Distorted Channel on a Rockford Fosgate Amplifier

[KilgoreCemetery]

I recently picked up a Rockford Fosgate T8004 amplifier. I was told it didn't work and, after a little investigation under the hood as it were, I realized that a few of the MOSFETs and a couple resistors were bad. After replacing them, I powered up the amp and was happy to find that both of the front channels were good. However the Rear Right channel has some very significant distortion. The Rear Left channel seems fine.

I believe that somewhere the audio signal is bleeding to the ground because you can hear the distorted audio signal it by probing the ground screws.

I've been comparing the Rear Left section to the Rear Right section to find some discrepancy and I think I've narrowed it down to about a 3" x 3" section (see picture), but I'm beginning to think I don't know enough about what I'm looking for.

It seems the chances of finding a schematic are pretty slim, but I feel like I'm one part swap away from having a good amp.

Can anyone point me in the right direction?

 

[kellys_eye]

Those two 10μF capacitors at the top left look a little 'bulgy' to me.

The good thing about stereo amplifiers is that they have IDENTICAL channels so you can compare signals/voltages from a good channel to find a fault in the bad channel.

 

[KilgoreCemetery]

Alright. I'll try swapping them with some known working ones and report back!

 

[KilgoreCemetery]

I swapped them, but there's no change. While I'm poking around in here I'm wondering how can I tell if the variances I find indicate the actual problem, or just symptoms of the problem? Is there a way to know or do I just have to start swapping out components until I find the right one?

 

[KilgoreCemetery]

Update: Rockford Fosgate customer service trolled me a bit. I requested a schematic and they responding, saying that one was enclosed, but it wasn't. Tried chatting with another rep and he said the schematic for my model hasn't been released yet.

In other news, I'm doing this Gargamel style (see pic). Every area crossed off either has similar voltage to a working channel or has components that have been swapped with a good channel. Also, they contain zero smurfs.

 

[kellys_eye]

Random(ish) testing on a comparative basis may not work in the manner you think it should - you really need to inject a signal and 'scope the signal path to find where distortion is being introduced.

 

[KilgoreCemetery]

Ah, that's quite possible. I don't have a scope, but I do have a tool that I made that's able to probe a point and output the signal to a small amp/speaker combo. I watched a video that showed me how to set it up so it won't destroy things. Hopefully that will get me where I need to go

 

[KilgoreCemetery]

OH, MAN! Turned out I somehow managed to skip an easy check and go on to all the tedious ones. The problem ended up being the large 0.1 ohm resistor at the top right corner of the picture. I must've checked one twice and that one not at all when I was checking the MOSFETs.

Thanks for all your help, kellys_eye! I finally have a working amp.