Help wanted with noisy mains transformer

[Nickthedentist]

Hi everyone,

I'm sorting out a tatty Cossor 524 Melody Maker at the moment - the one from 1955 with a Bakelite case, AM and FM, and a huge (10" x 6") elliptical speaker.

It looks like its been stored damp, and all the steel bits are rusty to a greater or lesser extent.

After doing my usual routine stuff (replacing Hunts and waxies, reforming reservoir/smoother, cleaning wavechange switches etc.), things were looking promising. Apart from dodgy speaker (see previous thread), everything worked surprisingly well, with plenty of sensitivity and good, loud audio on all bands.

However, I am having trouble with noise from the mains transformer. At switch on, it makes a brief clattering noise then while running, there's a low, deep hum (as opposed to a harsh buzz) which is transmitted via the chassis to the workbench. Slackening the fixing bolts seems to help, presumably because the mechanical coupling between it and the chassis is reduced. It's present when they are tight (or very tight).

When I received the set, the transformer's primary seemed to be o/c, but fortunately, it was just a case of "greenspot" on one of the connections to the voltage selector. I removed the fixing bolts, took off the top cover, and disconnected the voltage selector, permanently wiring the transformer for 240-250V operation via small wire-ended thermal fuse soldered directly to the tags on the top of the transformer. I then replaced the shroud and bolts as originally found.

Any suggestions please?

Nick.

[G6Tanuki]

If it's got a full-wave rectifier [two anodes, centre-tapped HT transformer] check that both halves are working: if one half is open-circuit (failed valve, green-spot on one HT half-secondary, bad valve base contact) the other half works twice as hard, and the DC component flowing through the secondary winding as a result causes it to magnetise causing nasty effects.

[Nickthedentist]

Ah, what a good idea, I'll check that, though the hum seems to start before the valves warm up.

And yes, it has exactly that arrangement with a rather tired-looking original Cossor 6V4 (EZ80 equivalent).

The transformer runs very cool BTW.

N.

[paulsherwin]

I take it you've checked that all the bolts are tight?

Are there any signs of gunge having emerged from the transformer in the past?

[Nickthedentist]

Hi Paul,

No signs of emerging gunge / "migration of wax" whatsoever (surprisingly), and the two bolts which hold everything together and fix it to the chassis are as tight as I can get them with a large screwdriver on top and a socket on the bottom.

Nick.

[Nickthedentist]

Another thought... When reassembling, I pushed the two arrowed black impregnated paper tags into a vertical position before fitting the top metal shroud. But I'm wondering whether they should be horizontal and trapped between the top of the lams and the bottom of the shroud?

[G8HQP Dave]

Some transformers are just noisy. Try leaving the bolts slightly looser, or add a rubber washer between the transformer and the chassis - but ensure electrical continuity.

[Nickthedentist]

Dave,

I was thinking I might have to resort to that!

It's a shame that the tx. is so mechanically noisy because there's no audible mains hum from the LS.

Nick.

[Leon Crampin]

If the laminations are not fully interleaved, you could try slackening the "through" bolts and fitting a large worm drive hose clip (with load spreaders at the corners) around the stack. Tightening the clip when the transformer is warm, then tightening the "through" bolts may yield a result - in some cases, the hose clip can be removed once the stack is fully interleaved.

Transformer noise is a very variable thing and I have used the hose clip trick to good effect on "C core" transformers, but it's so easy to do, it's worth a go. I managed to silence the shoddy transformer in my Quad 405 by this method.

Leon.

[Nickthedentist]

Thank you Leon, I remember you mentioning that before.

Have you got any pictures to illustrate where the clip would go, please?

[crackle]

Hi Nick
The photo in post #6 appears to show the transformer bolted down with out the top cover. The top cover can be seen in your first post.
If the transformer does not have a "frame" covering the laminations which the bolts pass through and so spread the load there will not be even pressure clamping the laminations together, thus maybe allowing them to vibrate.
Mike

[Nickthedentist]

Hi Mike,

For the photo in post 6, the top cover was removed while I was re-making the connections and fitting the aforementioned thermal fuse. I replaced the bolts loosely merely to stop the transformer dropping out when I turned the chassis on its sides, and straining its connections underneath.

The cover's now replaced and the two bolts refitted tightly, as originally found (as in the first pic in post 1).

N.

[crackle]

Have you tried placing card or rubber between the cover and the laminations to help spread the load?
Sometimes the cover gets distorted at the bolt holes resulting in pressure not being applied evenly around the laminations.

Mike

[winston_1]

Originally Posted by Leon Crampin

the hose clip can be removed once the stack is fully interleaved.

In fact it MUST be removed as it will be a shorted turn.

[Nickthedentist]

I will certainly try that.

[yesnaby]

Time for a 'Birmingham screwdriver'?

[Nickthedentist]

Ha ha!

[Herald1360]

Originally Posted by winston_1

In fact it MUST be removed as it will be a shorted turn.

It can only be a shorted turn if it goes round a limb of the core at right angles to the magnetic flux. Going round the ouside of the core there won't be any coupling.

[Refugee]

You are right about the hose clip not becoming a shorted turn.
20 years ago I made a small pump using a BSR deck motor with the mounting consisting of a large hose clip around the outside of the core holding the small gear pump assembly.
The motor coils last about 15 to 18 years running 24/7 with the rotor floating on a stack of washers to stop the bronze bearings going noisy.
It is still in use.

[Boater Sam]

If the transformer is electrically sound, I would remove it and dip it in varnish whilst warm.
Done this with dozens of mains transformers, silence is golden, the colour of varnish!