SWR Workingman's 15 - weird crackle

[lowell]

This amp sounds fine unless the GAIN knob is turned past 2 o'clock. Then, and only on certain notes, it has an awful crackle sound when a note is played and it only happens sometimes. If I plug a guitar into the amp the crackle ONLY happens on G and G# below middle C. I swapped speakers and it's not a speaker issue. This amp does not have any pre out/pwr in jacks unfortunately. I'm thinking of a bad solder joint somewhere, likely on a filter cap. Also maybe dirty pots? I'll check those but if someone's got a clue... uh, clue me in please.

[52 Bill]

I'm thinking of a bad solder joint somewhere, likely on a filter cap. Also maybe dirty pots? I'll check those but if someone's got a clue... uh, clue me in please.

I've seen a few of these amps develop broken output transistor leads right where they enter the plastic body. Press down on the pc board along the back edge where the outputs are soldered in.

I've also seen the driver transistors break legs from vibration as well.

May or may not be your problem, but just something to check.

[lowell]

on solder joint and broken lead inspection I discovered the fuse had a broken lead that was still touching the solder pad. Fixed it and voila. I have also repaired a broken leg on a different workingman amp on the driver transistor, however I don't think vibration causes it, I think it's when the PCB is removed and the power transistors are removed from the heat sink that the legs of those transistors get bent and become weakened. Anyone working on these just keep that in mind when disassembling the heatsink area.

[Enzo]

Lowell, I'd bet vibration did do it. You found only certain notes made this happen, but the solder crack was there no matter what note you play. The difference is that certain notes resonate in the chassis.

I find that in those G-K amps with the little switching power supply, the most common repair is broken off legs in the TO220 rectifiers. Even on amps that have never been apart.

[lowell]

wow, that's crazy. Bass cabs vibrate that much eh? So now for shock-absorbing transistor sockets.

[gblum19]

I had a similar crackling sound and used the suggestion of tapping the circuit board with a chopstick to isolate the area causing the problem. I found that the solder joint for one of two large black capacitors needed to be resoldered. Easy fix. Crackling gone