Rotel RSX-1056 amplifier. Troubleshoot a noisy channel

[Francesc V.]

Hi:

I've realized that one of the channels of this amplifier has no proper sound.


This is the signal at the speakers output.

As long as the audio coming in to the amplifier board (blue) looks ok although with some noise due to the wire, I think that there's nothing to look at the preamp section.




Now, I could start looking at components to see whether I find the guilty one.......but I would rather prefer to ask for advice. Here there is the amp. schema. Which points would you check with the oscilloscope to gather where the issue is coming from?



Thanks in advance,
Francesc.

[CapLeaker]

Is the channel equal in volume if you compare it to the other one or is it lower in volume too?
put your oscilloscope probe on C505 and have a look how the signal looks there.

[Francesc V.]

Hi:

Thanks for the response. I would say it's equal.........meaning that I am using a signal generator from the phone. i've been using both 1kHz and 440Hz signals in both chanels. So yes............in first picture at least they should be equal (ch1 and ch2).

I've been troubleshooting the schema and I've found up to 6 capacitors out of specifications, and more important, an open resistor.



As soon as I have the replacements, I will post the results and also I will gather some signals fom this C505.

Regards,
Francesc.

[CapLeaker]

C505 is a DC blocker cap, that’s why it is in series. If that cap is bad, you will find audio on one end and less audio on the other. It also gives you a good point to check the audio input to the amp itself.

[Francesc V.]

After replacing 7 capacitors and the opened resistor, the Left channel is back to live.



I didn't miss the opportunity to review and recap the other 4 channels from the amp.



I guess that the major problem was the open resistor rather than the old caps.

Regards,
Francesc.

[CapLeaker]

The bad caps didn’t help it any at all… but the bad resistor was the main culprit, agreed.