Suspect Mains Transformer in 1930s Radio P.S.U.

['LIVEWIRE?']

The RCD in question protects the sockets. Appliances always plugged in, include the TV, Freeview box, a VCR/DVD Player, Blu-ray Player, Computer, Riouter, Printer & Monitor, and a Hi-Fi System. All are switched off when not in use, except the Freeview box, which, as is usual with these devices, is left in standby, but none have ever caused the R.C.D. to trip. Other items left plugged in, include a Microwave Oven and Kettle, but these are switched off when not in use. I can obviously disconnect each appliance in turn to eliminate them, though I still feel there is a problem with the 80+ year old transformer, since the R.C.D. has never tripped before.

[teetoon]

Hi Livewire, Have you used the lamp limiter on anything else as I noticed you said it was built recently and that you had used a new 13 amp socket.
Could the socket be wired wrong or even the cable with the plug on be wired wrong as it sounds like a N/E reversal fault.
Forgive me if I'm wrong but otherwise there is no reason the RCD should be tripping.

David.

[crackle]

It has been mentioned that earth leakage faults are accumulative, many devices all leaking 1mA can eventually combine to cause an RCD to trip.
Also the fault can happen with leakage from either the live or the neutral to earth. Therefore to carry out earth tests you have to unplug all the devices on the circuit not simply switch them off.
We should not forget that electricity can also discharge through an apparent insulator to a small extent, and can appear to discharge into the atmosphere. Hence neon test screwdrivers glow even when you are wearing insulated shoes.
As the test lamp circuit wiring and components have all been double checked and as there is no earth connected to the equipment under test, then something else is happening to upset the RCD.
I dont confess to be expert on this but I would be looking to see if it could be a back emf caused by shorted turns in the transformer secondaries. Or additional voltage fed back onto one leg of the mains to cause an imbalance between the live and neutral.
I dont know what the leakage tester is that you used, is it a high voltage type, things may react differently under high voltages to low transistor type test equipment.
Have you checked your insulation tester probes and leads.
Mike

['LIVEWIRE?']

The Insulation Tester I use is a TMK TEM1555B, which I bought at the NVCF a couple of years ago. In the Megohm setting it applies 500v to the item under test, displaying the result on an analogue meter. As previously noted, when testing the transformer, readings show as >200Megohms (rising to almost infinity after a second or two). As this, admittedly old, instrument was designed as an insulation tester, and is in good working order, I'm inclined to accept it's readings. Regarding the lamp limiter, I built and wired it, and have re-checked my wiring. It may get re-built into a wooden or plastic box if and when I can find the time, but although it's presently on a wooden baseboard, it is correctly wired. The probes & leads are new Cat.III ones which I bought from CPC maybe a year ago. Each is fused @ 500mA. I mentioned all this in a thread about the TEM1555B a while back.

[cathoderay57]

Hi Livewire, the only things I can think of left to do are:
1. Replace your RCD with another one and see if the nuisance trip persists.
2. Purchase a 1:1 mains isolation transformer. You might find one cheaper than getting the vintage transformer in the set rewound. If the RCD still trips with an isolation transformer in circuit then move house!
Cheers, Jerry

[Herald1360]

Mildmartin's comment about asymmetric waveforms on top of other residual leakage from equipment on the same circuit sounds promising. The inrush current to a transformer primary at switch on is very asymmetric- huge spikes on top of one side of the sine as the core saturates until the the magnetic circuit settles down after several trips round the hysteresis curve!

It would be well worth connecting this setup, via a plug in dedicated RCD, to an unprotected feed. That will establish whether it's the set and its limiter combo or simply too many small leakages that is the problem.

FWIW, though, the lamp limiter will act as a reasonable "soft start" resistor to mitigate inrush current spikes down to maybe 10x FLC from possibly 100x or more.

['LIVEWIRE?']

Mea Culpa!!Despite checking my wiring two or thee times, I realized about half an hour ago, that, when fitting/wiring the switched mains socket to my Lamp Limiter, I had inadvertently wired the Neutral and earth leads from the 13A Plug the wrong way round!! i.e. Neutral to the Earth pin, and vice-versa. Must be getting old!!! There is still afault with the radio, though. When connected & powered, with only the rectifier and Pilot lamp connected, the lamp lights, but the rectifier isn't conducting. This is a N.O.S. 1821, which I bought about a year ago. The heater was O.K., when I metered it yesterday, so I now need to check the wiring to the valve base, and the base itself.

[Refugee]

Oops....
A badly wired plug.

[HamishBoxer]

Getting there that is all that matters.

Has I have always known the worst faults are those we put on ourselves.

[GRID LEAK]

Allah be praised!!

John

[Station X]

Occam's razor springs to mind.

['LIVEWIRE?']

I'm beginning to suspect the 1821. With only the rectifier valve plugged, I'm getting ca. 240-0-24oVAC across the HT Secondary, and just over 4v on the heater winding. The heater has continuity, but there is no DC o/p. The valve doesn't warm up, even if left powered for a few minutes, so no voltage is dropped via the limiter. I'm reluctant to plug in the incorrect rectifier valve (a VU39) which was in the set when I received it, as the Ih of the 39 is 4Amps, rather than the 1Amp the 1821 draws. Not good for the transformer!

[HamishBoxer]

Might be worth Carefully resoldering the pins on the rectifier valve.

[julie_m]

I guess the lesson we can all learn from this is, even if you're really convinced something is not the problem, check it again anyway.

Is this a case of that System #1 / System #2 thing someone mentioned earlier rearing its ugly head again? I guess human beings are just generally lousy judges of probabilities. Which is why some bookmakers are still in business .....

[Boater Sam]

Use a couple of diodes and a chunky 500 ohm resistor for the rectifier to test the rest of the set? Have you a UU5? Its a direct sub and better rated.

[ms660]

The UU5 is a different type, more heater current needed (2.3 amps) on account that it's an indirectly heated job, ditto for the VU39 (2.5amps)

1821 is directly heated (1 amp) so far as I know.

Lawrence.

[Boater Sam]

Presumably that is why the UU5 is much higher rated, bigger heater demand. Big jump from 1 to 2.3 Amps, thanks Lawrence, I didn't know that.
Can't think of another sub for 1821. You could build a plug in silicon diode substitute, then you could replace with a 1821 when you find one, needs that ballast resistor though.

How about a U10? There is one on ebay.

[Cobaltblue]

If the heater has continuity maybe the pins/socket are too dirty to make a connection.

It's usually hard to see the filaments on these 1821's since the whole valve is usually metalised.

Sometime visable in low light.

I would be surprised if the filament is lighting that there is absolutely no output especially "off load"

Cheers

Mike T

[Refugee]

I would power the heater at the correct voltage and test the warm valve with a diode tester.

['LIVEWIRE?']

Mike, the 1821 I have isn't metalized. Anyway I cautiously tried the VU39. Still no sign of life, so it could be poor contacts due to dirty valve sockets. I may make up a double diode silicon rectifier and try that out ( I was going to fit a solid state rectifier in the first place)