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Old 20th Jun 2018, 12:21 pm   #1
Joe Ingle
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Default Murphy A30 (c. 1935) mains transformer overheating?

Hi all,

I’ve got a Murphy A30, which I re-capped about 5 years ago and have been using fairly regularly ever since, with no problems.

A couple of weeks ago I was using it for a couple of hours at a stretch (which I have done before) when it started to make a burning smell, so I switched it off. When I powered it back up again on a lamp limiter (100 Watt bulb) the bulb came on instantly, fairly brightly.

Since then I’ve disconnected all the secondary windings, and still the light bulb glows immediately, fairly brightly. I’ve not tried running the set with the valves in, so I don’t know if it still ‘works’. There’s a bit of wax run out of the transformer too.

I’ve verified that all the windings (primary and secondary) are insulated from the TX body and chassis (using a resistance meter on R*10K). The DC resistance of the primary seems a little low (c. 30 Ohm), but according to the service data this is more or less right. The DC resistance of the HT secondaries seems OK (about 500 Ohm each), but the three heater secondaries have resistance too low to measure on my meter (I’d think this is OK).

My guess is that this strongly suggests that there’s an internal short in the transformer. My plan is to remove it completely from the radio so I know everything is disconnected, and then reconnect the primary to the mains by my limiter, and if it still lights up then that is conclusive, and rewinding / replacing is the only option (I’ll ask Ed Dinning, he’s helped me before)

Does this sound right? I don’t want to condemn the transformer unnecessarily. Could there be any other explanation for the lamp coming on so brightly? (I’ve tested it with my 30 Watt soldering iron and it doesn’t glow at all).

If I do get a new / repaired transformer, I’ll need to be careful to make sure there isn’t an underlying fault which caused the problem in the first place.

Thanks for any advice,

Joe
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Old 20th Jun 2018, 12:32 pm   #2
ColinB
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

No mains filter caps that may have shorted, or be leaking, across the line or to ground? No broken down wire insulation around the transformer?

Maybe worth checking from the secondaries down to ground before replacing the TX, or even wire in a spare one as a test.
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Old 20th Jun 2018, 12:40 pm   #3
Joe Ingle
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

I was wondering about those very points, that's why I thought the next stage is complete removal of the TX.

Unfortunately I don't have a spare... I think these TXs with 4v secondaries can be hard to come by.
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Old 20th Jun 2018, 12:58 pm   #4
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Looking at the diagram, there's nothing wired in the primary circuit apart from the mains switch, which has a cap wired across it, and the voltage selector.

Unless there's a short between the two wires in the mains lead, or it's shorting to the chassis, I'd say the transformer is toast. There is a remote possiblity that the OF/ON switch could be shorting to earth.

To test for these sort of things you really need a Megger as the battery in a DMM may not break down any insulation resistance.
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Old 20th Jun 2018, 1:20 pm   #5
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Worst scenario let Ed Dinning rewind.
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Old 20th Jun 2018, 3:04 pm   #6
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Hi Joe. Try running it with a lamp limiter won to Pri wires with all others disconnected.
Measure volts across Pri, which may only be a few 10's of volts.
Whatever it is use this (against 240v) as a ratio for the actual volts that you should see on the other windings.
Heaters should normally show the correct figure, but one side of the HT may be lower (and that will be the side with the short).
If all read ok then it is a Pri fault.

PM/email me the detail and picture and I'll see what can be done; I may even have a used tested transformer in my collection.

Ed
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Old 20th Jun 2018, 5:00 pm   #7
Peter.N.
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Try running it with the HT winding disconnected, if it still gets hot its either an HT winding short or the primary.

Peter
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Old 20th Jun 2018, 7:39 pm   #8
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

If there is an earthed interwinding screen, try disconnecting it. I've had windings go down to it before and with it dissed the transformer was usable.
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Old 20th Jun 2018, 8:15 pm   #9
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Try measuring the 2 anode voltages of the HT from the rectifier valve base to chassis with the rectifier removed. Measure both on AC obviously and both should be a similar voltage. If one is much less than this indicates a shorted turn on one of the HT secondary windings.

My A30C had the same fault. when the chassis was removed it was quite clear which one had failed as not only was the HT was low but the tape covering the windings on that bobbin had gone a darker colour.

Luckily the transformer can be dismantled once the 2BA bolts in each corner have been removed. The laminations slide out and the bobbins can then be removed.

If I can remember correctly there are 3 sets of bobbins on the transformer. The centre one being the primary (mains) winding and the other 2 bobbins sandwiching the primary winding bobbin, each having one HT winding plus a heater winding.

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Old 20th Jun 2018, 8:36 pm   #10
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

This is quite spooky my A30 is having exactly the same problem and I was contemplating having the transformer re-wound.
In fact it was today I spotted Mike Barkers service advertised in the latest Bulletin and got me thinking about it again.
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Old 21st Jun 2018, 9:16 pm   #11
Joe Ingle
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Thanks for all the replies, and I think I've homed in on the problem. The TX is now off the radio, all secondaries disconnected (including seperating the two HT secondaries which are connected together at ground).

I connected the mains through a lamp limiter and got the following:
Voltage on primary - 150v
Voltage on 'upper' HT secondary - 125, DC resistance c.275 Ohm
Voltage on 'lower' HT secondary - 200, DC resistance c.450 Ohm
All three of the Heater coils read around 2v, DC resistance too low to measure.

So it would seem to be the 'upper' HT secondary that's at fault, and I guess the only way forward is to get it re-wound, or replace the whole TX. I'll get in contact with Ed and take him up on his offer.

Only other issue is that I might want to undo the transformer myself to remove the faulty winding. Christopher (above) suggested that the laminations come apart easily - from what I've seen they're interlaced, and don't seem to want to come apart without bending. I didn't proceed!

I've attached a photo for the record!
Joe
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Old 21st Jun 2018, 9:47 pm   #12
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

The section next to the tagboard looks cooked.

The lams don't look fully interleaved, just in batches, so undoing the clamp screws and carefully tapping the outer set with a drift should start things moving.

Nothing to lose by trying, one way or another it has to come apart unless a compatible replacement can be found.
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Old 21st Jun 2018, 10:12 pm   #13
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Yours has failed exactly the same as mine. The top bobbin.

Once the 4 2BA bolts are removed the laminations can be removed The first couple are tight but then the rest come out easily. They are I and T laminations.

Once the bobbin has been removed the winding can be rewound fairly easily. I made a basic coil winder to hold the bobbin and a roll of wire. I counted all the turns unwinding it noting which direction the wire was wound on to the bobbin.

This is important as if it is wrong you will have both HT windings in phase and not 180 degrees out. This would cause hum as both sides of the HT rectifier would conduct on one mains half cycle and not conduct on the other half cycle rather than each of the 2 diodes alternately taking their turn.

Once all of the old wire is removed rewinding is fairly simple. The HT winding is the outer winding with one of the heater windings closer to the core.

Christopher Capener
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Old 22nd Jun 2018, 5:22 am   #14
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Quote:
This is important as if it is wrong you will have both HT windings in phase and not 180 degrees out. This would cause hum as both sides of the HT rectifier would conduct on one mains half cycle and not conduct on the other half cycle rather than each of the 2 diodes alternately taking their turn.
You can check you've got it right by connecting an AC voltmeter between the rectifier anodes. If you measure around twice the voltage of each HT winding it's right. If you measure close to zero, you need to reverse the connections to one of the windings.
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Old 22nd Jun 2018, 4:51 pm   #15
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

I have had exactly the same problem with the console version of the A30. It worked beautifully for years and then suddenly started stinking with a really hot transformer. I never did get to the bottom of it and reluctantly put a different set in my lounge for regular use. I am watching this thread with renewed enthusiasm for tackling my Murphy, it performed so well that I would like it back in service.
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Old 22nd Jun 2018, 9:44 pm   #16
Joe Ingle
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Christopher - I'll give the unwinding / re-winding a try (Ed Dinning also gave me some helpful hints). Before I embark on it, roughly how many turns do you think it needed? Also, what sort of wire did you use? I've never bought this sort of wire before so I don't know what's required.
Thanks,
Joe
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Old 22nd Jun 2018, 10:54 pm   #17
Mike Fulton
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Hi Chaps,
I'm pretty sure I have at least one chassis in the loft for the A30, possibly 2 but unfortunately my wife is currently in a hospice and I spend all my time with her so unable to search for it at the moment.
Should anyone wish to PM I can probably supply at least one replacement transformer at cost of postage only, should hand rewinding prove to be unsuccessful. If I find myself in a position to dig out the chassis(s) before then I can update anyone who might be interested. Given the circumstances I'm likely to forget so please feel free to jog my memory, should I not respond in a timely fashion.

Cheers,
Mike
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Old 22nd Jun 2018, 11:15 pm   #18
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

i repaired an A26C with the same problem. the transformer had a small gap underneath it , the sticky systoflex-covered ht secondary wires were wedged in the gap between transformer and chassis and charred black, i could see no direct contact of wires to chassis or each other so it seems the systoflex had become significantly conductive to draw enough current to damage the transformer ? i replaced the transformer with one from a Pye scrap chassis
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Old 23rd Jun 2018, 1:51 am   #19
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

I had an A26 with the same fault and must admit to doing the bodge of removing the offending winding in situ with a screwdriver and sharp knife, disconnecting the good one from chassis and fitting a pair of 1N4007s to use it in a bridge rectifier arrangement.
It seems happy enough.
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Old 23rd Jun 2018, 3:53 pm   #20
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Default Re: Murphy A30 (c. 1935) mains transformer overheating?

Hi Joe, for rewinding I'm afraid its a reverse engineering job, count turns and mic the wire.
Typically up to 1000turns of say 0.25mm wire, but varys a great deal depending on the transformer.

Ed
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