Hi all,
Just poking my head in to see if anyone smarter than me has ever dealt with this issue. I’m in the process of building two Music From Outer Space Dual AR modules (schematics here):
http://musicfromouterspace.com/analogsynth_new/DUALARGENERATOR/DUALARGENERATOR.html
And I’m having some issues with one of them that I just can’t seem to pin down. One module works perfectly, but the other has one envelope permanently stuck in the triggered position (the other envelope on the board works fine). Toggling switches and knobs does nothing to change this.
Normally I would just compare the working unit to the non-working one until I spot the difference, but they seem friggin identical as far as I can tell. The main clue that I think might hold the key to success is that the non-working envelope has about 6.4 volts on it’s input pin, when it should of course be at 0 volts. This immediately made me check on the diodes and transistors in the area, but they are all oriented correctly so I am at a loss. I also swapped the chips as a hail mary, no luck. Has anyone else dealt with this kind of thing? Any suggestions for things to check are most appreciated.
I don’t know if this will help but here are a few photos. The board is more or less divided down the middle, the non-working envelope is on the right side in the first photo. The off-board wiring looks good, and I definitely confirmed that the manual trigger button is not stuck closed.
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Where is that on the schematic? Do you mean the gate input jack tip pin?
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yes, your assumption is correct. thanks for replying and sorry for being vague!
And is S3 open or closed?
I guess it’s supposed to be closed for gated mode.
The only way I can see for any voltage to get to the tip pin of J2 (if nothing is plugged in) is if S3 is closed then it connects to the output of U1-A — but it connects through R16 and R18 (10.047M) with R19 and R20 (200k) to ground, so should be quite small. And negative. The only other possibility would be if S2 were bad and constantly closed. If not that, then something in there has to be misconnected or disconnected.
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I’ve been mostly testing in gated mode but switching to trig mode has no effect on any of the problems with the circuit unfortunately (to the best of my recollection). I had to storm off in a huff so I’m hiking now but I’ll be back at it in a few hours
Woohoo, I finally cracked it!
It turns out pins 3 and 4 of U5 were shorted together, so the +input on one of the op amps was connected straight to the +12v rail, which explains the weird voltage at the input, and the fact that the envelope was locked high at all times.
The bottom of the board looked perfect, and the short actually occurred on the other side of the board, sandwiched under the chip socket. Apparently I used a touch too much solder and they glommed together. I never would have seen it, had to poke around doing a continuity test with the multimeter.
This is a valuable lesson - from here on out I’ll be testing my sockets for unwanted continuity after I finish soldering, and if any of you are skipping that step, don’t end up like me! That simple test would have saved me hours of troubleshooting (and saved me replacing the chipset and 3 diodes that were probably perfectly fine >.<).
Analogoutput, thanks so much for your replies helping track this down. It’s really great to be able to turn to someone for a hand when I’ve hit a brick wall. It’s very much appreciated. Looks like this one falls under the category of “misconnected.”
I’ll be doing a demo of these bad boys at some point because they are really solid and super snappy.
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