Mysterious static in a preamp channel

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midwayfair

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I've got a modified Hamptone circuit that I have several channels of, and one of them is giving me some constant static in certain gain settings. The static is VERY loud -- around around -35dB at its loudest, so the channel's unusable.

The schematic below is modular, but it's the version with a volume pot (the 10K "Gain" pot hanging off the tail of Q2), transistors are all as shown (2N4401 in Q2) and it's running on 48V.


I've got two channels built identically to this one, and one that's identical except with a silicon transistor in Q3 instead of a second FET, but this is the only one with static. They all crackle when I turn the volume (completely expected) of course, because the wiper's changing DC.

The circuit does NOT exhibit static when the gain knob is maxed out, nor when the gain knob is backed off a couple detents from max. The static does get quieter when I turn the gain knob down, but it's actually loudest when the knob is around the center of its travel

If I take a big capacitor and silence various parts of the circuit, the first place the static goes away is the wiper of the gain pot. (NOT the emitter of Q2!)

If I pull Q2, the static goes away, so it's not coming from Q3 or later. I've replaced Q1 and Q2 with no changes. I tried a few different transistors in Q2 with no change.

I thought it might be C3 leaking, so I replaced it, and there was no change. I pulled and checked the filter caps relevant to Q2 and they're all still good.

I suppose, since this is the only channel doing this, that I should just build a new board and replace this one, but I'd really like to get some idea of what could cause this in case it happens again.
 

midwayfair

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I left it on overnight and it went away as suddenly as it appeared (it didn't used to be staticy, in fact I recorded some tracks with it a week ago). While this is good, it adds to my apprehension.

1. Good: It works and I know why
2. Bad: It doesn't work and I know why
3. Worse: It doesn't work and I don't know why
4. Hell: It works and I don't know why <----- we are here
 
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Ike Zimbel

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I would look for hairline cracks in the circuit board traces. These can be incredibly difficult to spot...magnification helps. Flexing the board helps. Cold spray might help. Where I have found these most often is at the transition point between traces and pads. If the circuit boards have solder mask, that adds another layer of complexity as the visual transition between the solder mask and the tinned pad can easily cover up the fault. The fact that it went away after being left on overnight also supports this since the copper would expand as it warms up, forcing the edges of a hairline crack together.
 

midwayfair

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That's a really good theory! We leave the heat off during the day most of the time so it's pretty chilly, I can fire it up a few times during the day and see if I can get it to go staticy again.

I think this board HAS been pulled at least once, so it's definitely possible that it has a hairline fracture from handling, though I did reflow solder joints so it would have to be a trace itself.
 

JohnRoberts

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look for bad (intermittent) solder connection... maybe tap around the circuit board with a pencil eraser looking for a response to percussive troubleshooting. [edit- another troubleshooting tool was a bottle of spray cold to force thermal cycles /edit]

Changing with gain pot symptom sometimes suggest circuit instability (oscillation). Check compensation cap, PS decoupling, etc.

JR
 
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