Right, just to be sure we are working from the same point (I am assuming like me you don't read Russian Cyrillic) the LFO board is Y7, which connects to the main board at X7. From what you are describing the LFO is not oscillating, so the modulation knobs for the VCOs, VCF and VCA will do either nothing or very little - at most it would add a constant voltage to the other CVs (envelopes and knobs). I expect the VCA pot being bumpy will be fixed with a hell of a lot of wiggling to remove the layers of dirt.
It is going to be difficult to exactly pinpoint the problem without me being there, so I am going to try my best to help you understand the circuit so you can figure it out for yourself. I am going to work backwards, telling you what I think isn't causing the problem so you can hopefully figure out what is.
looking at the schematics, the top section is noise generation and sample and hold - A1, A2 and A3 are just conditioning the noise generated by the transistor V1 to be at the correct voltages. When a pulse is sent to the transistor V3 the voltage from A3 is held for the length of the pulse, with A4 being used as a unity gain amplifier to buffer the signal.
The pulse for the V3 transistor comes from the V4 transistor, which is converting the positive portion of the square wave at A5 into 0.2ms pulses. The square wave at A5 is also driving the LED driver transistor V5. The square wave at A5 is passing through the V6 diode to remove the negative portion, with A7 being used as a unity gain amplifier to buffer the signal.
The rest is the section you need to pay attention to, which is much easier now we have reduced the number of parts to look at. What we have here is a comparator based square wave generator. This is an RC (resistor capacitor) circuit - it sounds like you might want to do a little reading on the basics of RC circuits, op amps and comparators.
If you take a look at this square wave generator example perhaps you can see the resemblance to our schematic - C is C7, R1 is R14, R2 is R13 and R is the combination of R15 and our frequency pot. The capacitor charges and discharges through R, creating a triangle wave. The variable value of R (thanks to the pot) dictates the rate of charge/discharge and hence the frequency of the triangle wave. A6 is being used as a unity gain amplifier to buffer the output of the capacitor. A5 looks at the triangle wave and outputs a positive voltage when it is above a certain threshold and a negative voltage when it is below a certain threshold.
My guess is it is probably NOT a component failure, its far more likely to be the connection between boards as this is a vulnerable point. Do all of this without power applied. I would remove the Y7 board and put your multimeter on continuity test. Check that at the X7 socket on the main board, pins 4 and 5 have continuity to the R107 pot. While the board is out, clean the contacts on both Y7 and the main board using isopropyl alcohol. Leave them apart for a few hours to be totally dry. Put board back in and check continuity between the R107 pot and pins 4 and 5 ON THE X7 SOCKET ON THE Y7 BOARD.
Beyond this you are looking at tracing tracks in case any have broken, or replacing components. The point is that without the RC relationship there is nothing to periodically trigger the rest of the circuit. Seeing as we have already checked the connections of the R part, the first one I would replace is C7 - easy enough, find an electrolytic capacitor of the same value. Folowing that I would replace A5, but that is where things might get awkward - how are you going to get a replacement russian part? It is just a single op amp being used as a comparator, so I think a TL071 would be fine but I can't be sure.
When I first got my Polivoks the LFO was working, but only some of the time. The rest of the time it was doing something similar to yours. My board was sorted with a damn good clean, but only after a day or two of head scratching, looking for something to fix that wasn't broken.
Please let me know how the exploring goes, I hope you figure it out. Good luck!
Thanks for this massive post! I will definitely print this out and add to my Polivoks data forder :)
ReplyDeleteAbout my faulty LFO - I got it work! Replaced 99% components on FLO module and it worked http://vaikus.eu/wp-content/gallery/polivoks/polivoks_remiont22.jpg. Yes, I made hard work to find critical components for replaceing. At least found company "Evita" (http://evita.lt), wrote to them and asked help for finding all those old russian components (KT815G. KP303V, KR140UD8B, KD521V). I got all I needed from them, very good service 5+. I replaced all those components and it did not help! It was bad day for me… Suddenly I decided to replace all capacitors left there, it was my last but weak hope, those components costs nothing basically. And it was it! My Polivoks LFO is working. Was it some mystical faulty capacitor or something more, I cant tell exactly. I was very suprised that some capacitor was faulty, because visually was all ok, also I tested them earlier with regular tester (seems in working order) but I do not have specific devide for cap testing.
I definitely learned a lot and had intersting times during this restoration process, which I personally enjoyed :) Also it is good feeling to give life back to something old and special. I wrote little post about restoration there (unfortunately it is in Estonian laguage, but there are some pictures also): http://vaikus.eu/?p=1094
I made other usual stuff also: Cleaning all around and inside, lubricateung keygoard, replaced 5DIN sockets, repaired some pots (remove, open, clean, close, solder back in), replaced LED's, installed IC power socket with GND.
Four knobs and front panel is missing, it is not a big problem but I keep my eye on eBay.
CV to MIDI board is arived allready. I have plan convert Polivoks for MIDI, but at the moment lets wait until snow arives :)
Will let You know how it goes.
Ciao!
Hello, respect for full interpreting polyvoks modulator schematic. I try to modificate modulation board to control frequency of modulation from external gate signal. I thought about disconnecting(by the 3pins switch) A5 opamp output, and connect 12,5V gate instead of opamp. Does it suitable solution? It will work ? best wishes, And
ReplyDeleteHey! I have half working Polivioks with missing modulator (lfo) and second osc module. I just soldered jack to the output pin in the main board and thus I can connect anything to modulate the synth or any of it's parts.
ReplyDeleteAt first I connected a free running oscillator (1 opamp multivibrator) proceeded with an adjustable lpf and volume. The output of generator itself is a square, but with lfo you can tune it much like a triangle.
But I've decided to go further. I've connected one of my usb soundcard analog outputs, set up LFO oscillator in ableton (with operator), and route it to dedicated output that is connected to polivoks jack mod i've described earlier .
I even can create a little pattern sequence of driving noises and silences in ableton, and modulate, for example, VCA with square, so i have staccato notes on polivoks while the key is pressed. Also, the most right and down switch plays the last note endlessly. So with the Polivoks analog filter and PC tempo syncing (on VCA) I can do some kind of analog (from polivoks VCO) acid lines, changing note pitch with the Polivoks keyboard.
I mean Jack to Modulator output pin in the main board the module was installed.
DeleteSo my next step is to clone or replace the second VCO (that is missing) with compatible one or just with another jack in to modulate 1 OSC with any musical source.
ReplyDeleteHello
ReplyDeleteThank you very much for your in depth explanation of the POlivoks LFO.
Since a couple days, my Polivoks LFO suddenly stopped to be able to change its speed when i turn the RATE knob : it seems to be very fast all the time, and turning the knob does nothing (it was working fine previously).
The led is keeping on lighting fast, and i hear the audio rate modulation, so i can say that it is sending some modulation signal to the rest of the synth, but i can not make it slowing down by turning the knob.
Do you have any idea what could be the cause ?
Should i try to open the case, and just try with wome cleaning ? Or can it be a faulty component ?
Thank you