I managed to build the Korg MS-20 VCA using the schematic from the service manual (and it worked great with a couple of very different LDRs, as long as I changed the resistor values of the second op amp to match the response of the LDR), but it got my head scratching when I built it on stripboard as I could not get any sound at all.
Here is the schematic and stripboard layout that I am working with. Note that I made a couple of changes around the second op amp in order to match the much lower dark resistance of the particular LDR that I used: 2.2k → 1.2k and 22k/10k → 2.2k. The VCA worked perfectly on breadboard, but when I transfered exactly the same components from the breadboard to stripboard, I got no sound at all.
Here’s the story and what I checked and tried so far:
The first culprit was a bad/shorted input jack. Chucked that away and replaced it with a working one.
I also checked all the solder joints. Nothing bad there.
I got an audio probe. I get sound as expected at the input jack, then at the input point (row D) of the stripboard, but nothing at the other end of the 100k resistor (row F). I checked the resistance between rows D and F and it’s correct at 100k. However, I got a signal at row A through that 75k resistor and again at E. Now I am scratching my head. How is it possible to not get a signal at the input of the 75k resistor but get a signal at the other side of it?
Perhaps there’s something obvious that I am missing here?
After a cursory glance, I don’t see anything wrong with the stripboard layout (however it’s been a while since I’ve worked with stripboard so I’m a bit rusty). Are you able to post a couple pictures of both sides of the board?
Row F/pin 2 of the opamp is held at a virtual ground, so any measurements taken there will read 0V. Because the non-inverting input is tied to ground, the output of the opamp will do everything it can to make sure the inverting input will be 0V, so this seems to be working as intended.
Thanks! I had a small breakthrough as I tried to get a couple of good pictures of the board. Under better light, I was able to spot a tiny solder bridge between rows E and F. Now it passes sound but extremely weakly. Moreover the CV input does not work.
I will probe further, but in the meanwhile here are a couple of pictures of the board.
Looking at it a bit more closely, it appears the emitter of the transistor is connected directly to the +12V rail. You may need to cut the copper at row I column G!
What resistance do you expect from your vactrol with maximum CV? You could try putting a fixed resistor of similar value in parallel with the vactrol resistor and see if signal appears at the output. If it does it means the vactrol’s never opening; if it doesn’t it means there’s a problem in the output stage.
Is the transistor collector voltage what you expect?
I think something around 1k although I could not measure it accurately on the breadboard. Dark resistance should be also low around 35k or so hence I adjusted R14/VR2 accordingly. I am not sure about the voltage at the collector, but I guess 12V across the 5mm green LED + 1k resistor, should be something like 8V?
That was it, thanks a lot! I have been looking at that layout for hours and spotted two mistakes from the first iteration, but could not spot that one!
Excellent, I’m glad you got it working! If you feel like updating the stripboard layout and posting it here (or on the verified stripboard layouts thread), it could help out others in the future if they want to make it for themselves.
Next time you have a circuit that “ought” to work, but doesn’t, try to trace the signal using a simple amplifier as described here. With one eye on the schematic and one on the circuit you will be able to easily follow the signal path where it begins and where it is interrupted.
Chucked that away and replaced it with a working one.
I have been looking at that layout for hours and spotted two mistakes from the first iteration, but could not spot that one!