Yamaha A-550 — left channel very low volume, full volume in mono — help diagnosing

[Space_Boy_Bo]

Hi all, hoping someone with A-550 experience can help me diagnose a left channel issue.

I purchased a a A-550 off of FB marketplace relatively recently. The amp has been working fine, but recently the left channel volume dropped to about 10% of what it should be. Right channel is perfectly normal.

Here's what I've already tried:

- Swapped speaker wires left/right at the amp — problem stays on the left channel, rules out speakers

- Tried multiple input sources — same result on all inputs

- Cycled the volume and balance pots repeatedly — no change

The interesting clue: when I switch to MONO, the left channel comes in at full volume.

From what I understand, this suggests something in the left channel signal path — possibly a failing coupling capacitor, a cold solder joint, or a differential pair transistor issue. But I wanted to ask here before touching anything since you folks know these amps much better than I do.

Amp is otherwise original, hasn't been recapped. No physical trauma.

Any known weak points on the A-550 that match these symptoms? Happy to take measurements or photos if that helps diagnose.

Thanks in advance.

 

[Sparkplug]

I’m fairly sure the issue is in one of the selectors or switches. I’d start by fully cleaning the Stereo/Mono switch itself first. Do not touch anything else at this point, especially any active components.

 

[mbz]

Don't know the amp...

Schematic shows the stereo/mono selector immediately after the input selector and before any preamp gain stage.
Indicates input selector or stereo/mono selector as per sparkplug. You could try working these switches like 20 times noting any change (good or bad) would point to that switch, cleaning would then be required.

 

[Asmodeus2112]

Did you get your issue figured out? I have an A-550 in my personal work queue. Not sure what I could help with at the moment, but I do have one of these.

 

[Space_Boy_Bo]

Problem solved, and I owe you all an apology for wasting your time.

After all the thoughtful suggestions — turns out it was a loose connection in the tonearm headshell on my turntable. I had methodically swapped speakers and tried multiple inputs, but embarrassingly never tried a completely separate source like a phone or CD player plugged directly into the amp.

The left channel on the amp is perfectly fine. The issue was upstream the whole time.

Lesson learned: always verify your source before diagnosing the amp. Thanks for the help and sorry for the false alarm.

 

[avionic]

I like easy fixes.