Engineer's Thumb Debug

[mremic01]

Well, it was working for a few days after I boxed it up... now I'm not getting much output. The most I can get out if it is about unity gain with the Level dimed and the Ratio turned down to keep it from compressing. The Ratio pot does work though.

I etched the board using this layout: http://www.freewebs.com/valvewizard2/engineersthumb.pdf

It's bone stock, with no bright switch and no millennium bypass circuit.

IC1
P1   4.45
P2   4.47
P3   4.07
P4   0
P5   4.07
P6   4.47
P7   4.48
P8   8.95

IC2
P1   4.78
P2   4.48
P3   hovers around 4v
P4   0
P5   4.26
P6   4.47
P7   4.48
P8   8.94

IC3
P1   1.086
P2   ?
P3   4.47
P4   4.47
P5   4.47
P6   0
P7   0
P8   0
P9   0
P10   0
P11   8.95
P12   bounces around
P13   bounces around
P14   bounces around
P15   bounces around
P16   0

Q1
C = 4.48
B = 4.77
E = 1.083

Any ideas where I should look?

[midwayfair]

First step: Pull it from the case to eliminate shorts as the source of the problem.

"Was working and now isn't" indicates either that you have a newly-broken connection, or a bad part (possibly due to a bad connection or short). I'd give the board a once-over and maybe even reflow solder before thinking about replacing parts.

When you say "bounces around" on one side of the LM13700, what are the voltages you're seeing? Are they really really low, or are they centered around 4.5v?

[jimilee]

And check the solder joints on the stomp itself. Sometimes these things happen to the best of us. I had that issue with 2 or three builds, apparently I decided to just decided to F up the stomp soldering, but I gave myself a stern talking to and straightened myself out.

[mremic01]

Hmmmm, now it's magically working again. I'm suspecting that one of the IC pins just needed some jostling, since that's all I really did to it since the last time I tried to play through it. Maybe a socket. I had swapped out all the ICs and the transistor before I made this thread.

From what I remember, the values that were bouncing around were something like 120mV and would rise when I took a reading, but go back down again when I removed the probe and tried reading it again.

Update: So it seems that this circuit plays nice with my Marshall and my Markbass heads, but doesn't like running into my Blackheart. For whatever reason, when I'm running it into my Blackheart, it squashes the signal down to the point where I need to run the level all the way up. That's odd, the Blackheart normally takes pedals very very well. Could it be an input impedance issue with the amp?