Debugging Pre-Revision 1 Sparkplug

[MeC]

I'm kind of at the end of my ability to debug the old version of the Sparkplug, having build and debugged many circuits I seem to be missing something "stupid".  There's a horrible "open ground" type noise when adjusting the volume, which turns into an oscillation when adjusting the cut.  At maximum volume and gain there is some output and the "open ground" noise and oscillation are minimized but the output is barely there.  I've checked the pots, caps and diodes, and have tried replacement ICs, have reflowed and inspected for solder bridges.  Any suggestions/ideas on what I'm missing?  Note: I wired the pots instead of using PC pin type. 

Voltages on IC

1  4.20       8  8.45
2  4.20       7  4.20
3  3.76       6  4.20
4  4.20       5  3.76

Input voltage: 9.20
Voltage post input diode: 8.45 (Vbb)
Voltage at (Vcc) is 4.20

Thanks for any ideas!

Cheers,
Mike

[jkokura]

You should have 0 on pin 4, pretty sure. That would be the first place to investigate. It looks like VB is leaking to ground.

Jacob

[MeC]

Oops, I do have 0v on pin 4, guess that mis-entered that one. 

[jkokura]

Next question I'd have is whether you have it in our out of an enclosure Have you got everything properly grounded?

Jacob

[MeC]

It is unboxed, always test before boxing.  All is grounded too, and am beginning to think it's wiring and/or hardware.  It's gotten a bit kluged up with my removing and reinstalling the transistors.  On my last adventure I noticed that the LEDs light when checking pin 3, light upon oscillation (makes sense), but am getting a "theramin" effect off the output jack, including LED illumination. Weird! With the voltages looking good, caps, resistors, and pots checked I am going to try replacing the pots and using a cleaner lead dress--not bad but am taking the next step.

Thanks for working through this with me, Jacob.
~Mike

[MeC]

The Sparkplug is up and working, sounds good too.

When I wrote, "... missing something "Stupid"", I was right.  After thinking about an ungrounded cap, having checked them multiple times and resorting to lead dressing what did I discover, that the polarity of the I/O leads were reversed. UGH, duh, but reminds me of day three of circuit analysis and the prof's story of solving a nonfunctioning transmitter with the insertion of the power cord.  Yeah!  Just a reminder to look at the simple things first!

Cheers,
~Mike