Peavey KB300 only hums

[The Dude]

I'm doubting those measurements. I agree with Enzo that Q10 & Q11 are not being turned on. But, you also have a 1.9V drop from B-E on Q6 & Q7, which doesn't make sense either. Typically, a working transistor should drop less than half that voltage across the junction.

[MichaelNuzum]

Hey all- so this amp being discussed here ended up on my bench. Turns out it had a blown diode (CR30 on the -16V supply) and two open resistors (R40 and R41 on the -42V rail for the output transistor section). Swapped those out and now the amp works beautifully. Dead silent at idle, good clean sine wave on the scope at output (also no DC at output), compressor light comes on at proper time and otherwise is off.

As the original poster mentioned, the backstory was that it was moved over rough ground, then hummed when plugged in next. I was expecting to see a bad solder joint, or cracked leg on a transistor. But it sure seems more of a power supply/ power surge type failure.

Anybody have an idea what sort of scenarios might cause the listed components to fail?

[Enzo]

But it sure seems more of a power supply/ power surge type failure.

Maybe, but I don't think so. Note in the 260 power amp there are two sets of 15v supplies. The "16v" ones are regulated and have the 15v raised by one drop CR30. Those two rails ONLY go to the preamp, they are not used on the power amp. On the layout at the umbilical connector, they are indeed labelled 16v. Unfortunately on the schematic, a little way above those power supplies is the connector to the preamp, says power amp in there. There it says 15v. It should say 16v.

So CR30 has nothing to do with the power amp. If those two resistors open, then the bottom end of the VAS is free to snap up to almost V+ rail, which puts that voltage on the output. DC on the speaker will make loud hum from ripple current.

Resistors rarely fail on their own, usually I look for a dead semi somewhere. But if they were indeed all that was wrong, I am OK with thinking the drop was involved.

And it is also possible after bouncing around, some loose nut or screw hopped up on the circuit board or even wedged under it, and shorted something out. In attempting repair, the OP cleared that so it no longer exists as a problem.

[MichaelNuzum]

Ah! I hadn't thought about a rogue piece of hardware. Good point.

I too thought I would find a bad semi, but all seem happy. Injecting a 100mV 1kH signal at input and diming it, I am getting 24VAC RMS at output into a 4 ohm load, so that puts it at 144 watts output for an amp spec'ed at 150. That seems pretty decent.

[Enzo]

Is that with the DDT on or was DDT off and you ran it to clip? Drawing says 42v, but with mains varying and who knows what else, we don't know what your rails are. But I'd think without DDT you could get within maybe 2v. SO maybe 28v? The DDT works very well, but I am not sure just how close to rail its threshold is. Might lose a few volts. ANyway I sure wouldn't quibble over 6 watts.

[MichaelNuzum]

....ANyway I sure wouldn't quibble over 6 watts.

My thoughts exactly.

I should mention I live in the mountains and our power is notoriously whacky. It fluctuates wildy, right now it is hovering around 124.5. It often rides up around 126, sometimes dropping as low as 121-ish. I always check where it is riding before I bias a tube amp, so I can know if I should lean towards the "hot" or "cold" side. I was curious once, so I checked with a variac, and I was amazed how much difference a few volts change from the wall can make in a tube amp's bias and output.

I would imagine a solid state amp would maybe be a bit more stable but still experience some similar changes in internal voltages and output power depending on the wall voltage it is being fed?

[Helmholtz]

It fluctuates wildy, right now it is hovering around 124.5. It often rides up around 126, sometimes dropping as low as 121-ish.

A daily mains voltage fluctuation of 5V or 4% isn't bad at all. And yes, max output power varies with supply voltage. Same for tube and SS amps.