Dirty Pot tricked me

[sonicmojo]

So I spent 3 hours chasing my tail trying to track down a constant super loud hiss in a GK 400B bass head from the 70s. I knew basically where the problem was in the circuit and spent hours trying to find the faulty resistor, capacitor, or transistor, etc.....

It turned out to be a dirty pot that did not change the hiss upon turning and still worked. I sprayed it and the hiss totally resolved. Live and learn and I think the moral of my story today is to try the easy things first!

[martin manning]

Was this pot noisy at all? I agree with the notion of cleaning things up and insuring good grounds is job 1.

[pompeiisneaks]

Sounds like the wiper was what had the noise causing gunk if it didn't change with orientation of the pot. Nice find.

that kind of thing can drive you nuts, on some of my earlier builds I was horrid with heating the pots too long and solder leaked down into the actual pot and messed up a few but it took me time to figure it out (with sluckey's help of course lol).

~Phil

[sonicmojo]

Was this pot noisy at all? I agree with the notion of cleaning things up and insuring good grounds is job 1.

None of the pots were noisy while turning. I honestly did not even think of the pot or pots as having anything to do with it. I was probing here and there for a while. The weirdest thing was when I removed one of the suspected coupling caps, all noise ceased. When I added the cap back, there was noise downstream and upstream of it. This circuit has +32/-32 rails on it and I think that contributed to my difficulty in pinpointing the problem. There are also transistors between pots in the tone stack, if you call it that on this unit. I’m not great on SS or bass amp designs but I do like to find and fix the problems! Luckily this unit has top and bottom panels that can be removed so it was easy access to both sides of the board. Many of the components have now been removed, tested and verified at least! I wish they still made things this way.....