SOLVED Thumbsucker - audio signal dead at D2 (user error)

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DigitalZombie

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I get next to nothing at the output unless I turn the Ratio to minimum. When I start to turn it up it abruptly cuts off to (almost) dead and I have to turn the level up all the way to hear anything at all. I started probing and the signal dies at the anode of D2. I checked continuity and voltage, getting about 2.5 volts there, so it's not grounding out somewhere. I get signal on the other path up to the Release control, but only to pin 1 if it's set to full CW. I reflowed solder to D2 and R12 but still nothing. As far as I can tell the work looks clean enough to work but maybe someone here can spot something I'm not seeing.
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Last edited:

giovanni

Well-known member
I see quite a bit of residual flux on the board, you can try and clean it with alcohol and a toothbrush. Also can you verify that the pots don’t touch the solder tits underneath? I can’t tell if they have a plastic cover or not.
 
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Feral Feline

Well-known member
Yes, apparently that's what they're really called.

Have you made sure R12 is 100r and not 100k? Hmm yes... 100r...

Have you checked the diode itself, maybe it got cracked while bending the leads. I like to socket my diodes, I tended to crack a lot of the glass ones when I first started out, less so now.
 
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PJS

Active member
A full analysis of the side chain is beyond me, but I can say that the purpose of the network between the threshold control and pin 1 of the LM13700 is to turn the audio signal into a DC current that controls the gain of the VCA. I would expect to lose audio signal fairly early in the process, so you may not be picking up a problem there.
 
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