blue smoke release

Yesterday, testing the most recent circuit, I had a bit of a setback.  First off, the section I had modified seemed to have practically no gain.  As in, just slightly over 1; maybe 2?  Switching from bypass to enabled, I could get a little more volume if the level was really high, but there was very little difference.  And, importantly, the gain pot seemed to do absolutely nothing.  No change in gain, from min to max.  So this seems like maybe I had a circuit error, relative to my latest schematic.  Kicking in the second section (not yet modified from the older circuit), I had a fair bit of gain, just like it used to behave.  So that section was still operating "normally".

But then, as I twiddled the gain and bias knobs, I seemed to hear a crackling sound, like too much DC on a pot (?), and then all of a sudden there was that dreaded, quiet little sound of something blowing up.  I cut the power.  There was a smell of too-hot, blown component.  But which one was it?

I ran some checks tonight with a DMM, and couldn't find anything out-of-spec in the area of the pots.  My best guess is, my new bias circuit with the two Zeners instead of the 100k fixed resistor, into the 250k bias pot, maybe let too much current flow and the pot partially fried, though maybe only in one spot which I didn't find with my power-off probing tonight.  My instinct is to put the 100k back into series with the Zeners.  But: what about the unexpectedly low gain?  Is that a separate issue, or related to the blow-out?  Occam's Razor says it's probably related, but I can't quite see how.  Again, I'm compromised without a scope (still trying to secure one, another story...).  Actually, I think I'll try to run the circuit with no bias network at all: at these new low operating voltages it should be able to find a stable place with no bias voltage injection, in theory.  Just like the transistor version.

Oh well, that's another day in the life of a circuit designer!  Never know initially if I made an error in the implementation of my design (wiring), or an error in the design itself.


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