If something blows fuses, what is the probability the transformer is toast?

[MrBungle]

I've got my eye on something that was working but suddenly blew the fuse but I'm slightly worried that the transformer is toast and am trading up the risks of a £30 expenditure and 3 hour round trip. The device's transformer is irreplaceable in this case as it has approx 1400v isolation between the heater winding and other taps as it has CRT bias. The rest of the device is 100% generic parts. I haven't seen any companies guaranteeing that for custom rewinds and I'm not sure it's actually worth rewinding or getting a new transformer made for. I may budge on that if it's not too much hassle; cost isn't really a problem as it's hard to justify not spending money when I do this regularly on technically pointless things

The device has standard linear transistor based power supplies on the secondary side so I suspect that it's dead smoothing capacitors rather than a transformer though.

Any comments or thoughts?

Has the fuse likely protected the windings at all?

Alternatively what is the probability I can rewind something like this myself? Taps are 840v, 6.3v, 250v, 24v I think.

[Nuvistor]

At this stage it's all guess work, it could even be the TX that is faulty and blew the fuse. Do you know how well the PSU is protected with fuses? Lightly fused primary and secondary windings and you could be in with a chance.

Not a lot of help, is it possible the seller could do some fault finding?

Frank

[MrBungle]

Just primary is fused. Secondaries have series limiting resistors apart from on one winding which I hope would sink until the fuses goes.

Seller doesn't even know what it is so no help there.

If I'm honest they make an appearance occasionally on eBay so may be worth it and turn two into one good one. The entire unit is very clean.

[jonnybear]

Have you got a ring tester, this would tell you if windings have failed, I would disconnect all the secondarys and place a 60 watt bulb across fuse in the primary circuit and see what happens,Mains transformers when they fail is usually O/C primary or they have been cooked by s/c secondary this you can smell or see the damage.
John

[DAC10 Collector]

A defective mains filter capacitor (if fitted) may be another possibility.

Mike

[MrBungle]

No mains filter capacitor on this by the looks. I don't have a ring tester - usually desolder the transformer winding use a DMM on low ohms and that's it.

[Radio Wrangler]

Dead rectifier diodes also happen quite frequently.

Fuse blowing might be telling you that the windings haven't gone open circuit.

I've seen more open circuit transformers than dead shorts.

David

[MrBungle]

Good point and interesting way of thinking about it.

Thanks everyone.

First hurdle sorted. Next hurdle SWMBO

[Boater Sam]

I would consider how robust the windings may be. Are they all a reasonable current, fairly thick wire, or is there a high voltage low current winding using tiny wire?

If the former, I would think that it stands a very good chance of being OK.
Consider too that fuses do age, inrush current on the transformer may just have opened it rather than popped it.

In a conventional transformer fed power supply there are lots of things that can go s/c without melting the transformer, I would be more worried if the fuse had not gone and the smoke had got out,

Has the owner replaced the fuse and tried again? If so, did he use the correct fuse? Or was he outstandingly sensible and left it alone? I would doubt that, most folk would try another fuse before selling it off on the off chance that it may work, the 2BA bolt may have been fitted, ask for an honest answer.

[MrBungle]

I know that the owner has not replaced the fuse. In fact I was told that they had powered it up and the CRT had come on for around 20 seconds and then the pilot light went out and they left it. No nasty smells or anything confirmed.

As for robsutness, entirely unknown.

[G6Tanuki]

I've had instances of a particular bit of gear where blowing of the mains-fuse simply showed that the over-voltage crowbar circuit in the voltage-regulator was doing its job (a failed-open Zener in the regulator being the culprit)

[MrBungle]

These are dumb zener/transistor regulators. Nothing that fancy.

[trh01uk]

Originally Posted by MrBungle

I've got my eye on something that was working but suddenly blew the fuse but I'm slightly worried that the transformer is toast and am trading up the risks of a £30 expenditure and 3 hour round trip. The device's transformer is irreplaceable in this case as it has approx 1400v isolation between the heater winding and other taps as it has CRT bias. The rest of the device is 100% generic parts. I haven't seen any companies guaranteeing that for custom rewinds and I'm not sure it's actually worth rewinding or getting a new transformer made for. I may budge on that if it's not too much hassle; cost isn't really a problem as it's hard to justify not spending money when I do this regularly on technically pointless things

The guy I work with regularly winds transformers like this. I am just testing one which produces 1300V at 200mA. Insulation isn't a problem provided you know what you are doing and have the right materials.

If you need a new transformer at some point just PM me.

Richard

[MrBungle]

Glad to know.

Have organised collection on Sunday. Fingers crossed

[CambridgeWorks]

I recently had a Racal 9301A rf millivoltmeter I disposed of that suddenly decided to blow the mains fuse (160mA) in a couple of seconds from switch on. Turned out it was a s/c tantalum, 47uF 25V!
Rob