Zapco z400c4-sl left rear channel low output

[Spuddd]

I'm new to the site but have read many of the threads that have been posted & I do not possess the same level of electronics knowledge as many of you. That being said hopefully can help point me in the right direction with this zapco. I bought it used off ebay for parts or not working. When I got it, I opened it up to find all 4 of the 50v 4700uf caps & the 5 16v 4700uf caps missing. After figuring out what the values to all of the missing caps were, I ordered new ones. After installing them, I powered up the amp & I've only got three out of four channels. After looking around a bit, I found some questionable solder joints, and a couple of fried surface mount resistors. After replacing those and cleaning up the solder joints, I got the 4th channel to put out sound, its just really low. Before I got it to play, when I first hooked up the amp, I had tried bridging those channels, hoping for sound but got nothing. Now after getting the low output from the 4th channel, those channels will actually work when bridged, and sounds just as good as the other 2 channels do, when bridged. So I'm close, I just don't know where go from here, so any help is greatly appreciated.

 

[S Merlington]

I'd try cleaning the switches for channels 3 and 4 first to see if you have a dirty switch ..

 

[Johnny Dior]

Unrelated to your question: Are the 50v 4700uf caps part of the power supply? Reason I'm asking is the main filter caps need to be selected carefully as ripple current needs to be accounted for as well as a low ESR.

If the cap manufacturer has a spec sheet online, it *should* list ripple current, etc. If they completely ignore these values and do not list them, they are probably not a good choice for power supply caps. (If you can find out the exact caps they used originally, chances are a spec sheet from those can help you select the right values)

If you are going into the amp blind (i.e. w/o a repair manual) and can't find the values anywhere, it's possible to ball-park.

Sorry if I'm being presumptuous here, would just hate to see it get working and have a cap fail later and figure I'd say something.