Sunday, March 24, 2013

Polivoks repair: dead outputs, 120 volt conversion, scaling issues

This unit came to me almost completely dead: neither oscillator working, just a power light and a slight hiss.

Bad paper and oil filter capacitors were the ultimate cause of the dead oscillators, but once they were replaced there were serious issues with the keyboard tracking.

At this point it became tricky since the only schematics that seem to be available are in Russian!

I made some headway by replacing the op-amps on the keyboard circuit board. I used a reference table found here,

Russian Ref
Europa/USA Ref
KR140UD8B uA740 , uA741, LF351,... Single standart AOP
KR140UD12 uA776, MC1776, LM4250,NTE888 Programmable low power AOP
KT315G 2N3904A NPN transistor

which states that the russian op amps cross with uA741's, which are super standard monolithic op amps.

This fixed many issues, however, there was a gradual detuning of every note if the sustain was turned up.

With no circuit descriptions (that were in English), I was ultimately able to use the international language of schematics to find the sample and hold circuit on that keyboard circuit board. It consists of a fet, and an op amp and a few capacitors. Essentially, while the key is depressed the voltage flows through the fet and charges the .22 capacitors. Then the key is let up, the fet opens up and the capacitor remains charged to the same voltage, and continues to flow through the op amp (a7 on the schematic below). Since the op amp has super high impedance, the capacitor discharges so slowly that it holds at the same voltage for a long time, and the op amp output stays at the sampled voltage for quite a while. A good description is available here.

The sample and hold circuit thus demands a fet based op amp, so that it doesn't draw too much current from the capacitor and cause the sample voltage to decay too quickly. The 741 op amp worked perfectly everywhere else as a pin for pin substitution for the russian op amp, but not in this sample and hold circuit!  I replaced it with an original russian one, and everything worked fine. (I am sure I could have replaced it with any fet based op amp, but I had russian ones left over since I had replaced quite a few on this circuit board, having use a shotgun approach.)

Interestingly, here is a website which shows a sample and hold circuit using the 741... notice the addition of the 10k resistor, which probably helps the 741 work without drawing too much from the capacitor. 

Here is a picture of the circuit in question

Once all this was settled, the keyboard worked well. I installed the mods located here for external CV and Gate, and I also swapped out the transformer with one that can convert 120 volts down to the 34 volts, center tapped, that the polivoks needs for power (thus you can run it off from regular wall power). I installed an IEC connector and it is all set!