One half of CGS LPG is totally not working!

chartrekhan · Joined: Jul 31, 2015 Posts: 3 Location: Adelaide, Australia

I built a CGS LPG module for Euro, putting the power connector so that it joined the two halfs together (opposite directions so the +12v lines up with the -12v) and the power seems fine. The top one is the one where the trace doesn't have to be cut, the bottom one is the one where the trace did have to be cut. The top's CV IN is normalled to the bottom's, and the top's SIG OUT is normalled to the bottoms SIG IN

The top half works perfectly fine except for the pot which is zero at full counter-clockwise and seems to be at full well before half-way, which I can probably fix with a resistor or something. (The trimpot just goes from bad to worse in terms of pot response)

The bottom half doesn't work at all. The LED from the vactrol is constantly on no matter what pot setting or CV is applied, and the audio doesn't pass through. I traced back the LED voltage to the "IN1-" on the TL074 where there is a mystery 2.5 volts even though I disconnected the pot totally from 12v and there is no CV in. I tried fiddling around with the top half's controls to see if normalisation affected it but nothing happened.

As for the audio not passing through, I traced that and it seemed to stop at the TL074 output.

I tried swapping the TL074 and Vactrol already but it didn't change anything. I looked for solder bridges extensively, to no avail.
I've already cut the trace and installed the jumper where I'm supposed to, of course.
I'll try going over the solder joints again but idk how this would solve the random 2.5v issue.
Also on the top, working half I've used a BC245B instead of a 2N3819, which works well. On the bottom I used a 2N3819 and it's in the right way according to the silkscreen, but the "D" and "S" are the wrong way around. I'm reluctant to fix this because apparently most JFETs are reversible anyway and I already tore up a pad on it.

Has anyone had a similar issue, or can figure out what could be going wrong from my description? Let me know if you need/want photos.

chartrekhan · Joined: Jul 31, 2015 Posts: 3 Location: Adelaide, Australia

well, now the bottom half seems to be working fine, after I re-soldered a bunch of joints but the top half has somehow been messed up. The top half now has the LED in the vactrol always on, and sound is passing through. I noticed on the working half if I turn the CV knob that I added all the way up it has a "feedback" effect a bit like a delay with feedback turned to above 100%, (although obviously where the CV slowly rises up to the maximum even with nothing plugged in. So maybe there is some kind of feedback loop going on in the LED driver.

wackelpeter · Posted: Mon Dec 28, 2015 2:46 pm Post subject:

Would suggest you post some pictures of both sides of the PCB... maybe someone can spot a error...

chartrekhan · Joined: Jul 31, 2015 Posts: 3 Location: Adelaide, Australia

Um, well actually I got it to work. It turned out I had a solder bridge between CV IN and 0V pads. I have no idea how/why this was causing the issue but it seems to be gone now.

I'm still having problems with pot control range and stuff though, It goes from 0 to full between about 7 o'clock and 9 o'clock and does absolutely nothing for the rest of the range, what could be the cause of this? I used a different diode to the one specified, but I think I accidentally used a 5.1v one instead of 3.9v. Could this be the issue due to different voltage range or something? Maybe it's clipping the CV somehow?

oh this thing sounds so good though, the low end is so deep.
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