Yamaha AX 500 unequal output

[Ariansdf]

Hey people,

I recently bought an AX 500 very very cheap. But as I started using it I felt the right channel was somehow more robust.
I've tested almost everything, changed speaker channels, changed speakers, etc. and I think the left channel is about 10db weaker (with the balance knob), especially in bass frequencies.
So tried to measure the outputs with a multimeter (not sure even if makes any sense! ) and the result was like this:
(not playing anything)
L: ~15mV (single) / ~10.3mV (A+B)
R: ~48mV (single) / ~26.2mV (A+B)
so the first question is, shouldn't they both be equal?

Then I opened it to see if there is any strange physical problem happening. I cleaned everything with contact cleaner. and also measured the R229 ad R230 and adjusted the idling current from about 18 to 10mV (what the service manual suggested). Didn't solve the problem.
I noticed that pushing the relay slightly when playing disconnected the channels. Soldering seems fine. Can the relay be the problem? or maybe the caps are just too old?

Generally, the amp is in good condition and sounds very nice and clear but this is driving me crazy

Thanks for your help!

 

[nopiano]

Welcome to the forum.

I’m not an EE by any stretch, and this forum isn’t necessarily a hotbed of DIY expertise, but a few thoughts:-
1. you have adjusted the DC offset, I think. That should be identical and minimal.
2. your amp must be getting on for 35years old and I’m sure lots of components will be well outside spec.
3. I fear there’s no quick fix and an overhaul is of questionable value as I don’t think it’s a collectors item.
4.We know now why it was very cheap!

 

[Gray]

Can the relay be the problem?

You can say for sure by bypassing its contacts (make sure it's the contacts and not its coil that you're bypassing).

Failing that:
Looks like you've got the pre power split with external links. If you cross the channels at that point (using a twin RCA lead), you will narrow down where to look for the problem.

After crossing them over (by feeding L pre to R power & R pre to L power) if the L output remains dodgy - your problem is in the power amp section.
But if the R output becomes the dodgy one, then the problem is in the preamp section.

It least that halves your search area.
The good news for you when faultfinding a single channel fault in a 2 channel amp, is that you've got an identical good channel for signal tracing / volt measurement comparison purposes.....,meaning that you can't fail to diagnose the problem 👍

(It's possible you're already there with the relay theory).