SWR Basic 350 with clipping distortion

[Axtman]

I recently bought a red face SWR Basic 350 bass amp head off Craigslist. i bought it cheap because the seller said the amp was making a "buzzy" sound. I took it home and sure enough it was making a horrible distorted guitar sound when I played bass through it.

I isolated the problem to the power amp section. I put the amp on the bench and ran a clean sine wave into the input. The output sine wave of the 'tuner out' is clean with no clipping. The output of the 'effects out' is clean (until you increase the gain knob then the wave clips evenly top and bottom). The output waveform of the 'speaker out' clips very early when the gain or the master is increased slightly. As you see from the attached photo, only the top of the waveform is clipped and it is clipped severely. Where should I start looking for problems? Thanks.

[AmpFix]

Looks like half of a class AB output is gone. Should be easy enough to trace it out to the output transistors.

[Enzo]

That doesn't look like clipping, that looks like a missing half of the waveform. The V+ side seems unable to pull the voltage up from rest position. Remove the speaker load, does the full waveform appear or is it still truncated?

[Axtman]

With the speaker load removed the waveform is still truncated.

[Enzo]

OK, so it isn;t a matter of current. Look at the bases of the outputs then the drivers, the predrivers, and so on. Is the full signal getting to the base of the V+ side outputs? Measure continuity from the emitters of the outputs to the speaker bus. Looking for open ballast resistors.

[Axtman]

Enzo you are correct. There was a ballast resistor that was burned open.

[Axtman]

As it turns out transistor (Q14) was microphonic. I have near seen this before. Possibly a bad solder joint. I ended up replacing the transistor. Mark Black told me that the resistor that was burned (R40) tends to be notorious undersized. So I upped the wattage and mounted the resistor so that it stood off the PCB. One hunch is that the close location to Q14. may have heat damaged the transistor.

[Axtman]

After working fine, I went to play the amp and it has a very low distorted output.

[g1]

Is it possible it's the same issue back again?
The previous find of a microphonic transistor did not sound like anything that could have cured the fault you had.
Maybe there is a bad connection somewhere that got wiggled and fixed the problem for awhile but has now returned?