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Old 18th Apr 2018, 10:42 am   #1
'LIVEWIRE?'
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Default Suspect Mains Transformer in 1930s Radio P.S.U.

When applying power -via a lamp limiter fitted with a 100watt filament bulb - to the PSU, which is wired to the rest of the radio, but no valves other than a N.O.S. 1821 rectifier fitted the RCD protecting the sockets in my consumer unit trips immediately. Having twice checked my wiring, and also checked the insulation between the transformer primary and chassis, and also between the primary and secondary windings of said transformer with my TMK TEM-1555B Insulation tester (Getting readings of >200 Megohms in all cases, b.t.w.), I'm beginning, nonetheless, to suspect the transformer, particularly, as when I first received the radio an incorrect rectifier valve, type VU39, whose heater is rated at 4 Amps, compared to the 1 Amp current drawn by the correct 1821 rectifier, had been fitted. I did not at any time, apply power withe VU39 fitted, but obviously don't know whether anyone else had so done! Measuring the resistance of the heater windings with a DMM yields no clue, since they obviously read, at most, 1 ohm each. The HT secondary is slightly unbalances at about 250 ohms from one end to the C.T., and ca. 265 ohms from the C.T. to the other end, which could, I suppose, indicate shorted turns. Would other members agree with me that the Tx is suspect. If so, the only option may be to get it re-wound, as it's most unlikely that a suitable substitute can be sourced from anywhere. Having already spent a considerable amount of time re-capping and rewiring, where needed, the radio, I'd hate to think that it cannot be restored to working order due to a faulty mains transformer ( I know that I should have checked this first,especially because of the incorrect rectifier, before buying a new valve and other components, but hindsight is a wonderful thing!!)
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Old 18th Apr 2018, 10:49 am   #2
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

MCB or RCD?

Lawrence.
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Old 18th Apr 2018, 10:51 am   #3
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

If the breaker trips even with a 100 watt lamp limiter it suggests an insulation breakdown somewhere.
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Old 18th Apr 2018, 11:17 am   #4
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

I checked my consumer unit before posting, Lawrence. Various circuits are protected by M.C.Bs, but the one which trips is, according to the label underneath, an R.C.D., which, I agree, does suggest an insulation breakdown. Not good news! Getting that transformer rewound is likely to be expensive, to say the least!
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Old 18th Apr 2018, 11:26 am   #5
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

Presuming that the lamp limiter is wired correctly (and it seems unlikely that an experienced member would make a mistake in something so basic) then a 100 watt lamp can not trip any likely type of MCB, no matter what may be in series with the lamp.
An MCB trips on overcurrent and the lamp limits the current to under half an amp.

I would therefore suggest that it is an RCD that is tripping due to earth leakage. If this set has an earthed chassis, and leakage to this is present that would trip an RCD.
A live chassis set to which someone has erroneously fitted an earth connection would also trip an RCD.
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Old 18th Apr 2018, 11:45 am   #6
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

The first thing I wuold try, is to disconnect the earth lead to the set and see if this stops the r.c.d. tripping. Then see if there is a.c. volts between the set chassis and the mains earth with a meter set to a.c. volts. This should show up any earth leakage with a bit of luck.

John.
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Old 18th Apr 2018, 12:04 pm   #7
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

It goes without saying that you should be very careful if disconnecting the earth connection to a radio with a suspected earth leakage fault.
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Old 18th Apr 2018, 12:41 pm   #8
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

This may be a daft suggestion but have you checked the set's on/off switch for earth leakage?

Alan
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Old 18th Apr 2018, 12:57 pm   #9
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

There is no mains earth to this set, John. When it arrived there was a mains earth lead fitted, but the original diagrams don't show an earth lead, so when I rewired the pluggable mains lead. I fitted two-core flex, assuming, I hope, rightly, that, despite having a double wound mains Tx, the makers never intended it to be mains-earthed. As it is over 80 years old, I felt it better to leave it that way. Alan, I've checked the insulation with the on/off switch disconnected, and the R.C.D. still trips. The only other things I can do are (a) check it wtho no valves at all in place, & (b) completely isolate the transformer from the rest of the circuit. As to my lamp limiter, at the moment it's somewhat clumsily wired on a piece of plywood, but it is correctly wired. Must get around to building it into a box!
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Old 18th Apr 2018, 1:21 pm   #10
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

If there is no earth connected then I don't see how it can trip an RCD if wired via a lamp limiter, as all it will be seen as, even if there is a short in the radio, is a load between live and neutral.
Have you got an external RF earth connected to the radio?

Mike
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Old 18th Apr 2018, 1:41 pm   #11
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

Quote:
Originally Posted by crackle View Post
If there is no earth connected then I dont see how it can trip an RCD
Could it be something daft like a short in the mains plug?
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Old 18th Apr 2018, 2:07 pm   #12
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

Either the line or the neutral MUST be shorted/leaking to earth to trip an RCD.
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Old 18th Apr 2018, 2:10 pm   #13
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

Anything earthed in the vicinity to the chassis? or possibly the bench itself?
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Old 18th Apr 2018, 2:26 pm   #14
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

I would connect the set to the lamp limiter and test with the insulation tester between the plug pins on the limiter with the set and limiter on an insulated surface and then work from there disconnecting things until the fault clears.
That will pick up all faults including silly mistakes.
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Old 18th Apr 2018, 6:17 pm   #15
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

The unbalance in resistances of the two halves of the HT winding is quite normal.

Even with identical numbers of turns, the first half to be wound will have a shorter average turn length. The half put on top will thus use more wire, and have slightly greater resistance.

There may still be a fault, of course, but this degree of mismatch is to be expected. Don't panic just yet!
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Old 18th Apr 2018, 9:18 pm   #16
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

Hi Livewire, you say you've recapped thus set. Are you absolutely certain that there are no leaky mains filter capacitors connected to live, neutral, chassis or a combo of all three? I must agree with previous comments that if you have no path between chassis and mains earth using a 2-wire supply, it is puzzling to understand why any form of mains circuit protection device us tripping off. Maybe you are getting a voltage spike at power-up? I did have a similar issue recently with a Pye QPAC mains radio after the mains transformer ha been rewound. I had fitted new suppressor capacitors between live and earth and between neutral and earth. Sometimes when switching off the set the mains earth leakage breaker would trip. I think I have mitigated the problem by rewiring such that there is only one suppressor between live and neutral. It hasn't tripped the breaker again (yet). Cheers, Jerry
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Old 18th Apr 2018, 9:18 pm   #17
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

I would also disconnect the earth from the lamp limiter board and also test the lamp limiter with a known-good radio.
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Old 18th Apr 2018, 9:29 pm   #18
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

Hi Livewire, you say you have recapped this set but are you absolutely certain that there are no mains filter capacitors lurking in there still connected to mains live, neutral, chassis or a combo of all three? If you are running a 2-wire supply it's hard to understand how a mains protection device is tripping unless you are getting a voltage spike on power-up which is upsetting something. I had an issue recently with a mains Pye QPAC radio that had a rewound mains transformer and 2 new filter capacitors one between live and chassis and the other between neutral and chassis. This tripped the mains circuit breaker sometimes at switch-off. I mitigated the problem by rewiring such that there was only one capacitor across live and neutral. It hasn't tripped again (yet). Cheers, Jerry
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Old 18th Apr 2018, 9:56 pm   #19
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

Have you got another low voltage AC source which you can connect to the heater windings to check them. I assume as its a 30's mains set then they will be 4 volt valves.
An idea, have you tried disconnecting the centre tap of the heater winding from chassis.

Mike
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Old 18th Apr 2018, 10:09 pm   #20
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Default Re: Suspect Mains Transformer in 1930s Radio P.S.U.

Rather clutching at straws, but back to basics in a way so apologies !

Are you positive it is an RCD which is tripping ? Does it have a test button and labelling on the device itself ? If so, an RCD will trip on any imbalance not only that due to earth leakage. For example, if the current returning via neutral bypasses the RCD due to a wiring error or fault (maybe in the CU or your test setup). The RCD test button introduces such a bypass and not an earth leakage.
Perhaps check your setup by connecting another lamp in place of the set and see if you still get a trip ?
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