Polivoks Restoration and Modifications

So, a few weeks ago I made a fantastic purchase - the legendary Polivoks analog synthesizer.  The one I got was in a very sorry state, but if it hadn't been I couldn't have afforded it.  I bought it from a guy in Belarus via Ebay.  It had been modified (badly) by a previous owner and he hadn't managed to get it back into working order.  It's keyboard had been hacked off and MIDI added, but it wasn't functioning correctly - the pitch was tracking poorly and it wouldn't always respond to a gate signal.  The LFO wasn't functioning and the amp envelopes sustain and release had no response.

The MIDI to CV board was a professionally made board, but there was very little information about it on the internet so I had to analyze it myself with my miniature digital scope while sending MIDI to it.  The Hz/V tracking needed calibrating, and the tuning of the synth could then be calibrated reasonably well - I get 4 octaves within 3 cents of accuracy.  There was a PWM out that responded to velocity which was connected to the VCA daughter board, but it didn't sound very nice.  I removed this entirely and replaced the solder bridge that it had been replacing and voila - sustain and release functioning again.  Although this synth should be switchable to duophonic - being able to play 2 notes at a time by assigning one note to each oscillator, this would only be possible with a duophonic keyboard or a 2 channel MIDI to CV converter.  This is not a big deal for me, I probably wouldn't have used it anyway.

The gate signal issue was resolved by adding a diode as seen in the gate/CV mods already published at technopop.  The LFO issue was resolved by cleaning the contacts on the LFO daughter board.  Despite some unfavorable comments elsewhere I was surprised to find the construction of this synth is actually very well done.  I don't understand Russian Cyrillic but I found it very easy to read the schematics.  The controls are mounted on two main boards screwed to the case, with a number of daughter boards mounted at right angles to the main boards, all held in with a large brass strip and a number of  screws.  Each daughter board is dedicated to a single function - 1 for each oscillator, one for VCF etc. This made troubleshooting much easier.

The rest of the restoration was pretty standard stuff - clear up all the dust, clean some contacts, Tighten up some screws and replace some really dirty pots with modern equivalents.   To fix the loose knobs I took a modern rubberized knob and peeled off the rubber, trimmed the plastic insert down and not glued them in place of the original plastic insert.  I have 4 knobs missing and it will be impossible to find exact replacements, I will have to do some browsing to find some vintage looking modern alternatives.  The power button cap is missing, I will probably replace the whole switch with a high quality rocker switch at some point.

And so having made the Polivoks fully functional again, I set to work looking for a few mods.  This synth features analog FM - you can modulate oscillator 1 with oscillator 2.  But the modulation doesn't go deep enough.  This was easily rectified by replacing a resistor (R64) with a short piece of wire.  With the knob turned fully counter clockwise, it sounds like an unmodified Polivoks.  At about 12 o clock it sounds like an unmodified Polivoks would fully clockwise.  And fully clockwise the FM sounds comparable to analog FM on my Eurorack modular VCOs.

The other mod was to extend the LFO rate.  This was also a pretty simple mod, simply replacing the original rate knob (R107) with a with a 1MEG pot.  With the knob fully clockwise the rate is the same as an unmodified Polivoks, with the knob fully counter clockwise the rate goes to about 0.5Hz.

I also replaced the main out, headphone out and expression pedal in jacks round the back - they originally used DIN jacks.  1/4 inch jacks can fit in the same holes with no case modifications.  The expression pedal input controls filter cutoff frequency.  with a 1/4 to 1/8 inch cable I can now modify the cutoff with my modular.  I have tested it with both unipolar and bipolar signals and it functions well.  The operating supply of the Polivoks is -+12.5V, so it is going to be safe with interfacing directly with most Euro racks as they are usually on -+12V, but be careful with a -+15V synth - module outputs shouldn't go rail to rail, but if they do they could damage the Polivoks.

In the future I will probably add pitch and gate CV inputs for interfacing with the modular, replace the plastic end panels with some polished wood end panels and replace the bottom plate with wood also - the leatherette bound metal case currently has a split right through it from when the keyboard was removed.  I also might replace the figure of 8 power supply jack with a 3 pin IEC/kettle lead, hooking up the ground supply to the body and the transformer to try and lower the noise floor - something I have already done, along with other mods to some soviet drum machines.  Details of these mods to follow in a later post.

PLEASE BE CAREFUL.  AS WITH ALL MY MODS I ACCEPT NO PERSONAL RESPONSIBILITY FOR DAMAGE TO YOURSELF OR YOUR GEAR.  ELECTRICITY IS DANGEROUS, IF YOU DON'T KNOW WHAT YOU ARE DOING DON'T ATTEMPT ANY OF MY MODS