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a new protection relay before it goes bad. Also, exactly which trim pots did you use? This is the first time that I've done this and noticed that the trim is very very touchy, and when I looked again after a few hours of use the right channel had crept up to 18mV. Should I also use deoxit on the trim pots? They are probably the only thing that I didn't deox.
Hi James,
Usually I replace all the trimmers in +30yo units (after full recap) with
multi-turns pots from Bourns, Murata, Vishay, etc. They are normally 20 or 25 turns, so much more precise than conventional/touchy/cheap/ trim pots and are used in most Hi End amps that require good stability. These pots come in a
variety of types (vertical, horizontal, lateral, square, side adjust., etc). You need to know the value of it for the A-500 (something around 2.2k I believe).
The drift/shift value you have (from 15mV to 18mV after few hours playing music) is not that much and, for an unit from 1980, it is still tolerable. I don't think deoxit the pots might help or make them be more precise/smooth. They are like this. Actually you might end up loosening them. The variation might happen because of some room temp changes/lack of ventilation, heat inside the unit, etc, for example. Or due to some aging components as well. In Summer your unit will run a bit hotter, but no worries.
If you want to replace them,
make sure to preset the new trim pots at least to the proper value in ohms by measuring the old ones first - in some units it's CW turn for one channel and CCW turn for the other channel -, otherwise you'll end up frying your driver transistors (and outputs) by allowing a lot of current into them.
But don't expect that changing the pots will solve the "problem" of bias variations in the right channel. It can still occur.
I made a certain assumption here, please correct me if I'm wrong. The service manual stated to check the idle voltage with no input to the amp, which is what I did. My assumtion was that turning the volume completely down and selecting both front panel speaker switches to off, and no speakers connected would also be the correct thing to do. Yes? No?
Yes, for the A-500: volume down, speakers off and disconnected, and balance, loudness, bass, treble in flat position.
I also noticed a surprising amount of source bleed through from one source to the other. For instance I connected a cd player to DAD, switched to Aux, and turned up the volume a bit and got a faint amount of music. I've never had this happen on a Yamaha amp. Is it common to these or a sign of a problem brewing?
For the signal bleeding across sources. Well, from dirty selectors and switches, to bad grounding, to some external interference, to input capacitors... we have a variety of causes. But it can occur also with brand new gear. Yes, it can be very annoying. One "quick solution" before you trace the problem is to turn off all the other components (tuner, cassette, etc) while you are listening to your CD player.