Noisy mains transformer

[Mick Bailey]

I have a pretty much butchered mid-70s Orange OR120 right now that's in need of a lot of work, but the main problems are an OT with a shorted turn and a mains transformer with an unusually loud buzz. Properly loud - like my old Wahl hair clippers held close to my ear. I've tightened everything down - mounts and end-bells, but no improvement. These are Parmeko transformers and I was hoping that they at least would be in good shape, but alas, not.

My previous experience of having a noisy transformer re-impregnated under vacuum showed only a minor improvement. Has anyone got any ideas on this?

[R.G.]

Nope.

You already know what's going on, though. Something inside is being banged back and forth with the magnetic field. Could be a loose winding, or a winding banging one of the insulation layers.

Vacuum impregnation may or may not help. It probably helps most if the transformer is potted in beeswax, not in varnish. If it's had varnish already, that varnish has hardened and may well prevent vacuum impregnation from helping by keeping the new stuff from getting in to where it's needed. Beeswax could at least be remelted slowly.

It might help you to measure the no-load (open secondaries) power use. If it's more than 5% of full load power, there's something in there eating too much current, and it's not long for the world. Vibrating things wear, and if the wearing surface is the wire insulation, it's only going to get worse.

Of course, it's a PT. You could find an exact-size replacement and put the old end bells on it.

[Mick Bailey]

It's pulling 15W with the secondaries disconnected, which doesn't look too bad to me.

The last OEM PT I bought for one of these was £166 about three years ago. The amp has had the wiring cut, got lifted pads from a very poor re-cap job (all needs removing and doing right), burnt output tube sockets, missing main filter can caps, busted fuseholders, missing voltage/impedance selectors, badly fitted mains lead, unusable output tubes, shorted OT.

So, even without any labour costs it's an expensive amp to fix/restore.

[J M Fahey]

Obviously it was not vacuum impregnated when it was made.
Friday afternoon job, I have seen transformer suppliers in a hurry just dunking the finished transformer in the varnish can, 10/15 seconds at most, just to cover the outside.

On my frowning face they said: "customer is in a hurry, no big deal"......

[Mick Bailey]

I used to do a post-mortem on each failed amp transformer, just to better understand failure modes. I've seen many with no impregnation of any kind, and some with just a crispy outer shell of varnish and loose windings within. The last Orange PT I took apart was from an early 90s OR80 and this had no varnish and no wax, but had a badly burnt HT winding.

I think tomorrow I'll remove the end-bells to see if this one is impregnated.

[stereomonostereo]

I had an old Peavey power transformer that was making the same noise. I tried tightening everything to no avail. Finally someone suggested dunking in Spar Varnish. I let it dry for 24 hours and reinstalled it. It's been running probably two years since with no issues