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ROTEL RA 980BX
Fault: No output from left channel. The right channel is working fine.
Added some photos and circuit for reference
While testing the amp noticed that the left channel is giving switch on off thump while powering ON and OFF the amplifier. It happens when there is DC voltage flow through the speakers. It shouldn't be there. And the thump is there only on the left channel. While testing the amplifier section found that all the transistors where idling at mentioned voltage. All Base pins where around +/- 0.5volt. and the output is almost 0.012mvdc.
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All I can say to you is look around where the mains goes in very carefully. Fuses are sometimes hidden or in a case or cover of some kind. Sometimes amps use fuseable resistors. If the PC board is marked up look for resistors with "RF" or different indication to a normal resistor. Otherwise fuses will have "f" markings on the board. Remember the power supply part might be enclosed in a metal case with holes in it, so might be hard to spot fuses.
Lets try the simple stuff first. This whole problem could be caused by a dirty switch contact. Pick up some electronics cleaner from a local electronics shop and spray the switch (from the inside of the unit). Work the switch on/off a number of times then respray one more time. That should clean out any dirt that is there. Try the unit again and hopefully the problem is resolved. If not, then the switch itself may need to be replaced. When you get the cleaner, get one that contains a lubricant as well. This will help to prevent the problem from coming back again.
I'd suspect that one of the two output transistors on that channel has gone bad. If you are unsure how to test or troubleshoot this, then you might try replace both the NPN and PNP outputs and catch the bad one that way. If your amp uses an output module then you would of course have to replace the module which would replace all the output transistors on both channels. Good luck. Another thing to look out for, is the emitter bias resistors. They are the large white ceramic ones near the outputs. Sometimes one of those will open up too. They will be of a very small resistance value like .5 ohms (point five ohms) or something in that general ballpark, and have a wattage value of typicaly 5 watts. So they are pretty big resistors. Be sure to check them all even if you do find a bad output transistor. They will often go out with the transistor. Good luck.