Help! Cure for transformer hum?

Chuck,

Thanks for this tip; I was hoping for something like this. Of course, I'm puzzled that there was no hum before, when the "old" transformer was installed in the original unit. Nor is there any hum in any other transformer-driven device in my home. But hey--I'll give it a try.

"It" being the "hum buster." What is that? I just searched Amazon and eBay, and I only found a few ground loop hum eliminators. This is NOT a ground hum, of course.

Can you name a brand for this device? A source for it? Cost is almost irrelevant; I'm willing to try anything within reason.

Thanks in any case for your answer.

So many helpful suggestions here! Thank you all. This is Audiogon at its very best. 

A few comments:

millercarbon: thanks yet again for the links to specific products. Your audio knowledge is vast, and I'm grateful for it. (BTW, although I do not share your enthusiasm for Schumann resonance generators, I nevertheless have one hooked up to my system.)

fiesta75: Sadly (for me), I suspect your advice is likely the most accurate regarding this problem. About the power supply caps, however: when the NAD service tech swapped out the transformers, he checked the caps, and they were OK (not leaking, at any rate; I'm not sure how thorough a test he made of them). In any case, no, they have not been replaced. I know they probably should be.

Erik: I am sure I have it plugged into the same circuit (via a power conditioner, btw). In any case, I've tried it in several different plugs, and I've removed it entirely from the system, plugged it in to a remote outlet via a good extension cord, opened it up, and stuck my ear up close; this is how I know the sound is coming from the transformer. In any case, before I made that long drive with both units, one hummed and the other didn't when plugged into the very same outlet.

tvad: thanks for the PSAudio troubleshooting guide link. I'll read it this afternoon (I've got a busy day until then).

Finally, vinylvalet: Yes, this is a mechanical hum, NOT a ground loop hum. There is no sound coming out of the speakers, and, as I just explained, my ear has located the source of the mechanical hum with good confidence as coming from the transformer. Your further comments tend to confirm millercarbon's advice, which is very encouraging.

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.

Don't know if anyone is still looking at this thread, but just in case:

I tried the kitchen wall outlets; no change in the hum. What does this mean? Does it mean that the problem is not "DC offset"? That I would not therefore benefit from a DC Blocker? (I notice that the "Humdinger" site says that device will NOT cure a hum caused by a faulty transformer, although it might reduce it.) Remember that the "old" unit never hummed in 30+ years of operation, and none of my other audio devices hum even now. So the problem does seem to be in the transformer, and not in the wall power. Still, I'd love to believe a DC blocker would help; that would be the easiest, and cheapest, solution.

chris_w_uk: thanks for your suggestion! I think, finally, it's my best bet here: a DIY "re-potting" with 555 varnish and an oven. But the link you provide is not, as far as I can tell, to a business that could do this for me. For one thing, it's in Mumbai! No way I'm sending this very heavy, and apparently vulnerable transformer to India. 

Some updates on this issue, for those who may be interested.

First, I tried a DC blocker (the iFi unit). No effect. Frankly, I had thought this was a long shot; after all, the unit didn't formerly make any noise--it had been dead quiet for 30+ years--and no other transformers on my home's power, audio or otherwise, make any noise.

The "dip and bake" suggestion seems to me to be the best proposal, short of just living with the noise or replacing my beloved old NAD unit with something newer. However, I'm not sure I want to mess with heating varnish in my kitchen's oven, for the obvious reasons. Chris_w_uk is the Audiogoner who described this process, identified the varnish I would need to use, and then even provided me with a link to a company that may be able to do this for me (at his suggestion, I tried to locate such a company in my area, but none of the likely candidates really understood what I needed). So I've emailed the company Chris sent me the link to, and I'm still waiting on their reply. But I want to thank Chris for his well-informed help.

Re-capping has also been suggested, and I know this is something I probably ought to do for lots of other reasons as well. But my intuition tells me it is not likely to resolve this particular problem. I won't go into my reasoning here, but if any of you think I'm wrong about this, let me know.

Finally...the "noise" is, after all, very subtle. I can't hear it at all while I'm listening to music (except between tracks), it does not seem to affect the SQ in any other way, and even with the room completely silent, the transformer hum is faint. My home's deluxe HVAC system is louder, and so is the refrigerator two rooms away in the kitchen. I have an audiophile friend with a designated listening room full of far more expensive equipment than mine, and he doesn't seem to be bothered by an ambient hum from some household device in the vicinity that is much louder than my transformer hum. So, really, this problem is more psychological than musical. And yet...we audiophiles are perfectionists. I can tolerate a noise in a fridge (although I chose one based on how quiet it was); I can tolerate wind noise in the trees outside, or birds, or the occasional plane overhead. These are not flaws in my audio system, unless the term "system" is construed broadly to include everything in the audio environment, including one's own ears (and who among us has perfect hearing, or doesn't suffer from minor tinnitus?). Be all that as it may, I am insanely intolerant of any perceived flaw in my audio equipment itself. So perhaps this "problem" is really mental. Maybe I should learn a lesson from it.

Thanks for everyone's contributions to this thread.