TV/VCR/Stereo Troubleshooting/Home stereo - dead channel

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Question
Hi there,

I have an Onkyo home stereo receiver - model TX-SR302.  It was working fine for several years until recently when the left channel went dead.  I carefully checked to be sure the problem is not with the external wiring or speakers.  All left side channel options are dead (including headphones, Speakers B / Surround).  After 2 weeks of being dead the left channel briefly worked, then died again, presumably for good until it gets fixed.

If the problem was somewhat intermittent as I just described, would you think it's more likely to be a corroded switch contact versus a fried component?

I have the service manual which shows lots of block diagrams and schematics, etc., though it's very short on troubleshooting procedures.  A few other forums I've browsed mention that the problem could be anywhere, and that one would need to start checking voltages at lots of different locations to find which ones are not within spec.

I'm ready to do this, though can you provide any suggestions of where I could check first?  Are there any components that are likely to have failed, where I can narrow my search?

The receiver does have a diagnostic mode via its LED display screen, which can test for protection mode and other things, though it did not appear to have a diagnostic mode to check for why a channel is dead - though please correct me if I'm wrong.

Thanks!!!
Eric E.

Answer
Yes, a dead channel can come from anywhere in the amplification chain and power supply feed lines.  In older units it is sometimes best to suspect a bad (short, open, intermittent) capacitor as they are often the first electronic component to fail.  Especially the electrolytic type which are found in the power supply and as ripple reducers on the voltage line to the output section of the amplifier.

Another cause of a dead channel that is very common is a shorted voice coil in the speaker connected to it.  Best way to check that is to swap the speakers from one channel to the other. If the speaker is the problem you will find the bad channel will switch over to the right side and the left side will then be good!  But, maybe you have already checked that out.  The reason I want to make sure on that is because the description you offered of the intermittent conditions and then a total failure is very common with the bass speaker voice coil.   And, especially when it has been driven hard with loud playback of music.

Since you have the service manual you should do some testing of the dc voltages around the amplification chain.  And, especially if you have a signal tracer you can put a signal in and just follow the signal along the pathway until it is lost then you know where it got 'stopped'.

An oscilloscope is a good signal tracer as you can put the probe along the chain of amplifiers and tell where it has failed, as above.

If the electrolytics and speaker checks don't come up with anything you should consider the output amplifier section as one of the firt to go since it is a heat generator and can often the be the cause of premature failure.  Replacing the output module or power transistors that are mounted on the heatsink is no easy job!

Hope this will keep you going towards a good solution.  

TV/VCR/Stereo Troubleshooting

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cleggsan

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Consumer Electronics of all kinds. Audio, esoteric audio systems and components, video, tv. Digital equipment for consumer use. Ham radio and automotive electronics. Note: I give advice on tv repair based on general consumer electronics engineering experience but I am not engaged in actual repair of sets. MAKE SURE YOU GIVE THE MAKE AND MODEL NUMBER AND AGE OF THE SET. BE SURE TO CHECK YOUR EMAIL FOR THE RETURN ANSWER DURING THE NEXT 24 HOURS AS I TRY TO REPLY TO ALL QUESTIONS WITHIN THE SAME DAY IF NOT THE SAME HOUR.

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