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Old 24th Jun 2018, 8:09 am   #21
Joe Ingle
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

I've got in to dismantling the TX and am making progress. It's T and U shaped lams. I can't budge any of the T lams - they're wedged into the core and there's no way I can move them. However, if I remove all the U lams then I can at least access the coils, and I'm in the process of manually unwinding the suspect core. It's a time consuming process, but I'm getting there.

Ed's estimate of 1000 turns seems likely (roughly). In terms of how thick the wire is, all I can say is that it feels thin (but I've never unwound a TX before, so I don't have anythng to compare it with!) Going for Ed's estimate of 0.25mm, I've found this on Amazon - does it look like the right stuff (I saw one of the users had used it for making coils...) I notice it doesn't seem to give a length, but I'd imagine 500g is enough for several TXs...

https://www.amazon.co.uk/ENAMELLED-C.../dp/B016QLWBGW

Joe

PS - thanks to Mike for the possible offer of a spare chassis (and sorry to hear about his wife). I'll see how I get on with the rewind first though.
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Old 24th Jun 2018, 7:44 pm   #22
Freya
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

You could always post a bit of the wire for someone to measure, I'm sure someone could help in that respect if need be.
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Old 24th Jun 2018, 8:14 pm   #23
vidjoman
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

If the wire gauge is wrong then, too thin and it’ll burn out or too thick and you won’t get the number of turns on. Prefer to leave it to the guys who know what to do rather than guess.
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Old 24th Jun 2018, 9:17 pm   #24
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Take a bit of the wire to the local technical college and ask the engineering guy to mike it for you, you need an accurate diameter measurement, unless you have access to a micrometer.
Publish it here, we need to calculate enamel thickness to find the actual wire size in SWG. You cannot guess it.
100gm should be plenty, weigh the old stuff.
If the original is interleaved with paper you need to do the same, I use PTFE tape, it's thin and easy.
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Old 25th Jun 2018, 10:02 am   #25
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

The diameter won't help much as it is dependent on the insulation thickness. You work out the gauge by measuring the resistance of a known length. There are copper wire resistance charts on the internet. You know the number of turns so you just need to multiply that by the average winding circumference to get the length you require.
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Old 6th Jul 2018, 1:14 pm   #26
Joe Ingle
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

I thought I'd put a concluding post here to say what I eventually did.

First, I couldn't remove the laminations properly - they were U and T shaped, alternately aligned. The centre bars of the 'T' members were all jammed through the main core of the TX, and there was no way of getting them out without doing some serious damage. However, it was easy enough to prise off the 'U' laminations, leaving the core exposed.

This meant that unwinding was possible but fiddly. The wire broke many times, but in the end I counted (about) 1270 turns.

To estimate the thickness of the wire (all I knew at this stage was that it was very thin) I built up the now empty former core with a thin sausage of Blu Tack to give a bed at about half the thickness of the coil when full. I then wound a few turns of wire onto the Blu Tack, the idea being that this would be representative of the average turn of wire on the full coil. I divided the length of wire in this test by the number of turns to get the length per (average) turn, and multiplied up by 1270 to get the length of wire on the whole coil.

Using the service data (corroborated by a measure of the surviving HT 2y) I got the resistance of the broken coil, and used it to calculate the resistance in Ohms / km of the wire.

Using one of the copper wire resistance charts mentioned by PJL above, it came out as .16mm, so I bought 125g of .16mm 'winding wire' from the Scientific Wire Company. I'd posted a sample to Ed Dinning, who kindly measured it and he came out with the same value, so my calcs must have been good.

The wire arrived, I wound it back on using PTFE as an insulator on the inside and outside of the coil, put everything back together and it worked (the radio runs, the lamp limiter remains very dim), and stays that way.

So thanks everyone for their help and suggestions - I'd definitely not have been able to do this without them.

Joe

Last edited by Joe Ingle; 6th Jul 2018 at 1:15 pm. Reason: Edited to remove stray text...
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Old 7th Jul 2018, 3:16 pm   #27
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?



Persistence admired!
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Old 9th Jul 2018, 9:28 am   #28
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

My sentiments exactly. That's the way to do it. Well done!
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Old 9th Jul 2018, 2:53 pm   #29
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Yes well done Joe. You got on with the job rather than just talking about it.
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