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[ultima]

I took the  Circuitbenders Sh 101 Mod Pdf and followed it through to every detail.
After assembling the  synth im having some troubles.

The filter input works , switching to Fm input works as well.
The Lfo Speed Mod Works.
Overdrive Works.
CV control input works.

Cant really tell with the PWM Source selector, FM Mods.

The reason i cant tell with the PWM+Fm is tha fact that after assembling i seem to have lost the VCO.
I can make the filter self oscillate and track the keyboard so i know the keyboard works.
I dont think the VCO is dead , Im just getting no output from the oscillator.
I can hear the vco's working on the FM mod , just slightly , enough to know it isnt dead

Im tempted to think that It might have something to do with either the PWM or the fm Mod as those are the ones that draw from the oscillator?
I could also have messed up the output from the vco's when inserting the Audio input but i doubt it .

So....My question is .

Has anyone done this mod ? Is there a known error in the PDF?
If not , care to take a guess as to what might have gone wrong?


My next step would be to desolder the PWM first and then the Fm mod but id rather not if someone has an idea.

Thanks
Arnar

ps. I love the filter input mod as well as the overdrive and LFo speed mod, those alone are worth the effort.
pps. If you are thinking about doing the mod then its pretty easy ( apart from my troubles) .
BUT.....im having a bit of trouble with mounting the keyboard as the Switches dont allow all the clearance i need so try and find the shallowest switches and pots you can find . The  small pots are no problem but the switch and rotary pots are hard to fit.

My fix for that is lowering the keyboard with washers.

[Circuitbenders]

hmmm, obviously fault diagnosis via a forum is a little tricky but i'd say your first move should be to check the obvious stuff like accidental solder bridges or shorts, that all the cabling is connected properly, or even if all your pots and switches etc are actually working. As far as i'm aware theres no error on the PDF as i've modded a few 101's using those guidelines so make absolutely sure you've followed them exactly. I've spoken to a couple of people who had problems because they weren't looking at the right area on the board and soldered mods in at the wrong points. Admittedly the images on the pdf are a little dodgy.

Its unlikely to be the PWM mod thats causing the fault as it draws its source from the LFO rather than the VCO. If you've lost the VCO then you won't be able to hear the PWM mod working anyway. Unless of course you have cut the wrong circuitboard trace when disconnecting the original LFO connection, or you have accidently cut more than one trace.

I think it says on the PDF that the most effective way to lower the keyboard is using slices of thick guitar cable or similar as washers. That way you get a bit of compression in the washers as you tighten the screws and you tend to get a better fit.

[ultima]

I have now taken all the mods off and im just aiming to get the VCo working again.
Im absolutely sure that i followed the instructions correctly
I have checked both the controller Board and Synth board for bad solder or cracks etc.. and cant find anything.

All the LFO shapes are working as well as the Noise and i can find them all with an oscilloscope at the LFO.

Do you know if i should be able to see the waveforms by putting the scope on the Cem 3340 VCO? I tried that and cant see anything except on 1 pin and its not a clean sqr or saw. Would you know how to test if the VCO is dead?

And the big question , can i get a replacement VCO if this one is busted??

edit.

After sleeping on this im starting to think that the Cem is dead....Any ideas how ro measure the cem to make sure?

Thanks a lot for your help.

Arnar

[Circuitbenders]

There a datasheet for the CEM here http://www.datasheets.org.uk/datasheet.php?article=1147818

According to that you should be getting a sawtootch at pin 8, pulse at pin 4 and a triangle (not actually used on the 101) at pin 10. These should be visible at these pins on a scope whenever the synth is powered up if the VCO is working properly.

CEM chips haven't been produced for many years but there are some synth repair places on the net that occassionally have spares available and you occassionally see them up for sale on ebay.

They usually have a lot of old CEM's available at http://www.synthchips.com/sparespage.html but they are not going to be cheap.

[ultima]

Thanks !

It seems that the chip is fried at least Im not getting these waves on the pins!?

I think I have found a replacement chip so Im hoping i can get it working again.

Thanks a lot for your help.

[ultima]

Apparently all the waveforms of the sh come from the VCO so if the lfo is working so should the vco.

im going to have abetter look at this

[Circuitbenders]

Well that suggests the obvious question, do you have your scope set up right?

[ultima]

I found the fault!!  Thanks  a lot for your help.

it was indeed a short after all , extremely small but i found it.
the squarewave was shorted at one of the mod points...damn...

The birght side is that i have traced almost the whole board and now know it inside out in fact i found a few other mod options , one of them being that unused triangle wave.

Back to the modding .

Thanks
Arnar

[Circuitbenders]

it was indeed a short after all , extremely small but i found it.
the squarewave was shorted at one of the mod points...damn...

9 times out of 10 its going to be something stupid like that. I have an old Paia modular system that had obviously sat in someones cupboard for 20 years after someone assembled the VCO with a short on the board and couldn't get it working.

I have the triangle wave option installed on my 101 but for some reason didn't include it on the bending guide, probably because i forgot about it and couldn't be bothered to open the thing up again.

I also have an LFO trigger input that restarts the LFO at the same point each time it receives a trigger. I didn't include that one on the guide as there is a very likely chance of destroying a chip or two if you don't know what you are doing and you need a DPDT jack socket which i couldn't find anywhere at the time.

[gmeredith]

Can you show how to do the triangle waveform mod? I have an SH101 and would like to add that one.

Cheers, Graham

[Circuitbenders]

Theres a basic way of doing it here http://www.hoohahrecords.com/resfreq/mods/sh101/index.html

I've heard it said that method of doing it can sometimes cause problems with tuning stability but i've never noticed it.