Weird Channel Output Problem - Onkyo TX-SR 508

[thitou]

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.
Jeff

[Audiorepair]

Dirty relays do that sometimes.

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

[cozza]

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.

[Kaluluka]

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

[raphaeltube]

A dry solder joint could do that.

 -R

[shakalnokturn]

+1 for checking solders (start with PCB interconnect solders) and cleaning output relays.

[thitou]

Hello everyone !

Thank you a lot for all your answers 

I'm really sorry I didn't respond sooner. I didn't get a notification about this post. My apologies.

I will check everything you have indicated.

If it is a relay I think I have some spares that would be compatible. By the way it makes me think that when the channel is working normally, if I go into the settings and deactivate it, I hear the relay click, then if I reactivate it clicks too but the speaker doesn't produce any sound (or really really low) until I increase the volume. There may indeed be a contact problem.

+1 for checking solders (start with PCB interconnect solders) and cleaning output relays.

How can i clean the relays ? I thought they were sealed and couldn't be taken apart


Thanks for the simplified schematic! It helps me to see more clearly, I never repaired an amplifier before this one.

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.

That's interesting, I didn't know that transistors could break like this. I'll definitively check that. I think you meant Q6035 and Q6045. i'm having trouble with the left rear surround channel, not the left surround chanel  .

I also noticed that there was a burnt trace on one of the resistors (see picture). I measured it and it seems to be ok anyway. Maybe it's because it's next to a transistor that may have heated up a lot.

[slbender]

Those white things look like dual fusible emitter resistors (if the have 3 pins out the bottom) one side may be open. I have a 2 channel “Korea” made receiver with an open resistor on one side.  I’m going to replace with two 0.27 ohm/2 watt discrete parts. Not sure of each resistor’s power rating.  Measure center pin to either end, to see if there is an open. An open resistor usually will mean a blown transistor also, my set uses Sanken 2SB1560 / 2SD2390 flatpak darlingtons, beta usually measures ~68 - 210, best if a matched pair.

Steven

[wasedadoc]

If one side of that dual resistor was open, there would be distorted sound or none at all. Neither is the reported symptom.

Check the relay by soldering a temporary wire link to bypass its switch contacts.

[cozza]

I would definitely replace or bypass the relay first. This does seem like it could be caused by a bad relay contact.

[shakalnokturn]

Yes this is closing in on a relay or other weak contact in the output path (speaker output terminal solders are worth checking too).
Some relays are sealed and usually the most reliable, others can have the cover removed, list of the time it's more convenient to desolder the relay for that.
I clean relays with contact spray and a small piece of paper, I'll make a closed-contact resistance measurement before and after.

That doesn't look like a burn mark on the resistor just ambient dust / smoke accumulation due to hot components.

[David_AVD]

Another vote for speaker relay. I see them quite often. Cleaning can work if you can't find a good replacement, but do not use any abrasive on the contacts as it will remove the plating and just make it worse long term. We use the contact cleaning strips (Electrolube?) for this and it works well if we can't replace the relay because it's too weird.

[thitou]

Hello everyone!

I have some good news, the problem was indeed a bad contact in the relay of the rear surround channel. I also noticed that there was a difference in sound level on another channel. It was also a bad contact in the relay. I changed both relays with compatible models that I had. Unfortunately I could not clean them, as they are sealed (Thanks anyway for giving me your cleaning techniques, I am sure it will be useful).
The amp work like a charm now.


I took the opportunity to redo the soldering of the speaker output terminal card.

Those white things look like dual fusible emitter resistors (if the have 3 pins out the bottom) one side may be open. I have a 2 channel “Korea” made receiver with an open resistor on one side.  I’m going to replace with two 0.27 ohm/2 watt discrete parts. Not sure of each resistor’s power rating.  Measure center pin to either end, to see if there is an open. An open resistor usually will mean a blown transistor also, my set uses Sanken 2SB1560 / 2SD2390 flatpak darlingtons, beta usually measures ~68 - 210, best if a matched pair.

Steven

You are right! There are 3 pins on the bottom. If I remember correctly, I had tested them and I didn't find any anomaly. The power is 2 watts per resistor i think, according to the service manual. I have seen that some people replace them with two resistors as you are about to do.

That doesn't look like a burn mark on the resistor just ambient dust / smoke accumulation due to hot components.

i don't know what this is, I've tried to cleaned it but i wasn't able to get ride of it. But the resistor is fine ! l’m happy with that.

Thanks a lot for all your answers ! 
Merry Christmas everyone