Elby Synthacon VCF Trouble

[skweeegor]

Hey all, been lurking for a few months, lots of great info here. I've been doing the DIY thing for a bit now, and have never been stumped this badly, so I would really appreciate a hand here.

The following works:

- Resonance + Frequency knobs do their thing
- Filter self oscillates
- HP input works fine and filters fine

The following doesn't work
- LP input doesn't pass any signal
- BP input doesn't pass any signal
- CV input doesn't pass any signal

So far I've checked the following:
- No solder bridges
- No dodgy traces
- All polarity sensitive components in the right way
- Resistor values match silk screen
- ICs facing the right way

I also went over EVERY joint on the board by reheating the joint and flowing a bit more solder to ensure connections are good, being careful with heat sensitive components.

After all this I have no change.

I'm fairly new to reading schematics but I was able to figure out the following:

Using this circuit schematic:
http://www.cgs.synth.net/modules/pic/schem_cgs35euro_steiner.gif

And this board schematic for the PCB I'm using:
http://www.elby-designs.com/panther/cgs735-synthacon/cgs735-schematics.pdf

Based on those two documents, I've ascertained (hopefully correctly) that the three points on the board that represent the three filter inputs are:

R101 - HP Input
R104 - BP Input
R108 - LP Input

I tried taking an audio signal and probing with a 3.5mm jack to those three points, and R101 (HP input) passes a signal as expected and filters, whereas R104 and R108 do not pass any signal at all. This leads me to believe that there's nothing wrong with the jacks or their connections and the issue is on the board itself.

At this point I really have no clue where to proceed so I would appreciate a nudge in the right direction.

Thanks

[slacker]

Feed a signal into the BP and LP inputs and then probe the tip of the input socket to make sure the signal is getting that far. Then trace from the tip of the socket to the 100k resistor (where it says BP IN and LP IN on the schematic) to see if you've still got signal at that point.
Next, turn the "level" pot fully clockwise and measure the resistance across it, this is to check that you've wired the pot correctly, it should be around 100k ohms. If it is then probe for the signal at pins 7 and 8 of the TL074, that's assuming the PCB matches the schematic.
Hopefully doing that will show you where the signal is disappearing which will give a better idea of what the problem is.
I'd say the most likely problem is between the jacks and main board probably with the ribbon connectors.

Welcome to the forum

[skweeegor]

Hey, thanks for the reply.

I think we're getting somewhere. The jacks test fine, and the signal gets to the 100k resistor so I think we're good there. TL074 checks out fine too.

The pots are a different story. Both the LP and BP pots are connected to ground across all 3 points. Also, both the tip and shield of the CV in jack are connected to ground as well. I tried different cables and went over the solder points on the pot/cv in jack board but they're all good. Measuring the resistance, I get a clear reading with the HP pot of ~100k, and the LP and BP pots give a quick reading that changes everytime and then drop to 0 each time so I'm guessing they're shorting out somehow, which would make sense if all 3 points are going to ground, correct?

What could be causing this?

I had what now seems like a similar problem with the ChaQuo. Two of the output jacks seemed to be shorting out so I just wired them directly to the main board and all was good after that.

[skweeegor]

Well, it was the pot board! I never figured out what the hell was wrong with it, but I was having the same problem with the ChaQuo build too. Did the same thing I did there - wired all the pots directly to the main board. Now it works great, including the CV In.

Thanks for pointing me in the right direction, I didn't suspect the pots at all.