Quelle surprise.  These are the first things I go for in an analogue synth, those and TL072s.  They are in one of two states - failed, or just about to fail.

Well done on getting it running!
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: chamelerus on March 17, 2009, 01:25:31 AM
hi. i have no idea about electronics or circuit bending, i just came here searching for info about the mb33.

i own a mk2 and a mk1, both run with 12V AC, although on the back of the mk1 it says 15V DC. (i got it with the AC)

now the mk1 isn't working correctly anymore and i really thought i can never operate it again. it shows the same symptoms as described above and coincidentally i found out that if i switch one of the midi-jumpers and re-switch it again, the mb33 starts to work again. and here comes the curious: as soon as i touch the midi-cable or the midi-plug, the sound stops. when i do the midi-jumper-trick, it works again until the midi-cable is moved a bit..

what do you think? is it just a defective contact that can be repaired easily?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on March 17, 2009, 02:22:41 AM
It's probably worth cleaning up and resoldering the pins on the MIDI socket.  I cured the intermittent MIDI problems on my mate's Sequential Sixtrak by doing that.  There was no visible dry joint, but somehow the pins were oxidised and not sticking to the solder.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on March 17, 2009, 05:18:39 PM
Same goes for the midi channel DIP switches. As i say, i've fixed problems on a couple of these machines just by replacing the DIP switches.

You might get away just with using some contact cleaner on them but otherwise just replace or resolder the switches, if resoldering the midi plug doesn't work.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: chamelerus on March 21, 2009, 09:59:57 AM
thx very much guys! i opened the mb33 and saw that 3 pins of the midi connector were 'lose'. it seems it got a punch sometime.. dunno. so i went to the store, got myself a cheap soldering iron and did the job. it's fully functioning again! :-)
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on March 21, 2009, 06:46:25 PM
it's fully functioning again! :-)

Very satisfying feeling, isn't it?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: DOXXOD on March 31, 2009, 09:41:43 PM
Any one got any details on the FM mod? still not worked out how to do it on mine, i'm pretty luck ive still not fried it ( or myself!)

Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: dirtycircuits on April 05, 2009, 01:58:55 PM
Hi folks, I've been modding my fb-383 for the last few days. I'm lucky enough to have a fully functional machine, with 12v ac/ac adapter.

Firstly, thanks for sharing the midi switch key. It's saved me alot of hassle.

The filter - when I plug in an external audio source (drum machine) , nothing happens until I hit play on cubase and the 383 is playing a sequence. Then the sound from my drum machine can then be heard with some envelope.
Is this normal? Would it be possible to feed my audio source straight into the filter, bypassing the envelope section?

I'm also thinking about adding modular outputs for cv/gate and also the accent + porta, so that I can midi-fy an mc-202 on the cheap. Has anyone tried this yet? Are there any issues I should be aware of before try this?

As usual, thanks for advice and help . . . . .  ;)
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on April 17, 2009, 02:14:41 PM
The filter never stays open on an Freebass. You have to trigger it with a midi note and you will get the envelope on it every time.

As i said further up this thread, a while back i had a completely destroyed freebass that i rebuilt into a filter rack. On that one i managed to find the filter trigger input and feed it a constant 5v so that it stayed permanently open and you could control the cutoff with the knobs. Unfortunately i lost my notes on how to do it and i've just managed to kill another freebass in an attempt to redo that mod.

On this occassion the probelm is definately with the firmware EPROM as i stuck in another one from a working machine and it works fine. I've taken a dump from the working firmware chip which is a 27C256 but i'm having a bit of difficultly burning it successfully to a new EPROM. Is anyone able to check out the dump i've got, or even better get a dump of the EPROM in their machine.

My OS dump is at:
http://www.mediafire.com/?u4kkx9659e9tk6r (http://www.mediafire.com/?u4kkx9659e9tk6r)

UPDATE:

Problem solved. That OS dump works fine, i just had my programmer set up wrong. I've erased and reprogrammed the EPROM on the non-working freebass and now it works fine, so theres something else to look at if you machine is misbehaving.

On another note. From messing around with the filter triggering it appears that the midi LED on the front panel doesn't actually represent midi activity at all. In fact it appears to represent the envelope opening and closing. If you trigger the filter/envelope without using midi the LED comes on while the envelope is active and then goes off when its completely closed again. It takes about 5 seconds to completely decay.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on April 17, 2009, 06:46:30 PM
It shouldn't be hard to figure out what's going on inside the Freebass.  I doubt if it would even take long to draw out the complete circuit.  Most of it will be much the same as the TB-303 circuit anyway...
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on April 19, 2009, 02:49:31 AM
Parts of it certainly seem very similar indeed. I think the main differences is the cheaper more easily obtainable components such as the 2N3904 and 2N3906's instead of the 2SA733P and 2SC536F's you find in the TB303 and the x0xb0x. I'm wondering if it would be worthwhile swapping out any transisters in the VCO and filter with original TB components and seeing if it sounded any more 303-like. The filter on the Freebass certainly isn't all that it should be.

I just had another look at exactly whats in there and there appear to be a whole load of original 303 parts including a   2SK30, a 2SC2291 and two  2SC1583's which are all supposedly rare parts according to the x0xb0x BOM.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on April 19, 2009, 04:28:33 PM
IIRC those are listed as equivalents.  I doubt that swapping the trannies for the "proper" ones would make much of a difference - the tolerances are huge anyway, so the difference between two different units is probably greater.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on July 17, 2009, 03:00:13 AM
well, i finally tried swapping some transistors in the VCO and it does make a fairly big difference to the sound of the squarewave.

You can make the squarewave less 'hollow' sounding and possibly a little more 303 like by swapping the 2SK30AGR in the VCO for a 2SK30AY as found in a real 303 VCO but to be honest i prefer the sound of the original.  :-\
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on March 11, 2010, 02:49:15 PM
Resurrecting the longest Freebass / MB33 thread on the net once again  ::)

Seeing as schematics for any MAM device are stupidly rare i thought i'd upload the set i have for the MB33 mkII that i found the other day. Take a look here to download them:

http://www.mediafire.com/?fomzjmzqnyq (http://www.mediafire.com/?fomzjmzqnyq)

Now who wants to post the mkI schematics, they must be out there somewhere.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: omarko on March 17, 2010, 02:26:15 AM
Hi All, interesting topic - my fb383 midi's stopped working :( im going to try the tips about resoldering the joints (esp around the midi connector), its strange! The midi light wont indicate a midi signal is present.

Anyone worked out the FM Mods for it yet?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on March 17, 2010, 01:37:13 AM
If it starts up fine and does the autotune correctly, what i'd try is:

1: Resolder the midi socket
2: Clean or replace the Midi channel selection DIP switches.
3: Test that the voltage regulators are working properly
4: Burn the firmware onto a new EPROM and try replacing the one in the Freebass.

Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: omarko on May 19, 2010, 09:37:21 AM
Hi All, Ive managed to fix my fb383  :D

I think the problem was due to a dodgy dip switch.

Thanks for the help, much apreciated.

I was thinking of adding a filter trigger pot to the unit - has any body any idea on what value (ohms) the pot should be?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: leeroylee on September 12, 2010, 08:33:25 PM
hello, i had a problem with my freebass, when it started up the midi led would stay on and i woundnt get any sound, so i bought  some electronic cleaner and sprayed the midi port contacts and the midi dip switch, it now works ok again.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: benergy on December 12, 2010, 02:00:56 AM
Hello every one, new member here....
love my fb383, it's one of the few items thats come around the world with me.
I was wondering if anyone can shed some light on a small issue
whenever vca is triggered, an audible 'pop' happens regardless how how low the filter is
i want to put a resistor or cap in to soften the instant attack just enough to solve this .
does anyone know a starting point, schematics seem a little thin on the ground

regards
B
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on December 12, 2010, 09:45:06 AM
Well, it's supposed to be a clone of the TB303, isn't it?  So I'd be trying to figure out from the board what the equivalent of the TB303's IC15 and friends is.  On the real 303 circuit, it *looks like* (I could be wrong) the envelope attack is set by how quickly Q32 can discharge C42 through R130 and R132.  This is getting into the realms of conjecture because I haven't played with building this part of the circuit yet, but it looks like Q32 is pulsed on briefly to discharge C42, which (once Q32 turns off again) slowly charges up through R123.  This gradually turns off Q31 (PNP, so the base goes positive to the same potential as the emitter), reducing the voltage on the gain control pin of the VCA chip.

Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on December 12, 2010, 02:02:38 PM
You'd probably have more luck with the x0xb0x schematic as they are a very similar circuit but a lot easier to read than the TB303 diagrams.

On an x0xb0x i had that clicked like a bastard i stuck a 86nF cap between the junction of R131 & R133, and ground. I also shorted D35. These part numbers are from the x0xb0x schematic but you can probably relate this to the FB383 circuit with a bit of comparison. This pretty much got rid of the click but didn't remove it completely. You can try different values for the cap but it seemed that anything higher than 86nF started to effect the decay.

The Freebass really shouldn't click at all, at least not enough to be an issue.

Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on December 12, 2010, 04:14:54 PM
I wonder if clicking is a symptom of a dodgy VCA chip, or possibly DC leaking through one of the capacitors in the signal path.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: AndyF on January 27, 2011, 08:56:09 PM
Just found this thread and website via google..

Really interesting reading, luckily my FB383 Freebass is working fine. Nice to know what to look out for though.

Im actually posting because ive borrowed a MB33 mk1 (black/silver) from a mate and have been comparing it to my FB (orange/silver)

It seems the filters are different.. (or at least calibrated differently)

The FB filter does not reach as far down and doesnt open as much as the MB.. resulting in it sounding duller with no resonance set.

But the MB resonance doesnt go as high and filter envelope doesnt open as much as on the FB.
With full resonance the FB sounds much brighter and filter envelope snaps more aggressively.

Im just wondering if anyone else has noticed this?
Does anyone know if these things can be changed by tuning pots inside?
Or do they use a few different components?

Im more curious than anything 8)
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on January 28, 2011, 02:58:33 AM
The later Freebass and MB33 boards are identical, in fact if you open up a later Freebass it actually has MB33 printed on the circuitboard. Some of the early Freebass' have one sided brown circuitboards rather than the green double sided boards you see on later models and MB33's. They also have much better steel sheet cases rather than the normal aluminium, along with better quality sealed pots. These earlier ones do seem to sound slightly fatter and more organic to my ears but i haven't done a direct compariison of components to find out why. They do have supposedly 50K pots instead of 47K ones, though i can't see that making much audble difference.

There are two trimmer pots behind the cutoff front panel pot on the green board models, if i recall correctly they both affect the cutoff width and range to some extent but i haven't fiddled with them for a while. They will probably be around the same area on the brown board version.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: heretic on March 05, 2011, 08:42:50 PM
any schematics available for the MB33 mkI?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on April 26, 2011, 02:53:38 PM
Finally got a FAT Freebass manual and made it into a PDF if anyone wants a copy.

http://www.circuitbenders.co.uk/hostedstuff/Freebass-manual.pdf (http://www.circuitbenders.co.uk/hostedstuff/Freebass-manual.pdf)
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: 01012k7 on April 03, 2012, 03:12:55 AM
Its not actually a problem with the power supplies as it turns out it doesn't actually matter if you use 12v AC or 15v DC. Once its gone through the circuitry and voltage regulators in the power supply area of the board its all the same anyway.

We get at least two or three emails every month saying exactly what you have just said and i'm afraid to say that at the moment i don't actually know how to fix it. This appears to be a very common fault with all versions of the freebass. I have had 2 freebasses and an MB33 that displayed the same symptoms i.e you turn it on, the midi LED comes on and it makes a noise like its going to do the autotune start up it normally does but the midi LED never goes off, the autotune doesn't happen and the VCO doesn't respond to any midi messages.

The problem isn't with the firmware EPROM as i swapped the OS chip with one from a working Freebass and the MB33 still didn't work. I've been through the circuit looking for dead components a few times and have come to the conclusion that its most probably something corrupted in the big Philips PCF80C31BH-3 12P microcontroller chip near the power input, but have yet to get around to trying to replace it as its a bit of a monster.

A while back someone got in touch with me saying the same thing and said that they had somehow got in contact with an original MAM engineer who claimed they could fix this problem but wanted to charge more than a secondhand freebass to do so!

If we could actually work out whats wrong with these things we could save a whole lot of machines from the scrap heap.
hi guys i am new here i just wanted to ask i have a freebass fb383 which has this problem but i have open it up and found it is a Philips PCF80C31BH-3 16p microcontroller chip  40 pin job any ideas if you can still get them ...
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on April 03, 2012, 11:40:35 AM
i assume you've read the rest of this thread?

Its most likely the DIP switches for the midi channel selection.

Don't go near that micro-controller unless its as a last resort. You can still get them from ebay but i've only ended up having to replace one of those on two occasions, and one of those was when someone else had tried to do it previously and had put in an incorrect replacement.

If nothing else, removing that chip without a proper heated desoldering tool will be one of the least fun things you'll ever do.   :-\
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on April 03, 2012, 08:26:23 PM
i assume you've read the rest of this thread?

Its most likely the DIP switches for the midi channel selection.

Don't go near that micro-controller unless its as a last resort. You can still get them from eBay but I've only ended up having to replace one of those on two occasions, and one of those was when someone else had tried to do it previously and had put in an incorrect replacement.

If nothing else, removing that chip without a proper heated desoldering tool will be one of the least fun things you'll ever do.   :-\
Tbh thxs for your  help i  did read this topic and sort of read most of it but start to fall a sleep lol
i got it from ebay fs383 did not understand the problems i would be facing but when i got it was like midi port look like a cable had been pulled out the wrong way it had damage the pins so took the midi socket out as i could see some track had come away so i decided to get my angle grinder to it ( only joking) i got some fine wet and dry rubbed the track to show the copper then i bridged the track to the midi connection then i came here .. i order a new dip switch as one was not making a connection also was like loose when you switch it .. but i did see one of the track was lifted so done a temp job till i have time which will be over the weekend ..it sort of works but i need to do a better job also want to do some mods my self the leds and the power switch will be first as keep having pull the plug out is a pain
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on April 08, 2012, 11:35:40 AM
Hi thinking of options as it work for like 10 mins and its given up again so think of things i can do with it

I have order a Meeblip micro kit which should be here in a week or 2  so my ideas which i am not shure will work is maybe link the meeblip micro up with the freebass filter ( not shure if this will work) too is just user the rack for the mee blip micro and then sell the bits on or may keep the bits just in case thing change on repairing it

parts that have been change on the free 383 so far

dip switch
cyn-17
power cap 470uf to 470 uf 25 volts at a temp job
Done some track work also repaird the midi ssocket  but may order a new one ..i have per order both  5 v and 12 v power reg think it if it done't work then go with a option here
Title: Re: FAT Freebass / MAM MB33 Midi channel settings
Post by: 01012k7 on May 02, 2012, 10:26:33 AM
Its not actually a problem with the power supplies as it turns out it doesn't actually matter if you use 12v AC or 15v DC. Once its gone through the circuitry and voltage regulators in the power supply area of the board its all the same anyway.

We get at least two or three emails every month saying exactly what you have just said and i'm afraid to say that at the moment i don't actually know how to fix it. This appears to be a very common fault with all versions of the freebass. I have had 2 freebasses and an MB33 that displayed the same symptoms i.e you turn it on, the midi LED comes on and it makes a noise like its going to do the autotune start up it normally does but the midi LED never goes off, the autotune doesn't happen and the VCO doesn't respond to any midi messages.

The problem isn't with the firmware EPROM as i swapped the OS chip with one from a working Freebass and the MB33 still didn't work. I've been through the circuit looking for dead components a few times and have come to the conclusion that its most probably something corrupted in the big Philips PCF80C31BH-3 12P microcontroller chip near the power input, but have yet to get around to trying to replace it as its a bit of a monster.

A while back someone got in touch with me saying the same thing and said that they had somehow got in contact with an original MAM engineer who claimed they could fix this problem but wanted to charge more than a secondhand freebass to do so!

If we could actually work out whats wrong with these things we could save a whole lot of machines from the scrap heap.
hi is the Philips PCF80C31BH-2 12P  the same i look at the data sheets same pin out also i have a PCF80C31BH-3 16P just looking at options at the moment ...
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on May 02, 2012, 11:41:28 AM
If you have one already and you've decided you're going to attempt to remove the old one, just give it a go, its not going to hurt the freebass, unless you make a complete mess of removing it.

I've just got a load of PCB80C31BH-3-16P's from somewhere in the US, and they certainly work, so i don't see why a PCF80C31BH-3-16P wouldn't. Its seems to be the 80C31 part thats the important bit. The  12P / 16P bit seems to indicate the maximum mhz speed, but i'd imagine that higher than the original would be ok, but lower wouldn't.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on May 13, 2012, 08:12:41 PM
well i decided to give the thing one last go  ;D to day i took out the chip 40 pin thing also clean the crap work i did before ok so i installed a metal case midi socket as the plasic one was bit knackered from when i got it also replaced the cny17-2 again just in case from over heating so installed a 6pin dip socket and also a 40 pin sockets  here some pictures i also clean some of the pain off as the soldering was not great so wanted to make sure of some tracks
(http://i805.photobucket.com/albums/yy334/ucapolaris/Analog%20Synthesizers%20Projects/IMAG0285.jpg)

was please with the way the chip came out

(http://i805.photobucket.com/albums/yy334/ucapolaris/Analog%20Synthesizers%20Projects/IMAG0286.jpg)

think i may have a problem reusing this IC
(http://i805.photobucket.com/albums/yy334/ucapolaris/Analog%20Synthesizers%20Projects/IMAG0288.jpg)

Also with the ic sockets installed i done a route check to see if the links to other trackes work  so just waiting for the main chip now if this don't fix it not much else i can do unless a dry joint on the board ..
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on May 13, 2012, 09:04:20 PM
To be fair, a dry joint would be the *first* thing to look for ;-)

If you don't get it going, can I buy it from you?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on May 13, 2012, 09:48:40 PM
To be fair, a dry joint would be the *first* thing to look for ;-)

If you don't get it going, can I buy it from you?
i been over it checking for dry joints but i be using the filter anyway so will not be selling it
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on May 14, 2012, 10:36:38 PM
Well if anyone else has a dead or dying FAT Freebase, MAM MB33 or similar they want to part with, I'll buy it.  Ideally one with working analogue bits but a dead CPU, since what I want to do is gank out the old chip and devise a modern replacement, that eliminates the bloody stupid faulty dipswitch problem once and for all.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on June 06, 2012, 12:39:05 PM
Heres something weird. We've had  two Freebasses in for modding recently, both from Germany, which is pretty odd in itself as most mainland European Freebasses are MB33's. Whats strange is that both owners had complained that their units never sounded as good as the ones they had seen on Youtube videos etc.

Having checked out both pretty extensively i discovered that in both of them the 47K log pot for the cutoff (mysteriously labelled D47K) was only measuring to be around 40K. Thats measuring them with the pots out of the circuit. This gave a pretty feeble amount of response to the cutoff knob and you just couldn't get the filter even remotely closed

An attempt had been made on both to widen the cutoff response using the trimmer pots behind the cutoff knob. I assume this must have been done at the factory, but it was nowhere near as wide as is should have been. Replacing the cutoff with a proper A47K pot fixed the problem on both synths.

I've never seen this one a UK Freebass. I can only assume that MAM or whoever actually built these things must have got hold of a load of dodgy pots at some point and used them in units destined for mainland Europe. It would suggest that theres a whole load of people out there who are wondering why their Freebass sounds crap!
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on June 06, 2012, 06:24:56 PM
it sounds like the units were made in some one shed :) also i going to be replacing the pots thxs for the info
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on July 30, 2012, 01:42:40 AM
i wound like to thx paul for helping me out with the tigger for the freebass ( i love this guy and not in a gay way :D) 
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: manneokoko on February 02, 2013, 09:04:06 PM
Hello, another MB33 mk1 owner here!
Ive read the whole discussion and can't find the answer to my problem:

Evertything works great except the auto tune button. When i press it the midi LED lights up and it stops reacting to incoming midi, nothing happens. Strangely enough the auto tune seems to work when i power up the unit: the Midi LED lights up for some time then its goes dark and the unit is running properly.

Any help?

Also it might be worth mentioning that i opened it up and changed one resistor to get some more resonance, this one (http://www.flickr.com/photos/supersetsounds/2696636025/#in/set-72157606341206967). I changed from the original 2,2k to a 3,9k. Sounds great. But this shouldn't affect auto tune should it?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on February 02, 2013, 11:09:38 PM
It depends on if it also tunes the filter.  Does auto tune work if you swap the resistor back?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: manneokoko on February 03, 2013, 05:03:07 PM
Ive tried changing back to the original resistor on the filter, the auto tune button still freezes it. Strange that the auto tune  works when it is first turned on, but not when using the button.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: manneokoko on February 03, 2013, 05:36:49 PM
I noticed when I accidentally used a 9V power adaptor that the midi LED stayed on/ freezed. So maybe the auto tune problems is related to voltage? According to the manual it needs 15V AC - but on the unit itself it says 12V AC…
What do you think, is it safe to try using a 15V AC adaptor?

Also, the resonance mod:
According to the instructions for the mod Im supposed to just bridge the given resistor. To my finding this result in decreased resonance, not increased. But when putting in a higher resistor than the original the resonance is boosted. Can this harm the unit in any way?


Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on February 03, 2013, 10:04:26 PM
I noticed when I accidentally used a 9V power adaptor that the midi LED stayed on/ freezed. So maybe the auto tune problems is related to voltage? According to the manual it needs 15V AC - but on the unit itself it says 12V AC…
What do you think, is it safe to try using a 15V AC adaptor?

Also, the resonance mod:
According to the instructions for the mod Im supposed to just bridge the given resistor. To my finding this result in decreased resonance, not increased. But when putting in a higher resistor than the original the resonance is boosted. Can this harm the unit in any way?



i my self would not play with the unit they they tend to die some times
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on February 03, 2013, 10:08:18 PM
Its 15v DC or 12v AC, either will work fine.

Bridging the resistor thats been removed on that link won't achieve that much as far as i recall. You want to go for the third one back from the rear of the resonance pot, not the second one. You don't need to take it out, just solder a switch to either side of it.

When you press the autotune button you won't actually hear it doing the tuning like you do on startup as it doesn't trigger the envelope. It takes about 40 seconds or so from when you press the button until the midi LED goes out again. While its doing the tuning the unit won't respond to midi.

Why do you even need to do the autotune?

Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: manneokoko on February 04, 2013, 01:30:01 PM
When changing the resistor that is pointed out in the instructions on the net (http://www.supersetsounds.com/2008/11/fb-383-freebass-mods/) i certainly got a resonance boost.
But according to the instructions, the resistor should be bridged - I instead put in a higher value resistor.

I will try as you say bridging the third resistor instead.

Also, I have a MB33 newer version with green PCB - not the one shown in the modding instructions.


Well, i dont know if Im gonna need the autotune button, I just bought it. I just thought that since there is a button there, the machine will get out of tune every now and then.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: manneokoko on February 04, 2013, 06:33:57 PM
OK, so i tried doing as you said and bridged the other resistor - it also made a resonance boost.  I liked the res mod by changing the other resistor more, it gave another kind of resonance boost.. different sound. I guess it affects something else than just pure resonance, maybe accent or env amount or something.

So I will change back to the way i made the mod first - that is changing the second resistor back from the rear of the resonance pot to 3,9k and the third one I just use the original (or maybe I lower it a bit to have a little of booth resonance mods).

Can you please advice, will changing the second one back from the rear of the resonance pot give any damage to the unit?

And do you think its these resonance mods that damaged the autotune button?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on February 05, 2013, 02:00:26 PM
To be honest i'm not entirely sure why the autotune button is there. I've never needed it, and even if you did, you could just restart the thing and it'd go through the autotune sequence anyway.

Without a schematic of the filter it'd be difficult to tell exactly what the second resistor is doing, but i can't imagine increasing it would cause any issues, especially not by such a small amount.

I don't think the filter tracks the keyboard so its unlikely to have any effect on the tuning.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: manneokoko on February 08, 2013, 09:19:37 AM
I bow down in thanks for your support!
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on February 12, 2013, 06:47:12 PM
not shure if i ask you guys before any one know if you can connect a midi to cv adaptor to the VCO of the freebass i know you can do if for the vcf and have info was thinking ( and yes it does hurt to think):) if this possable was thinking maybe using one ...
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on February 12, 2013, 06:47:59 PM
I bow down in thanks for your support!
Paul great ..
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: darkest_star on February 21, 2013, 09:34:14 PM
First of all: This is a really great threat. Never get so much troubleshooting information about the great MB33.
My MB33 MK1 has the following behaviour:

After switching on I can hear a bass sound which is slowly getting quiet after 1-3 seconds. After about 5-10 seconds the midi-LED goes out. But if I send midi data to the device I NEVER see a flashing midi-LED or hear any sound of my MB33. I have checked a lot with my oscilloscope. The DIP-switch seems to be ok. For testing I have measured the voltages on the CPU pins where the DIP-switches are connected: All DIPs work fine. Further I have seen a perfect TTL signal on the midi-input at the CPU while playing a midi song. So, optocoupler and midi-input socket should also to be ok. During the "warm up phase" (midi-LED on) I have accidentlly touched pin4 of the CPU and have heared a bass sound, which can be manipulated by the knobs on the front. Perhaps pin4 is the gate signal?
Therefore I would like to ask, if someone could send me the schematics of a MAM MB33 or FB-383 MK1? It would be very helpful for me.

Could it be, that my EPROM has partially lost it's memory? Or wouldn't then go my midi-LED not off after some seconds? I have a EPROM programmer available and would like to try to program a another EPROM chip for a retest. But unfortunately the posted link to the EPROM dump of a MB33MK1 (many threads before) isn't working anymore. So, if someone could send my this dumpfile, it would also very helpful for me to investigate respectively isolate the problem of my MB33.

Thank you very much in advance for support!


Regards,
Uwe
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on February 23, 2013, 06:01:09 PM
I've just uploaded the EPROM firmware again. You can fin it here http://www.mediafire.com/?u4kkx9659e9tk6r (http://www.mediafire.com/?u4kkx9659e9tk6r)

I've never found the schematic for the mkI anywhere. I asked Touched By Sound a while back and they said they were in the process of uploading them, but then they stopped answering emails. The schematic might be out there, but i've never seen it.

I'd be willing to bet that you have a problem with the CPU, they seem to die on a fairly regular basis.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on February 23, 2013, 06:57:32 PM
cheers for that! ive had a search and cant seem to find one anywhere!

can only find AC/DC!

anychance you would know where id be able to get one?

cheers again, muchly appreciated
lots on ebay
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: darkest_star on February 23, 2013, 09:10:36 PM
@crustypaul: Thank you very much for uploading the EPROM file! I will reprogram a second EPROM on Monday.

But, if the midi-LED goes off after some seconds I assume, that the autotune procedure was successful. Which means, the CPU has already a working program in EPROM. Changing one of the ICs (e.g. CPU) would no problem for me. I've done this a lot in my life (the first time in the 80s for my C64). But doing this without any success in sight I wouldn't do this. :-)
Probably it is one of the multiplexers (4051, 4053)?

But I could really remove all IC's from digital part, solder for each a IC socket and then retry with replaced ICs.

If you send me the email address where you have tried to get the schematic I could retry to get it. I think there are a lot of MB33-hobbyists around which would be happy with that schematic.

Thanks very much!
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on February 23, 2013, 10:15:47 PM
Don't just shotgun-replace parts.  It's worth checking the outputs of the multiplexer with a 'scope to see if they seem sane.  They do fail, quite frequently.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on February 24, 2013, 01:06:28 PM
Don't just shotgun-replace parts.  It's worth checking the outputs of the multiplexer with a 'scope to see if they seem sane.  They do fail, quite frequently.
going to have another look at the unit after reading your post.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: darkest_star on February 26, 2013, 02:12:46 PM
Unfortunately still had no time to reprogram the EPROM because I'm on business trip.
I will check the outputs of both multiplexers too. But this is also a kind of reversal engineering due to missing schematics.  :(
But back to the symtoms: My mb33 seems to perform a succesful autotune because the midi-LED goes off after some seconds. As I understand the posts in this thread the LED would stay on if an error occurred? Further I can press the autotune button without any reaction. Not during running device and not while swithing it on (where it should play a demo melody).

Very strange...  ::)
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: darkest_star on February 26, 2013, 09:29:08 PM
After the hours I've spent trying to find the error I really assume to "disable" the digital part, add CV and gate input sockets and connect it directly with my Doepfer Dark Time.  ;)
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on February 26, 2013, 10:03:05 PM
After the hours I've spent trying to find the error I really assume to "disable" the digital part, add CV and gate input sockets and connect it directly with my Doepfer Dark Time.  ;)
i had an idea with adding a meeblip  this what they told me

_______________________________________________________________________________
My question to one of the guys

can another filter be connect to a Meeblip like anther vcf filter ??? as i have korg ms 20 vcf clone i am making ..
_________________________________________________________________________________
Reply
Yes, it's possible to drive an analog filter from Meeblip. There are two 8-bit outputs on the DAC chip - they're currently both used to generate the audio waveform. It's possible to use just one output to generate an 8-bit waveform. That leave the other output to provide 0-5V cv to control filter cutoff on an analog filter.

It works even better with a higher resolution DAC. Or so a little bird tells me
-----------------------------------------------------------------------------------------------------

any idea if this would work for the freebass
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on February 28, 2013, 01:59:23 AM
I've just checked a Freebass. Pin 4 of the CPU does appear to be the gate signal. You should see pulses of about 750mv whenever you play a note, for the duration of the note. Pin 14 drives the transistor for the Midi activity LED, and pin 10 is the midi input.

The pitch CV seems to appear at pin 13 of the 4051 behind the autotune button. It seems to be some kind of an inverted log scaling with around +5v at the lowest C and about 80mv at the highest C.

If the midi is getting to the CPU and it looks fine, but you aren't seeing the gate signal on pin 4, then i guess you have a CPU issue, or possibly an EPROM problem, but as you say, if the EPROM was corrupted i doubt it'd even get past the startup sequence. If you are seeing the gate signal but no pitch CV, then the first thing i'd try would be replacing the 4051.

Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on March 02, 2013, 11:14:06 AM
i am going to have a go and  trying to get the vcf to work with my 8 Step  Analog Sequencer if no joy i decided to give up with it and break it still worth a bit to me as i can use the 2SC2291 2SC1583 out of it for x0xb0x think this the way forward.

I do have 2 cpu think 16mhz and 12 mhz which is a old one none have been solder into boards just installed in ic sockets also i guess knobs , and some of the plastic pots
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on March 03, 2013, 12:00:02 PM
If you can't get it working at all *and it is complete* then I'm quite keen to buy it as a project, replacing the seemingly-fragile CPU with something more modern.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on March 03, 2013, 12:22:49 PM
If you can't get it working at all *and it is complete* then I'm quite keen to buy it as a project, replacing the seemingly-fragile CPU with something more modern.
I decided to build a x0xb0x  now  i have a lot of parts for x0xb0x also have 2SC2291 2SC1583 out of the freebass and i guess the knobs i can sell too people who are bending the freebass.
also  the case will be for another project

i have just got main board and the socket board with usb solder on and also main cpu with os
i do have all the  carbon resistors and some of the other resistors and 2SA733P and 2SC536F i also have some IC also hardware and some caps ..think its the way to go for me now i have also order another 2SC1583...
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on March 03, 2013, 12:34:00 PM
Why not just buy some new parts for an x0xb0x and get the freebass working in some manner, or sell it to gordon?  :-\

It seems a shame to pointlessly demolish a Freebass when it'd only cost a few £'s to just buy the parts you need for the x0xb0x new, rather than harvesting them from the freebass. You can get rack cases from loads of places.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: 01012k7 on March 03, 2013, 03:21:56 PM
Why not just buy some new parts for an x0xb0x and get the freebass working in some manner, or sell it to gordon?  :-\

It seems a shame to pointlessly demolish a Freebass when it'd only cost a few £'s to just buy the parts you need for the x0xb0x new, rather than harvesting them from the freebass. You can get rack cases from loads of places.
ok i understand what you are saying i would want £30 + post as parts that i could use would be  like £25 pounds to replace also this one is the one with the bad circuilt boards you look at it and the tracks fall off some damage around midi port when i got the unit the midi socket was like inside the unit had come off it work for about 5 hours then gave up after putting it on also have a 500ma ac/ac power adaptor with it .. ...
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Gordonjcp on March 03, 2013, 11:36:51 PM
Work out the postage, and mail me off-forum.  30 quid sounds about right.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: darkest_star on March 12, 2013, 12:13:51 PM
A reprogramming of the EPROM had no positive effect for my MB33.  :(
Now I have ordered a CPU and the both multiplexers 4051,4053 for a about 3 Euros in sum. I would starting to replace the multiplexers and finally the CPU. Of course I will solder socket. :-)
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: darkest_star on March 16, 2013, 09:28:53 PM
Like crustypaul I had also the suspicion that the CPU is the cause for my mb33-problem. So, I have started to replace the CPU first. It took only 10 minutes work (cut out the old CPU, remove the old pins, solder in a 40pin socket). But the behaviour is still the same. The CPU was not the problem. Therefore now I will replace the multiplexers 4051 and 4053.

But one question to all:
What happens, if you press the Autotune button while switching on the device (MB33 MK1)? On my system I hear no demo song; I hear nothing. But the midi-LED stays ON forever. If I switch on my MB33 without pressing the Autotune button the midi-LED goes OFF after about 5-10 seconds.
Thanks very much!
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: darkest_star on March 17, 2013, 12:56:47 PM
After replacing the two multiplexers 4051 4053 the MB33 behaves as before.  >:( If I remove them from their sockets the midi-LED stays on. So, the Autotune procedure during power on seems to work.
I have resoldered every point in the digital part, without effect.
What I have found out accidentally is, that if I plug the 6,3mm cable into LineOut only half (not pushing up against) I can hear the Autotune process (without any manipulations inside the MB33). I'm not sure how a Autotune must sound. On my device I hear two parts: First I hear a constant sound (which can be manipulated e.g. with the waveform potentiometer) for few seconds. Then I hear a falling tone pitch sound. After this the midi-LED goes off. Seems to be ok, isn't it?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on March 17, 2013, 02:52:51 PM
I explained what happens when you press the autotune a couple of pages back.

When you press the autotune button you won't actually hear it doing the tuning like you do on startup as it doesn't trigger the envelope. It takes about 40 seconds or so from when you press the button until the midi LED goes out again. While its doing the tuning the unit won't respond to midi.

Are you actually getting the CV and gate signals on the pins i mentioned in a previous post?

What you've described is the correct startup procedure. Why would you press the autotune as it starts up?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: darkest_star on March 18, 2013, 09:26:38 PM
Quote
Are you actually getting the CV and gate signals on the pins i mentioned in a previous post?
I will check this again.

Quote
Why would you press the autotune as it starts up?
In the German manual (my native language) I can find a statement, that a demo song will be played if one presses the Auto-tune button while powering on the device. But on my MB33 I can hear nothing.  ???

If I have time again, I will measure the CV/gate signals you proposed and resolder all points on the board.

Btw, do you know about the three unused jumpers (one beside the Autotune jumper, and two in the analogue part) on the board?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: darkest_star on March 18, 2013, 11:22:32 PM
So, now my MB33 is working again as it should.
First of all, I'm very sorry! The last few days I have done two big mistakes:
1. Since my MB33 was completely open I was shure that the audio out is the one direct beside the midi-in. My audio cable was plugged in VCF-in.  :o
2. I had reprogrammed the EPROM, but it was faulty. Now I have burned another EPROM (a different IC) and my MB33 blinks when receiving midi-in.

So, the real cause seems to be the original EPROM-IC. Because the old one is still not generating sounds... Even, if erased and reprogrrammed whith crustypaul's image.

And with the new EPROM-IC (burned with crustypaul's EPROM-image) I can hear the demo song (or better demo sequence) after the Autotune operation.  ;)

Strange, but true.  :D

Now I want to thank you very much for help and support!!!!!
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: Circuitbenders on March 20, 2013, 12:07:15 PM
I've never noticed it does that demo sequence before. How odd, there doesn't seem much point to it.
If you start the thing up with the jumpers shorted near where the autotune button connects, it seems to go into a kind of test mode where it just plays a note endlessly. The jumpers to the rear on the right seem to give every note an accent when you remove the jumper block. I'm not sure what the jumpers in the filter section do.

What do you have to do to a synth to corrupt the EPROM? I would have thought that was pretty much impossible, especially with the rest of the synth surviving intact, and yet it does seem to happen on these things more often than you'd imagine.

It could be that you had a combination of problems on this synth. Maybe changing the CPU and the EPROM fixed it, but either on its own wouldn't have done.
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: darkest_star on March 20, 2013, 09:04:45 PM
Quote
I've never noticed it does that demo sequence before. How odd, there doesn't seem much point to it.
I think, it's good for testing the sound equipment (mixer, amps) or isolate failure causes because you don't need a midi device to send notes to the synth to get a sound output.

Quote
What do you have to do to a synth to corrupt the EPROM?
I've bought it at ebay as defective. So, I don't know about the history of the MB33. I find it interesting that the verify operation after erasing and reburning the old EPROM hasn't found any problem. It could also be a contact problem of the EPROM socket. To use another EPROM chip was completely accidentally, because I have found a similar EPROM (a 27256 instead of 27C256) in my box of spare parts and I've thought: The proof of the pudding is in the eating.  :D

Quote
It could be that you had a combination of problems on this synth. Maybe changing the CPU and the EPROM fixed it, but either on its own wouldn't have done.
Maybe. Now I have a working synth, where I could try out some circuit bendings. If I destroy something in the digital part I can replace the ICs very quickly.  ;)
I'm very happy to found a forum of synth hobbyists and thank you very much for your support. Also thanks for the hints regarding the jumpers. I will try this out in the next few days. PErhaps I could find out the reason of the jumper in the filter section?
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: chebru on August 18, 2016, 09:20:47 AM
Hi,

I just received a Freebass FB383 yesterday. I was gearing myself up for some serious troubleshooting as the seller said that it was broken and didn't respond to midi messages. When it arrived I was pleasantly surprised to find out that the unit worked perfectly, as long as the midi channel was set up correctly - I'm guessing the seller didn't have access to the midi channel set up key posted on the first page of this thread. Thanks a million for posting it!

So, now I have a fully functioning Freebass, which is awesome, but I'm not so sure about those ugly black and orange plastic knobs, especially when I see the beautiful retro style knobs on the units on this site.

Where would I get hold of knobs like that, and what size would fit this synth?

Thanks in advance, and thanks once again for all the great info in this thread!
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: bujbot on January 14, 2018, 03:51:13 PM
I repair a lot of stuff and often don't share what I did, thinking it's too obscure a problem so no one cares ... so I'm trying to fix that going forward in case there's someone out there stuck and googling like crazy.

I fixed my MAM MB33 MkII that was showing the classic power on issue (Light never turns off).  Here's what I did to fix it up (in 2 parts).

1) There was no clock signal going to U86, this is because of U91:B, so I tuned TP18 until the U91 could operate as usual and send that pulse.

2) make sure to get the proper power supply if you are missing it.  Doesn't need to be that one that came with the unit, just the correct voltage with enough current overhead.  It will work with one slightly off, but it sounds a bit buggy.

Happy bending

*the MB33 mkii schematic is in this thread somewhere I believe.  U91 is LM393 and TP18 is the furthest trimpot on the bottom left when viewing the PCB from the top (across from the wave shape selector switch).
Title: Re: FAT Freebass / MAM MB33 Problems + Midi channel settings
Post by: belzrebuth on May 08, 2021, 02:01:22 AM
Reviving this thread once again.
New user; first post and seeking for help with my MB33 II.
I have a strange issue with main OUT volume dropping as soon as I turn the distortion pot CW.
The volume drops a LOT as soon as I engage the pot and then goes to zero about 1/3 of the distortion pot's travel.
As I understand from the schematic there's some passive current mixing going on before the signal hits the main OUT opamp which seems like it's only a I-V converter from the sumed current  leaving the OTAs.
So far do good but I don't see any good enough reason for the volume to rapidly drop as soon as the pot is turned.
I get just a little bit of distortion before the signal vanishes.
I have a hunch that it's probably a DC bias issue at play which manages to collapse the whole 13700 but I could be wrong of course.
I haven't actually had a proper look on the circuit but thought to post here in case anyone might have any suggestions.
Possibly help others as well..
Cheers.