Gallien Krueger 1001RB Crackling

[dmeek]

I have a Gallien Krueger 1001RB that I present here just as a puzzle since I have just about given up on it.
It has intermittent crackling not affected by heat gun or freeze spray so I can't track down what's
causing it. It crackles for a few seconds then stops for a few seconds then starts again etc.

The first photo is the output, speaker unplugged. For the second photo I disconnected C9 to kill the AC gain.
Notice the output shifts almost 200mV negative while it's crackling, otherwise it's at zero volts.

I'm guessing the problem is in the input stage so I replaced Q2/2A as a best guess - no luck.

Anyone have any ideas how to pinpoint the cause?

https://music-electronics-forum.com/...1&d=1574821757

[The Dude]

Does the noise appear on both the low amp and high amp outputs, or low amp only?

[Enzo]

Is your scope on DC coupling or AC coupling?

[dmeek]

Scope is DC coupled, and the noise is only in the low amp. The problem in locating the noise is that almost every component is within the feedback loop.
Sometimes with a tube amp I will disconnect the feedback to locate a problem in the output stage. Not sure I could do that here.
If I lift R10 and connect it to a pot to manually set the voltage at the base of Q2A for zero at the output, assuming the open-loop gain isn't too high,
I still won't know if it's in the positive side or negative side. Plus it's a 4 rail power supply.
I might try some experiments for the hell of it but it's definitely not a money-maker.

[nevetslab]

Scope is DC coupled, and the noise is only in the low amp. The problem in locating the noise is that almost every component is within the feedback loop.
Sometimes with a tube amp I will disconnect the feedback to locate a problem in the output stage. Not sure I could do that here.
If I lift R10 and connect it to a pot to manually set the voltage at the base of Q2A for zero at the output, assuming the open-loop gain isn't too high,
I still won't know if it's in the positive side or negative side. Plus it's a 4 rail power supply.
I might try some experiments for the hell of it but it's definitely not a money-maker.

One of the problems I have had continually with the 1001RB, 2001RB is the solder joints on the bridge rectifiers, as well as the AC Secondary header plug that runs the two-tier bipolar supplies. And, with GK's poor practice of using tiny solder pads, small solder mask patterns to limit good solder flow to those high-current colder connections. Not to mention their practice of cutting all of the component terminals nearly flush to the PCB, so going in to do re-solder surgery is a real PITA. I've had to unsolder those connections, including the buss cap connections, then scrape off all the excess solder mask on those pads, then re-solder so there's more surface area to flow the solder onto. But, half of the solder connections having any substance are on the component side of the board, with virtually no access do to parts density.



I have had success in restoring the intermittent and crackling from that area. Just a thought.

[dmeek]

Thanks for that information. I tried re-soldering the power diodes, bridge rectifiers, header plug, and all power transistors from the bottom of the board but no luck, crackling is about the same.
I guess if the hole plating is defective I would have to solder from the top as well. Not sure if I'll do that, I'm about ready to quit.
I tried my idea of breaking the feedback loop, but as I suspected the amp is too unstable in that mode for testing. I was able to measure the open-loop gain at about 300.

[dmeek]

I made one last attempt and soldered the diodes and bridge rectifiers from the top - still no luck. I didn't see any noise on the power rails.

btw I don't know how this thread ended up in Schematic Requests, I thought I posted in Maintenance and Troubleshooting, I'll try to be more careful next time.