1970s Peavey Bass 300 - popping/shorting when load connected

[Drummer4gc]

Look at the circuit diagram, understand what to expect and test accordingly.

This is why I am reaching out for help. For an intermittent type issue like this, I do not at this point have enough knowledge to test in a particularly targeted way and recognize an abnormality that points me in the right direction. I’d love to take suggestions for a logical sequence of tests and ask for assistance analyzing the results. Otherwise, you’re right - I’m shotgunning a bit.

I tried replacing the 2n4249s, no change so I put them back. I have a set of tip29/tip30 that I could replace the 40409/40410 set with just as a test, but they don’t look to be adequate replacement options, so I won’t try that unless someone thinks it’s a good idea.

Jon, do you have a suggestion for a next test?

[Drummer4gc]

Coming back for more....

I thought I had this sorted yesterday. I resoldered all of the connections at the to-3 sockets as well as the pin connectors on the PCB. It sounded silent. Played through it for a bit at volume with a real cab and it sounded great.

I put it back in it’s box and ran it through a bit of a stress test - 4 ohm dummy load, an albums worth of rock and roll played nearly at clipping, scope on the output. Interestingly, as I set this up and adjusted volume, the signal cut out a few times and was then restored by turning the volume down slightly. Once, I lost just the top half of the signal. It sorted out quickly and looked to be passing signal properly during the test. When the album finished, the signal looked clean. I let it cool down for a couple hours, plugged it back into a cab with a bass, and low and behold, crackly mess all over again.

My best guessing at this point is an intermittent connection that seems to have some correlation with heat. I noticed this past week that it seemed quieter in the afternoons than it did in the mornings. But man, I’m basically at a loss. Putting out one last request for thoughts and suggestions before I call it.

Matt

[Drummer4gc]

I lied, back once more. I replaced the 40409/40410 pre-driver pair with 2N3440 and 2N5415 with press on heat sinks. Still occasionally getting popping and crackling.

All of this work has certainly tamed the issue since I first posted, but the amp is still unplayable with this unpredictable-but-predictable noise. It seems to be worse the first time I turn it on during the day. But, I just ran it for ~30 min on a dummy load to warm it up again and it was crackling/popping right after when I plugged it back into a test speaker. So, maybe related to cold, or heat, or both? It ran hard and fine on the dummy load btw, although there wasn’t really a way for me to see if it was “popping” through the test signal (AC/DC) - I can only assume it was though.

I've cleaned every mechanical connection in the amp, resoldered the whole power amp pcb and all off-board connections, tried replacing every output and driver device, replaced all power amp electrolytics, and cleaned every connection again. Can’t get the noise to reliably stop or start through chop sticking all over.

What am I missing?