FAT Freebass / MAM MB33 Problems + Midi channel settings

[Circuitbenders]

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.

[Signal:Noise]

How hard are the freebass' to mod? I've been eyeing one up for ages for all my acid need.

[Circuitbenders]

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.

[jonesie196]

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?

[Circuitbenders]

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.

[jonesie196]

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

[Circuitbenders]

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.

[jonesie196]

just placed my order for one! will be nice to get the beast back working!

cheers for all the help 

[adam]

Which resistor is the one for the resonance boost? Thinking about trying this and just about to start repairing my FB!

Thanks in advance.

[Circuitbenders]

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.

[adam]

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!

[Circuitbenders]

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?

[spk]

 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?

[Circuitbenders]

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.

[catweazle]

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))