Cracklin' Fender Bassman 135

[jonathan_kea]

I am helping a friend with her Fender Bassman 135. After a brief warmup period where the amp works correctly (2-3 min), turning the volume beyond 4 while playing causes it to crackle and cut out. We tried it on multiple cabinets to rule out speakers. We tried the amp in her practice space as well at my house and the issue remains.

She recently replaced/biased the output, PI, and preamp tubes and had the amp tech recap the power supply. The tech could not replicate the issue in his shop (so he says), and she spent about $600.

This morning I cleaned all of the sockets, pots, and jacks with deoxit, however, it still crackles and cuts in/out after a few minutes. Any thoughts on what to try next?

[shooter]

crackles n cut-outs are classic for bad solder joints, so NO POWER - bust out the magnifying glass, wiggle n jiggle, including UNDER board jumpers, pots grounds, etal.  once you've found it, fix it, play it, bang it n repeat as necessary

[jonathan_kea]

That sounds like a nightmare :). But yes, I will try that. Thanks!

[WimWalther]

crackles n cut-outs are classic for bad solder joints, so NO POWER (...)

Where's the fun in that? ;-)

If you have the luxury of a Variac, run the amp at the lowest voltage where it will still reliably pass signal (usually around 50-60Vac) and probe around with a plastic rod or bamboo chopstick.

[jonathan_kea]

Is there a way to test (bypass) the bright and deep switches to see if they are the culprit?

[mresistor]

Hi  it might help to post some high resolution photos of the insides of the amp so people could see it.  You will probably have to host the pics somewhere first.  Would like to see the quality of the work that was done by the "tech" that has worked on it and the parts he used. The pics have to be good enought resolution to be able to see areas and scrutinize them.
I would first check every ground connection in the amp and use a magnifying glass because the solder jounts can sometimes have hairline cracks that you just can't see well with the naked eye.
As Wim above has mentioned also use a non-conductive tool like he has mentioned, i use a plastic one but a chop stick would do, and probe around the LIVE circuitry listening for any clues, keep one hand behind your back while doing this please.


Sometimes the eyelet boards can be conductive or there is solder underneath that is contacting other parts or ground that can cause the problem.   

[jonathan_kea]

Problem solved!

I just went through and retouched every solder joint I could reach in the tone and preamp sections. I resoldered a couple of the wires from the output transformer as well that didn't look well connected mechanically. Voila! The amp works perfectly.

Thanks for all the advice!