Bad, bad Polivoks (or builder) [SOLVED]

Hello guys,

Trying to bring the Polivoks Shruthi to Romania, and not much luck until now… :expressionless:

  • main issue: very, very low signal on the output (50dB of preamp gain needed to barely hear any sound)

  • secondary issue (now gone, without any clear reason why): I accidentally put the output cable that was going into my audio interface in the “Input” jack while doing the first tests; and it actually put out sound, but (75% certainty) it was unaffected by the cutoff/reso controls (I doubt there were any shorts to carry the OSC> signal straight to the input jack… quite paranoid about these things, always checking)

  • had a 10k log pot in the R2 (now it’s just shorted), and it didn’t do anything to the signal

  • it might seem that, even lower in volume than the already very low filtered output, there might be some unfiltered OSC> signal bleed

  • LEDs/LCD light up, all the pots/encoder/switches are fully functional, so my guess is the control board is quite fine

  • the “pvks” setting is on

  • it receives notes and all via MIDI

  • the cutoff/reso work properly, so do the Dist and FM feedback (audibly), LP/BP settings work

  • supply voltages are good (+/-5V), with or without control board on

  • F, Q, VCA and OSC have been all checked (there was a post about how to do this), all good

  • all ICs have been socketed, checked somthing like 10 times for the right orientation

  • solder joints seem good (had quite a few small builds before, never a cold solder joint in sight)

  • transistors have been checked for orientation and positioning, trouble free

  • even though my OCD prevents me from doing any stupid “swap around the values” mistakes, I checked at least half of all the soldered resistors, and they’re all good

Hmm, quite the short summary for 6-or-so-hours of fruitless troubleshooting. :stuck_out_tongue: Any help, please? :slight_smile:

All the best,
Mihai Toma

Check the schematics and probe the:

  • Mixer output (IC4 pin 1). Working?
  • LP/BP nodes of the filter (IC5 / IC6 pin6). Working?
  • Filter mode selector IC output (IC9 pin 14). Working?
  • VCA/Output amp output (IC4 pin 7). Working?

I actually label things on the schematics! How nice!

Yes, thanks for the labeling. Very useful. :slight_smile:

So, after a hearing test:

  1. Mixer output - strong signal
  2. LP/BP nodes - both dead (impossible, I guess) or even lower than the already abysmally low output (I can definitely hear a difference when changing between BP/LP, and the filter actually seems to work, so I guess they’re just REALLY low)
  3. Good signal, but a bit lower than OSC> or mixer output
  4. Really low signal, about the same as my output.

(BTW, I do know a bit of electronics, at least to the point that I understand schematics and [sometimes] what role most components have; maybe this helps you out in talking easier]

As per our e-mail exchange, fast as the wind and really nice and helpful. My hat’s off to you! :slight_smile:

Could you post photos of your board?

https://skydrive.live.com/?cid=ae6c634f9751f211&id=AE6C634F9751F211!109

Here you go, kind sir. :slight_smile:

Disregard the cable - it’s just what’s left after removing the output attenuation pot. Signal is the same before and after it.

Checked all resistor and non-electrolytic capacitor values. All the same.

and they are?

The right values all across the (PCB) board. :slight_smile:

Come to think of it - most likely what I’m hearing as very low output level is just the actual, processed/filtered signal seeping into the ground… So that could mean a short to ground, after the LM4250s?..

So, please correct me if I’m wrong (I’m trying to figure how this blood-red thing works):
ICG - controls VCA gain (by the IC3OTA1) from VCA>, after buffering by TL084 and 2N3904
ICF - controls filter frequency (by the LM4250s) from F>, after buffering by TL084 and 2N3904
ICQ - controls resonance (by the IC3OTA2) from Q>, after buffering by TL084 and 2N3906

MIX - OSC> + input jack, buffered by TL072P1

BP (IC5), LP (IC6) - signal post LM4250s (post resonance and filter frequency processing)
—>
BP and LP go into the 4053LN multiplexer and come out via VCA OD (overdrive setting on) or VCA IN (no distortion), then carry forward into the IC3OTA1, then into the output buffer TL072P2

Yes, except that the transistor circuits generating ICG/ICF/ICQ are not ‘buffers’ but controlled current sources.

Thanks… There my electronic ends. And now expands. :slight_smile:

Checked all the V+/- (VCC/VEEs, whatever) of all ICs - all good, in the +/-5V range (0.05V tolerance).

All values DC, rechecked as of now:
Q> from control board - 0 (min res.) - 5V (max res.)
F> from control board - 0 (min freq.) - 3.1V (max freq.)

Questions, odd stuff:

  1. Pin 14 of IC2 - 0 (MAXIMUM res.) - 3.3V (min res.) is this right? - in a post, you mentioned -3.3V to 0V
  2. How could I check VCA>?

Shit! moment:

  • pins 6 of both LM4250 do have correct output signal! I may have been stupid enough to measure pin 7 instead the first time around. (4 hours sleep/day = BAD!)
  • pins 14, 15 of 4053E (VCA In, VCA OD) output the right signal, according to the DIST setting
  • pin 3 of LM137000N has the right signal, albeit lower in volume then the above (thanks to the resitors, I guess)
  • pin 5 of LM13700N outputs just some clicks (at the right time of the MIDI notes), pin 6 of TL072N (output buffer input) is exactly the same (as they should be, as they’re linked directly)
  • pin 7 of TL072 (output buffer output) has the right signal! (I was so tired I must’ve measured pins wrong last night)… signal goes after the output cap

OK! SOLVED!

The fuckin’ (pardon my French) link between the cap’s - end and the output jack is broken…

Your voltage reading on IC2 pin 14 is correct.

Can be put down as “solved due to idiocy”. I had lifted the lead going from the output cap to the jack, and didn’t see it, somehow.

First Romanian Shruthi, now online and making nasty noises. :slight_smile: