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MidiDAC 3 troubleshooting
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author MidiDAC 3 troubleshooting
gs3333
Hi guys

I've just started testing my newly built MidiDAC and I'm getting some unexpected behaviour. The gate signal is working perfectly but the pitch CV output doesn't seem to respond to MIDI. It's stuck up at around 12V or so with the INIT trimmer roughly in the centre of its travel, although I can adjust it down to about 10V. Either way it's constantly on and doesn't seem to respond at all to MIDI notes.

If it's relevant here I have a 2u2 63V electrolytic for C2 (-ve lead towards the bottom of the board).

I'm just about to start checking out the schematic. Any suggestions of where to start fault finding would be much appreciated.

Thanks

Greg
Synthbuilder
gs3333 wrote:
If it's relevant here I have a 2u2 63V electrolytic for C2


This probably isn't the cause of your fault but you need to get that changed for something else. Electrolytics are not going to work well there. Too much leakage leading to pitch droop at high slide times.

But the problem sounds like a bad DAC or DAC reference. But the midiDAC is a hard one to fault find if you don't have a scope. Do you have one?

If not, then make sure pin 1 U2 is reading -5V.

Tony
gs3333
No scope unfortunately. Pin 1 of U2 measures -5V.
Synthbuilder
Without a scope it's going to be tricky.

Do the other CVs work at all? Try checking the mod wheel output first since that's pretty easy to check. It should go from 0V to around 10V. If that works the DAC and multiplex circuitry is working. This means the fault probably lies in the slide circuit or U12.

If the mod wheel output is also stuck at 12V - then I would suspect a bad DAC, 4051 or 4104. But do check for a board short near the pins of those ICs before replacing them.

Tony
gs3333
I've just checked and the modulation, velocity, CC and pitch bend outputs all seem to be operating correctly.

There are no shorts around U12 so I'll investigate the slide circuit now. Is the value of C2 critical or would a 1uF polyester be a suitable substitute?
Synthbuilder
gs3333 wrote:
Is the value of C2 critical or would a 1uF polyester be a suitable substitute?


1uF will work but it will reduce the maximum glide time by 1.5. The 1uF will be worth popping in if you have one just to check though.
gs3333
I've replaced C2 with a 1uF and a 0.47uF polyester in parallel for now.

There's no obvious problem around U12, although I haven't swapped the chip itself yet.

So, I got busy with the multimeter looking for unusual voltages...

First I checked the slide output itself. I have 0V at pin 4 of CN1 until I trigger slide by pressing 2 keys at once, at which point I get 5V. The legato LED also seems fine.

With the INIT trimmer fully anti-clockwise, the CV output is 11.22V with no keys pressed. If I press the highest MIDI note I can, this goes up to 11.30V and stays there. If I press the lowest note I can, it drops back to 11.22V. Turning the slide pot up to full makes this 11.22V signal rise up to 11.32V until I turn the slide pot down again.

With the V/OCT trimpot fully anti-clockwise I get -12.8V at pin 1 of U1. With it fully clockwise I get -14.5V. This is certainly a long way from the recommended 10.58V!

Does this tell us anything or is there anywhere else I should check?

Thanks

Greg
gs3333
Pin 1 of U12 measures 0V when I push midi note 0 and rises by about 0.9V per octave. I guess this means we can rule out the chip.
Synthbuilder
gs3333 wrote:
Pin 1 of U12 measures 0V when I push midi note 0 and rises by about 0.9V per octave.


Yes this means the DAC, de-mpx and sample and holds are working just fine.

Measure the voltage at pin 1 of U4. This should track the pin 1, U12 voltage exactly. If not the problem lies within the slide circuitry. The 4066 would be the prime suspect in this case.

If it does track the voltage then the problem is around U1.

Tony
gs3333
Pin 1 of U4 is stuck at 13.94V.
gs3333
I replaced the 4066 and the problem seems to be solved. I must have damaged it at some point. The converter's not fully calibrated yet but for now at least everything looks good.

Thanks for your help.
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