Electronics > Repair

Weird Channel Output Problem - Onkyo TX-SR 508

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Hello everyone, I recently picked up an Onkyo TX-SR 508 7.1 AVR, so I don't know much about its history. it works very well but I have a small problem on one of the output channels.

On the left rear surround channel, the sound is less loud than on the other channels. But I can make it come back to normal by increasing and decreasing the volume.
Let's say that the volume level is set to "10", all channels work but the left rear surround channel is weaker than the others. However, if I briefly set the volume to "20" and then lower it back to "10", the problem disappears and the channel produces the same sound level as the others. It seems that increasing the volume "unblocks" something in the amplifier.

I've already tried with another speaker, and another cable, but the problem is still there. I also tested the transistors, I did not see any anomaly. I also removed the dust inside. All connectors seem to be good.

Have you ever had this kind of problem on an amplifier?

All answers are welcome! I thank you in advance for your help :)

Here is the service manual, if you want to see it (need to scroll down a little for the DL link) : https://elektrotanya.com/onkyo_tx-sr508.pdf/download.html

(Ps: little update about my post for my oscilloscope, unfortunately I didn't have time to take care of it, but I will keep you informed!)

Jeff eelcr:
Good visual inspection will most likley not find this type of problem.
Check the input with an oscilloscope first then follow the signal to the 
problem..... capacitor or whatever.

Dirty relays do that sometimes.

You could try tapping them to see if you can make the volume go up and down.

I had a similar fault in an Onkyo TX-SR 607 receiver. One of the surround channels was lower than the other, with intermittent crackling  / popping occasionally then normal level would resume.

The cause was a defective driver transistor, either the 2SA1930 or the 2SC5171 (can't remember which polarity) which had an intermittent open circuit bond connection internally.

In the surround left channel of your amp, that would be Q6033 or Q6043. It was an unusual mode of transistor failure, which resulted in the output either losing the positive or negative half of the waveform.

You could swap the two transistors from another channel (one at a time) and see if the fault moves with them. Your amplifier uses the same type of transistors so it may possibly be the same issue if you're lucky.

Hier, from german forum, is a simplified flow of the left front channel (other channels are similar) on the pic.


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