Help! Cure for transformer hum?

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.

millercarbon9,401 posts06-01-2021 12:18pmThe sort of mechanical vibration hum you have is caused by DC offset. Our homes are run off 240V but instead of running everything at 240V like they do in a lot of countries someone here decided that was way too dangerous for us fraidy cat Americans and so all our electrical panels are split right down the middle with only 120V going to each circuit.

Thank you Sir.

Here I was thinking having 240v was an impediment limiting purchasing choices.

I used to work for a company that repaired and rewound electric motors and generators. See if you can find a company that does this. They can do what they do with the motor stators after rewinding. They put them in an oven and heat them, then dip them in 555 varnish until they cool, lift them out and leave them to drip then put them back in the oven to slowly bake overnight. We did this in batches every night, so it should be quite inexpensive for them to just pop your transformer in with all the other stuff. If it is a shrouded transformer you will need to remove that first. https://nsshah.in/product/jyoti-555-fd/

As an aside, if the transformer is vibrating, this can also be the windings vibrating as well as the iron laminations. Over time this can cause the windings to short. A dip and bake will stop that.

If you don’t mind upsetting your good lady (assumption mad here) you can buy the 555 varnish and do it yourself. The oven only needs to be 80 to 90 degrees to thin the varnish when dipped to allow it to penetrate. Then leave to drip and bake at 80 to 90 degrees for an hour or so. Do it on a fine day as you will need the windows and doors open 555 is smelly stuff :^)