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Yamaha A1 restoration 2021

The "instantaneous" current will be about 50Volts/R, so using 1ohm will give 50A!!! using 1MEG will give you 50microA.
The instantaneous power dissipation will be 2500/R, so depending on what you have on hand, eg, a 1W 2.2K, but this
will be a bit slow (22mA), I'd be going with 200-1kohm, 5W. I simply hold the resistor body with some pliers and then hold
across cap solder joints, ok, a bit clumsy, so use clips/wire.

Is the amp working normal before you start the refurb?
 
Some patience... and they both go down to 0mV.
However, the right one climbs very slowly, about 1mV every 10 seconds,
The unit isn’t plugged into mains power.
Toggling the power switch on and off on the unit, with it not connected to mains power, makes no difference to this climbing back up activity.
Does it say something about the condition of these capacitors?
 
Thanks Martin, I remember that comment from him.
Just measured again and it’s now got to 114mV of its own accord, and still climbing.
 
I’ll go along with that.
It’s now at 179mV.
Look forward to hearing any contributions as to why this might be happening. Also whether that capacitor can perform as intended when it’s in operation with power supplied.
 
Maybe.
Got to 257mV.
Will now clear it to zero, and see what happens if I leave it until tomorrow.
Will let you know what happens. Thanks again.
 
So, 11 hours later the readings are:
Right is 534mV
Left is 17mV
Cleared them both to 0.
However, the right channel again continues to rise at the same rate of progress as before.
Will leave it a little while and see where it gets to.
 
Yes, it’s a puzzle.
After clearing both channels about 50 mins ago, the readings have again climbed.
Right: 84mV
Left: 3mV
Wondering if there could be a ‘bad’ cap somewhere else and/or a bad diode allowing current to flow back into the right channel capacitor? No bulging caps, but everything in this unit is original and at 40+ years there may be a compromised component or two.
Maybe it’s a question for @avionic
Surely he’s come across this phenomenon before?
 
No, doesn't work like that. Is this a working A1 you're just checking on or does it have an issue?
Are you using the multimeter properly?
 
Have only just purchased the amp.
Was told that it would power up, play sound through the speakers but then cut out after a short period if the volume was turned up.
So, having got it home I haven’t powered it up myself. I wanted to clean out all the dust and see if there were any obvious physical issues on the boards etc. and thought I should discharge the big caps ...just in case... and that’s how I’ve discovered this puzzling issue.
 
Thanks Martin. Yes, it may well be, good thinking. I’ve spotted those crusty joints, as posted already, so will attend to those. Will also try to access the other boards and reflow if necessary.
Then switch it on and see what happens.
Will try to get an early reading and adjustment on offset and idle, in case an incorrect level is what’s causing the cut out, and then recalibrate as it warms up. Apparently the relay switch itself can cause problems, so if I can access it I’ll check and clean the contacts if possible.
Any other advice gratefully accepted.
Thanks again.
 
Yes, I noticed some mention of that in the tail end of that other thread. In the meantime...
 

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