Another big thanks to everyone. You guys are great.
So, as to particular suggestions and questions:
fiesta75: Yes, I know the opinion among many is that electrolytic capacitors should be replaced after even 20 years. But then, others have told me (and they're usually the ones who might actually get paid to do the job, so are motivated to recommend it) that if they're not leaking, and if they measure within spec, there's little point in "re-capping." I've been repeatedly assured that SQ will not change, and that, especially if they have not been warmed and cooled much from lots of use (and this unit was stored for most of its life, as far as one can tell and as the seller claimed), electrolytic caps can last 50 years or more. Of course, they're now 34 years old, so it makes sense to follow your advice and have them replaced. I'll do that if I have to take it back to the tech, but I'd rather solve the problem myself without making another 400 mile drive.
As for the "strange metal strap/bracket on the top" of the transformer: wow, thank you for going to so much trouble as to search out photos of the guts of this thing. I have my own photos, as you can imagine, and would be happy to send them to you if you think a virtual visual inspection might result in more advice. Let me say, though, that I have indeed checked to be sure the transformer is securely fixed to the chassis. I was tempted to swap out the two of them myself, but didn't want to screw things up; as I've mentioned, the unit does work in every other respect, and superlatively well.
BTW, I did keep the "old" transformer--which is really the newer transformer, since it was originally mounted in the unit that saw much less use. If it can be "re-potted," I would definitely want to do that. In fact, I hinted at that as a possible solution in my original post, except that I described it ineptly. Do you know anyone who does that kind of thing?
jea48: You're probably right that it's not a toroidal transformer. It's disappointing to learn that millercarbon's original suggestion--a DC offset filter--wouldn't solve the problem if that's the case.
I will try the test you suggest: plugging it in over the range in the kitchen. But I don't understand quite how this would be diagnostically meaningful. Is it that a 20 amp circuit wired with #12awg copper won't have the DC issue? If it doesn't hum in the kitchen, does that mean that a DC blocker would, or wouldn't, help? If it doesn't hum in the kitchen, does that indicate whether or not it's a toroidal transformer?
As for whether or not it hummed in the repair shop (which fuzztone also wonders about): well, I can't say! You should have seen that shop! A beautiful mess in an industrial park, old equipment everywhere (for parts). The repair guy was an old Japanese man on the verge of retirement who has been fixing audio equipment since the seventies, and has been an authorized NAD tech for most of that time. But the place was noisy; traffic, lots of other distractions. And we're talking about a fairly subtle sound here. I'm lucky to live in the country, where I rarely hear anything other than breezes in the trees and birds. In my listening room, I have complete silence, and no nearby neighbors. The noise would be obvious to almost anyone here at home, but in the tech's shop, I wouldn't have been able to hear it if I'd tried.
So, to sum up: 1) do you guys think a DC blocker is a likely fix, or not? 2) How might I get the newer transformer "re-potted"? 3) If the unit still hums when plugged into the kitchen outlet, what do I conclude from that?
Thanks in advance for your sleuthing and speculating.
So, as to particular suggestions and questions:
fiesta75: Yes, I know the opinion among many is that electrolytic capacitors should be replaced after even 20 years. But then, others have told me (and they're usually the ones who might actually get paid to do the job, so are motivated to recommend it) that if they're not leaking, and if they measure within spec, there's little point in "re-capping." I've been repeatedly assured that SQ will not change, and that, especially if they have not been warmed and cooled much from lots of use (and this unit was stored for most of its life, as far as one can tell and as the seller claimed), electrolytic caps can last 50 years or more. Of course, they're now 34 years old, so it makes sense to follow your advice and have them replaced. I'll do that if I have to take it back to the tech, but I'd rather solve the problem myself without making another 400 mile drive.
As for the "strange metal strap/bracket on the top" of the transformer: wow, thank you for going to so much trouble as to search out photos of the guts of this thing. I have my own photos, as you can imagine, and would be happy to send them to you if you think a virtual visual inspection might result in more advice. Let me say, though, that I have indeed checked to be sure the transformer is securely fixed to the chassis. I was tempted to swap out the two of them myself, but didn't want to screw things up; as I've mentioned, the unit does work in every other respect, and superlatively well.
BTW, I did keep the "old" transformer--which is really the newer transformer, since it was originally mounted in the unit that saw much less use. If it can be "re-potted," I would definitely want to do that. In fact, I hinted at that as a possible solution in my original post, except that I described it ineptly. Do you know anyone who does that kind of thing?
jea48: You're probably right that it's not a toroidal transformer. It's disappointing to learn that millercarbon's original suggestion--a DC offset filter--wouldn't solve the problem if that's the case.
I will try the test you suggest: plugging it in over the range in the kitchen. But I don't understand quite how this would be diagnostically meaningful. Is it that a 20 amp circuit wired with #12awg copper won't have the DC issue? If it doesn't hum in the kitchen, does that mean that a DC blocker would, or wouldn't, help? If it doesn't hum in the kitchen, does that indicate whether or not it's a toroidal transformer?
As for whether or not it hummed in the repair shop (which fuzztone also wonders about): well, I can't say! You should have seen that shop! A beautiful mess in an industrial park, old equipment everywhere (for parts). The repair guy was an old Japanese man on the verge of retirement who has been fixing audio equipment since the seventies, and has been an authorized NAD tech for most of that time. But the place was noisy; traffic, lots of other distractions. And we're talking about a fairly subtle sound here. I'm lucky to live in the country, where I rarely hear anything other than breezes in the trees and birds. In my listening room, I have complete silence, and no nearby neighbors. The noise would be obvious to almost anyone here at home, but in the tech's shop, I wouldn't have been able to hear it if I'd tried.
So, to sum up: 1) do you guys think a DC blocker is a likely fix, or not? 2) How might I get the newer transformer "re-potted"? 3) If the unit still hums when plugged into the kitchen outlet, what do I conclude from that?
Thanks in advance for your sleuthing and speculating.