Help repairing a dead channel on a Sylvania RQ-3747 receiver

[angelaudio]

Hey folks I got this quadraphonic receiver a while back and I would like to fix it but I don’t know where to begin with the diagnosis and repair.

It’s a quadraphonic receiver that can operate in 2-Channel bridged mode so I’m not sure if diagnosis and repair will be as straight forward as a stereo receiver. The only diagnosis I’ve done so far is: 1) Plugged a working source and preamp into the main in and found that the left channel is still out. This quadraphonic receiver has two sets of pre-out/main-in jacks and when I plugged in a preamp I did so only on one of these pre-out/main-in jacks. Is that correct or am I doing something wrong there? There is a switch on the back which changes the receiver from 4-channel to 2-channel and there is also a printed diagram above that switch which shows how to wire the speakers when running 2-channel bridged mode. I’m wondering if someone previously wired their speakers up incorrectly and maybe blew transistors or maybe a speaker protection fuse or something. I’ve attached some pictures below including one picture of a small white paper with some components drawn on it that is stapled to the inside of the cabinet. Any help would be greatly appreciated. Thank you.

 

[tcdriver]

Congratulations on acquiring a Sylvania quadraphonic receiver.

1. DeOxite to clean all controls, including the 2 channel / 4 channel switch on the back of the receiver.

2. With the unit turned off, set the bridge switch to 4 channel mode. Turn on and check each channel individually by disconnecting the pre-amp out / amp in jumpers and feeding your external preamp into each of the channels in turn. This should tell you which channels are good and which are bad.

 

[gadget73]

^^ exactly what I'd do, figure out which, if any, of the 4 output amps are working first.

If all 4 come up OK, see which output from the preamp is not working.

I'm a big fan of breaking things into "blocks" for troubleshooting purposes. Makes the whole process a lot easier than trying to look at it as a whole.