Yamaha RX-V 596 RDS Receiver Buzzing Sound

[fermyon]

My Yamaha RX-V 596 RDS Receiver has a buzzing sound. The unit is approximately 20 years old and I obtained it second hand. I just noticed the buzzing, but it may have been there from the beginning.

The buzzing can be heard on all speakers and the headphone port, but it is loudest on the front left (recording is attached). It becomes louder when connecting the optical input. At normal listening volume, it is barely audible. The volume increases with the main volume knob. However, it disappears once I am close to full volume.

Service manual: https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Yamaha-RXV-596-Service-Manual.pdf

I also connected my "oscilloscope" to the speaker terminals of the left speaker and recorded the waveform.
Without optical input: see first image.
With optical input: see second image.

Without optical input, there are pulses every 10 ms. With optical input, the pulses are wider and bigger every and occur every ~12.5ms with additional smaller pulses (not sure which interval).
Due the 10ms pulses I first thought the power supply capacitors may be faulty, but since the frequency changes when the optical input is turned on, it seems more likely that the issue is in the decoder/electronics.

I also tried grounding the receiver since its plug does not connect to earth, but that did not change anything.

Is there anything else I can check? What do you think might be the cause? To be honest, I am not very knowledgable about this sort of thing.

[ambrosia heart]

First thing to do is to isolate the audio path. See pm.

Don't write too lengthy. No time to read composition. I am now playing a game. I wrongly chose Claire as first to play, leading to great difficulty in second play.  I must start from Leon with no choice and play it from start.  Oh dear!

A good opening is a key to success,  not only in troubleshooting but also in gaming. 

[SpecialK]

Well the inverse of 10ms is 100Hz.  Since Germany is a 50Hz country, recification would create 100Hz products that the power supply capacitors are mean to filter.  So I would look at the power supply section.  Maybe find the regulator that powers the op amp circuitry.  Look at the electolytics around the area.

[fermyon]

Thanks for your suggestions. The issue (gradually?) disappeared while I was measuring an internal left channel pin. I might have accidentally connected it to the right channel with the probe, but not sure.

[mzacharias]

I'd always suspect power supplies and ground paths first.

'Scope the power supply rails for similar noise;

Make sure all the grounds are confirmed good.