Help troubleshooting a SS power amp section

[kingnimrod]

I bought a Sunn Beta Bass combo the other night, and need some help troubleshooting. It was working fine until I plugged the master accessory OUT from a beta lead head into the master accessory IN on the beta bass (to try out slaving it). I got no sound from the bass speaker, so i unhooked the beta lead.


now all i get from the beta bass combo is a muffled, distorted sound. If I strum hard, the sound is louder, but it seems like it's trying to fight its way through, like the sound you get from a distortion pedal with a dying battery.

I'm sitting here with it right now, with the head open, doing some testing.

Here's what it's doing:

Channel A preamp line out and Channel B preamp line out send a good signal -- I am running a line into the return on a small practice amp here. All preamp controls seem to work fine, and I get a good signal from each discrete channel, plus signal from both when plugged into the "both" input.

Master Accessory line out gives a good signal.

Patching any of the preamp outs into a power amp in on the Beta itself gives the same crappy fizzing sound, as does running an external preamp into any of the power amp inputs.

So it does sound like something is going on with the power amp section. Looking at the power amp circuit board, nothing looks fried to my eyes. Fuses are in fine shape, and at a first glance no caps are leaking or bulged.

Tracing back to when this problem originated the other night, it happened immediately when I plugged the preamp out from my Beta Lead head into the master accessory line in, so my questions are:

Could running an external signal into the power amp do something to cause this? Are there any prime candidates for malfunctioning in this situation?

Could it be something as simple as a cold solder joint on the line in input jack that could cause it? Or are the symptoms indicative of something else?

I am not an amp tech but have built some small solid state stuff, repaired effects, etc. I'm hoping it's an easy fix...