GEC BRT400 - suspected duff mains transformer

[cmjones01]

I'm trying to bring this behemoth of a receiver back from what seems to have been a watery grave. Having removed the worst of the mouse droppings and dead spiders, I did some basic safety and continuity checks to make sure there were no obvious disasters waiting to happen, removed all the valves and applied mains via a 100W bulb. The bulb lit and stayed lit at nearly full brightness.

The bulb was dropping about 190V, leaving about 50V across the set. Wishfully thinking that perhaps the enormous mains transformer needed a really hefty magnetising current, I tentatively tried full mains armed with the voltmeter probe in one hand and the other on the 'off' switch. Oh, and the other hand in my pocket, you understand.

Result? The set draws about 1.6A from the mains (that can't be right), the mains transformer gets warm and makes ominous fizzing noises. Meanwhile its outputs seem nearly perfect: the dial lights all work, there's 5V on the rectifier heaters, 6.3V everywhere else, and 290-0-280V on the rectifier's anodes.

This is with all the valves removed (including the rectifier) and even the metal rectifier which generates the -120V rail disconnected.

In my dreams, someone will tell me that there's a perfectly simple and easily-fixed explanation for the heavy current draw, heat and fizzing. In reality, I need a new mains transformer, don't I?

Chris

[Colinaps]

Hi Chris,

Which model do you have?
The book quotes a power consumption of something like 80 or 90W as I recall, so if everything really is disconnected, you're probably right. Mind. if you feel the heat generated by a BRT400 in action, the figures are hard to believe - the lids probably all acquired that concave appearance because of hypothermic erks warming their posteriors on them in chilly nissen huts.
Simply unplugging the facilities plug on the back should isolate everything form the transformer if the wiring hasn't been bodged.

I had contemplated using a transformer from a scrap RA17 with a small 240/12 volt transformer wired as a step-up from a spare 6.3v winding to generate the -120v, if mine were to fail.

[cmjones01]

It's a BRT400B, and it's the early version full of Loctal valves (which have the unexpected advantage that they're really easy to spot in boxes of mixed valves!). I did try to remove the facilities plug but the screws on its cover have rusted solid. In any case, the heater wiring has been modified to bypass the plug , which is a little odd because the rusted-up plug still seems to connect the HT wiring OK.

A little Googling reveals that a suitable new replacement transformer, though not original, wouldn't be too hard to come by. I've seen one advertised as suitable for stereo Mullard 5-10 amplifiers which has 300-0-300 at 250mA, 6.3-0-6.3 at 6A or so, plus a 5V winding for the rectifier. The -120V could come from either a (shock!) silicon rectifier and zener diode, or the primary winding of a small mains transformer used as an autotransformer from half the HT secondary. It only needs to supply a couple of milliamps as far as I can see.

I'm not too fussed about the loss of originality (someone's already replaced the output valve with a 6L6, for example) but if anyone has a BRT400 mains transformer lying under the bench, I'd be interested...

Chris

[G8HQP Dave]

In my dreams, someone will tell me that there's a perfectly simple and easily-fixed explanation for the heavy current draw, heat and fizzing. In reality, I need a new mains transformer, don't I?

It sounds like a shorted turn (i.e. insulation break-down) in the transformer primary (or possibly the inter-winding screen). If you left it switched on you would probably see the blue smoke escaping from the transformer, and then it would stop working altogether - don't do this at home! Unfortunately the primary is often the innermost winding, so rewinding it would involve stripping the lot off and starting from scratch. Probably not worth it.

[Station X]

You talk of a "watery grave" is it possible that water has got into the transformer? If so keeping it in a warm place for a few weeks may dry it out.

[cmjones01]

Yes, the set's certainly been damp at some point in the past judging by the amount of rust on most of the bare steel surfaces. As far as I know it's been dry stored for several weeks in its more recent history, and I haven't found any moisture in it while cleaning and poking around.

The transformer itself has very rusty laminations and looks like it was originally coated with black pitch (or something similar) which has dried up and cracked off. Lumps are still falling off.

Shorted turns are my prime suspect, too. The 400W (!) going in from the mains is definitely coming out of the transformer as heat judging by the temperature it got to after last night's brief experiments. Nothing else seems to be getting hot.

It looks like it's time to start getting inventive with replacement ideas...

[Station X]

Have you measured the DC resistance of the primary? The reading should be low, but not approaching a short circuit. Ohms Law doesn't apply directly here, as it's the inductance of the winding that governs the current drawn from the mains.

My DMM has an inductance range and I've often wondered how the inductance of the primary winding of a good transformer would compare with that of one with shorted turns.

[cmjones01]

The primary (set for 240V) measures 6.6 ohms on both my Fluke 12 and Avo 8. I have no direct means of measuring inductance here, but the cheapo plug-in kWh meter I'd plugged the receiver into shows a power factor of 0.99 and current of 1.6A (ish). That doesn't seem very inductive to me! It's consistent with a transformer delivering power into a resistive load, though.

[Station X]

If the load were purely resistive the current would be given by 240/6.6 which is about 36A, so there must be some inductance there.

When set for 240V the entire winding will be used. You could try a lower voltage setting and maybe prove the fault to a particular portion of the winding,

[ukcol]

Originally Posted by Station X

When set for 240V the entire winding will be used. You could try a lower voltage setting and maybe prove the fault to a particular portion of the winding,

I am very reluctant to question Station X, but surely this method would not prove anything.

If, for example, the fault was in the primary in part of the winding between the 220volt tapping and the 240 volt tapping; by running the transformer on 220 volts you would still get current flowing in the shorted turn(s) in the 220 -240v part of the winding due to transformer action. This fault current would still be "seen" at the good part of the primary as over current and reduced inductance, or am I missing something?

[Station X]

You are quite right. It would prove nothing. I was wrong

I've just measured the primary of an AR88 Mains transformer at 8.4 ohms including 2 metres of mains lead.

Info on testing for shorted turns here:-

https://www.vintage-radio.net/forum/...ad.php?t=14317

[cmjones01]

I think we're differing in our definitions of 'resistive'.

Consider a simple 240V:240V mains isolation transformer with a resistive 500W load, say a toaster, connected to its secondary winding. The primary winding might have a DC resistance of, say, 10 ohms, but when connected to a 240V AC supply it would draw 500W, 2.1A - an impedance of about 115 ohms, but with a power factor of 1, assuming a perfect transformer.

I think that's analogous to what I'm seeing. Replace the toaster with shorted turns (crucially, it doesn't matter which winding they're in, as long as they're in the magnetic field). Actually, maybe this transformer would make a good toaster...

[Station X]

Quite right, but I was assuming that nothing was connected to the secondary. If the primary is in good order, then when connected to an AC supply the current will be limited by the inductance, or more correctly by the vector sum of inductance and resistance. If there are shorted turns then the inductance drops to a low value and the current depends more on the resistance.

[cmjones01]

Agreed. Shorted turns are really just a secondary winding with a short-circuit across it, and have all the corresponding effects.

[Station X]

I just did some sums. With 240V, 1.6A and 6.6R the inductance will be about 0.48H. The AR88 transformer has an inductance of 0.52H.

[Station X]

Thinking about this a SINGLE short circuited turn in the primary winding will make little difference to its resistance, so measuring the resistance of the primary and comparing it with that of a good transformer of the same type isn't much of a test. It would be different if several turns were short circuit.

In fact the resistance is small when compared with the inductive reactance and can almost be ignored when calculating the primary current with the secondary open circuit.

So it comes down to saying "this transformer gets hot when off load, so it must have shorted turns", or using some kind of inductance measurement to detect shorted turns.

[Skywave]

Originally Posted by cmjones01

The set draws about 1.6A from the mains, the mains transformer gets warm and makes ominous fizzing noises. Meanwhile its outputs seem nearly perfect: the dial lights all work, there's 5V on the rectifier heaters, 6.3V everywhere else, and 290-0-280V on the rectifier's anodes.

This is with all the valves removed (including the rectifier) and even the metal rectifier which generates the -120V rail disconnected.

Chris

So, let's re-cap on that.

In effect, in the test above, the only load that you had on the transformer was the dial lights? (All the valves were removed & the metal rect.). But the set insisted on drawing 1.6 A from 240-v a.c. mains. And made fizzing noises. And got warm.

Sounds to me like the transformer is kaput; where is that 1.6 A going? It's got to be powering something; it's far too much for the dial lights alone or the mag. current.

I suggest that you remove the transformer from the set, run it on no-load from a suitably fused supply - say a 2A fuse - and put it on 'soak test', keeping a close eye on its temperature rise. It'll soon tell you if it's unhappy.

Al. / Skywave.

[cmjones01]

Yes, indeed - I think that'll be my next move, when I've drawn a map of where the multitudinous wires on the transformer's tagstrip should go.

Thank you to all for the advice and inspiration.

[Ed_Dinning]

Hi Chris, a long shot when you get the tranny out, but try it with the screen isolated.
The other thing to look at is the lam securing bolts. These usually have insulating washers under their heads. Without them it is possible to get the effect of a shorted turn. Try taking them out and see if it makes any difference.
Failing that its a re-wind.

Ed

[glowinganode]

Originally Posted by Ed_Dinning

Hi Chris, a long shot when you get the tranny out, but try it with the screen isolated.

Not such a long shot, I had it recently on a second hand item. Tested the transformer on the (wooden) bench ok, without an earth connection. Fitted the transformer, only to find the HT winding had a short to the e/s screen. Unfortunately I had no choice but to scrap the tx.
I like the idea of bolts causing a s/c turn, though I doubt rusty steel bolts would have a low enough contact resistance to cause enough current to flow to dissipate 400W. Assuming 5 turns per volt, you would have a total of 2000A flowing.
Almost certainly a shorted turn to screen/frame/turn. Which ever way it's going to need a rewind or replacement, bit like the 300KVA generator alternator that went bang at work today. Quite scary watching 6 tonne of generator hop, bang and smoke across the yard!
Rob.