Made in the USSR in the 1980's, this synth was always one I had wanted. It was the design and the low end bass that attracted me. People say it's built like a tank, well sort of. It 'looks' like it is but the plastic is actually not that strong, still better than a Pro one. I got this great Soviet era synth from a friend in St Petersburg in Russia. It was sold with a case and foot pedal. It was quite reasonable to buy in Russia but once postage and tax was factored in, it was not so much of a bargain, but still worth it.

It is nicely designed. Each circuit has it's own card which can be removed from the back plane. Makes troubleshooting much easier.

power supply voltage adjust trims. The -12.5 rail on mine was dodgy and made the synth lose tuning, so replaced it with a new/old stock open trim of the same kind

 

CONDITION

The synth was cosmetically good, no broken rocker switches or big scratches. The knobs looks slightly off line. This seems to be normal amongst Polivoks. I removed one and noticed 2 little metal shims fall out, so be careful they don't get lost of the knob will be loose.

The machine played but was way out of tune. I removed the cover and got it basically in tune, but will do this properly once I have checked the synth over. The keys all worked but the keyboard has a plastic feel to it. I believe it uses reed switches. It is responsive though.

I pulled a few electrolytic's from the PCB's and they all checked out fine on a meter, so I think this synth has had all these changed at some point. They were also not a Russian make.

The main issue I could see was corrosion. This had started to damage some of the PCB tracks and legs of components. I gave the boards a clean with Isopropyl alchol.

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MODS

These are just my own experiences, and may not be the perfect way to achieve the desired results, so are not binding. Mod this synth at your own risk.

 

LFO

There is an issue with this synth that the LFO does not go slow. I think it's the same for all Polivoks. I decided to see if I could change this. One option would be to replace the current LFO potentiometer with a bigger one, say a 1M. I didn't;t want to have to remove the entire from assembly and the circuit board. I decided to have a look at the modulation card, y7. This has a generator circuit comprising of R15 and c7. The capacitor charges/discharges creating a square wave.

 

I removed the resistor and wired the new pot across the pads below. Notice the corrosion which I had to clean off, weird as this synth does not have a microprocessor, therefore no battery.

I decided to replace R15 with a 1m pot and mount it on the side of the synth, in the recess where the hand hold is. The pot allows me to shift the range and get some quite interesting effects. The pot allows me to reduce the rate considerably, to about 2 seconds. Still not slow enough, so I plan to experiment with the capacitor value. The knob is subtle and does not change the look of the synth too much.

 

FILTER CUTOFF RANGE

The filter seems to have an odd range. Cutoff goes fully open very early in the pot travel. I decided to fix this. I added a 91k resistor to the existing R87, 47k resistor.

Snip the bottom end and add a 82k or close resistor to the end of the existing resistor. Wire the other new resistor leg to the central cutoff terminal, just below the resistor.

You could of course replace R87 with a 120k or something close. I was just too lazy to remove the entire front panel to do this. The range is now much better. I got this idea from a post on Muffwiggler.

 

 

X-MOD RANGE

The X-MOD where oscillator one modulates oscillator 2 was a bit limited. I saw a mod somewhere that allowed this to go a bit deeper. Locate R64 and replace with a wire link. I just bypassed the resistor, as I am still experimenting with this.

 

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TUNING

 

There are many guides on how to do this online. One thing I noticed is that the top range can sometimes be out of reach and there's not enough travel on the high end pot to reach A440. The solution is to adjust the trimmer shown below on the oscillator board in question. Adjusting this will give one more room to work with when tuning from the front panel. Perhaps adjust it with a bit of overspill so that when tuning one is not constantly trying to reach the A440 at the very end of travel on the front pot.

 

 

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THINGS TO WATCH OUT FOR

 

losing shims from pot knobs

potentiometers not working, hard to get inside these without removal

internal trimmers gunked up, due to never being moved.

Power supply rail voltages off, should be + and - 12.5

boards not making contact, remove and clean contacts.

tuning

corroded component legs, at least on mine

 

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Of course having enjoyed this synth for a few days, it decided typically to play up. Oscillator 2 gave up, making only low clicking noises. I swapped the card over to oscillator 1's slot and the problem moved. I knew then that the card had a fault.


First up, reseated the card and probed voltages, then  I  checked for any IC's getting too hot. They all seemed ok. Next I desoldered the K140UD8 silver can opamps. One leg came off on one device, due to corrosion.  I managed to repair this though. These checked out ok on my tester. Next it was the resistors. They all were in spec. Then it was the capacitors. Again all ok, bar one ceramic which value was swinging all over the place. I replaced this , but it's probably not critical.
Finally I knew I had to tackle the two 14 pin DIL IC's. One was a precision opamp KP551 and the other the heart of the oscillator, a resistor network KR198NT1A, a Soviet version of the CA3086, with different pin layout. A 3086 will not work as a straight swap in this synth.

I had no spares of these, so my options were, order from Ukraine and wait 2 weeks, or swap over the chips from the other working oscillator. I chose the latter, fitting decent IC sockets in the process.


Well it turned out that the KR198NT1A resistor network was faulty! So I ordered 2x NOS ones from Russia, curing the problem.


I think running the synth for a few days after possibly years of inactivity may have produced the fault.