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Repair help: Yamaha AX-540 amp

L

Ludd

New Member
Love my old AX but the left channel sounds slightly "dull" compared to right channel. At first I thought it was just a volume difference, but it also lacks some brilliance compared to the other channel. Sitting at ideal listening position, the right channel sounds brighter and louder.

I have switched speakers and cables to eliminate external sources. The fault is present on all inputs and both A and B speaker channels. Bypassing the EQ does not help.

I recently cleaned and re-lubed all switches and pots and they work fine. I also calibrated the idle voltage according to the service manual. The left channel had notably lower idle voltage, so for a moment I thought I had found the problem, but calibrating the voltage did not help. I don't have much experience with fixing amplifiers so would like some advice to what my next step should be.

Manual link:
Yamaha AX-540 Service Manual

Any help much appreciated!
 
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Welcome aboard Ludd



Sounds like a dirty switch ( Input Select switch ) or dirty control.(volume , balance , etc.)
 
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Thank you Avionic.

Yes, they were in absolute terrible condition. I have cleaned all pots and switches though, so that's not it I'm afraid.
 
Ludd said:
Thank you Avionic.

Yes, they were in absolute terrible condition. I have cleaned all pots and switches though, so that's not it I'm afraid.
Click to expand...
Did you dis-assemble the switches ? Could also be dirty protect relay contacts.
 
I did not disassemble them, but carefully injected a lubricant cleaning spray into the switches and pots and gave them a good workout. This was about a year ago and they still operate reliably and without any signs of scratches or glitches.

The A and B channels each have their own relays and the problem is present on both channels, so it shouldn't be the relays.

I'm quite positive this is a pre-amp or EQ issue. Since defeating the EQ doesn't make any difference, I'm leaning towards the pre-amp section. Also, the problem doesn't seem to exist in the headphone output, only the speaker output.

The fault is subtle, i.e. the bad channel still sounds pretty OK but when A/B comparing to the good channel, it's quite obvious that it's lacking presence and brilliance.
 
Ludd said:
The fault is subtle, i.e. the bad channel still sounds pretty OK but when A/B comparing to the good channel, it's quite obvious that it's lacking presence and brilliance.
Click to expand...
Going to need a scope to find that. By comparing the two channels. Somewhere in this area. I see a handful of coupling caps , a couple op-amps and two muting transistors.


1704857814482.png
 
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avionic said:
Going to need a scope to find that. By comparing the two channels. Somewhere in this area. I see a handful of coupling caps , a couple op-amps and two muting transistors.
Click to expand...

Thanks for looking into it. I don't own a scope unfortunately. Would it make sense to start swapping op-amps or transistors and replacing caps in this area, in order to pinpoint the issue?
 
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