Yamaha M-50 power supply voltage adjustment

I got this amp at my bench with just the question of giving it a check.

I already learned from here the issue with the infamous c607 cap, but there's still some thing that made me wonder;

DC offset at right was ok 1,4mV, but at left 62,3mV. I could correct that without a problem.

The bias voltage for both left and right was low, 2,6mV, that could be adjusted to 10mV, although these trimpots are not the easiest to adjust.

But then the power supply voltage (B- measured). That should be around -55 Volt, but it was -66,4 Volt and could not be adjusted lower than -57,8 Volt. It even made noise that diminished
with lowering the voltage (the transformer that is I guess). Could that have anything to do with this cap being bad?

It was clear someone else already worked on this amp, one of the bridge rectifiers had been replaced and it was missing some screws (I hate that!), so I just wonder if the original PS cap is still
in (I think so, since I can see bursts inside), so would this be the original?

Could that have anything to do with this cap being bad?

Click to expand...

No. The cap is OEM Rifa .If that cap was bad . It would be smoke'n/burnt up by now.
What is your measured input AC mains voltage ?

I thought of that, but then rejected the idea, because that difference imo could not cause it.
You still think it would? It's 230V, exactly even, a 4,5% difference compared to "vintage days" .

Is this a multi-voltage unit? And if so, is the voltage switch set to the correct mains voltage? What voltages are you getting on the main filter capacitors? Are they balanced?

-bob

jeromach said:

I thought of that, but then rejected the idea, because that difference imo could not cause it.
You still think it would? It's 230V, exactly even, a 4,5% difference compared to "vintage days" .

Click to expand...

How does the amplifier sound at 57.8v ?
Were you using 2 multimeters to adjust the PS B- ? VR601 and VR602 need to be adjust together, they enter-react with each other.
These X-power circuits can be tricky. Page 8 of the service manual has a PS t-shooting procedure if needed.

If the amplifier sounds ok and the photocoupler adjustment is within spec. Your probably good to go.

Solved.

Initially I seperately adjusted the photocoupler adjustment (first) to 1.0 volt, I then adjusted the B- voltage and could not get it lower than -57.8.

But indeed (and the manual does mention that, I should learn to read ) when connecting two multi meters it appeared the photocoupler adjustment was at 0.8 volt after lowering the B- voltage.

By adjusting the two together I could get it to 1.0 / -54.5 volt, that is exactly according to specs. However, the B- trimpot is almost at its lowest position. I assume this indeed is because of the 230 volt instead of 220 (@bob, it doesn't have main voltage options - unfortunately).

It does sound good now, a lot of power (125 Watt before clipping @ 1kHz sinus/8 ohm dummy load).

Thanks guys, for your support!