Philips A460A Is my mains transformer dead?

[Wishiknewmore]

Hello
I have just got a Philips A460 A 1946, with a Bakelite chassis and side contact valves.
The valve line up is: ECH3,EBF2,EF9,EBL3,EM34 and AZ1
It is a French set and is in France, which may mean that at some point it has been set at 110v and connected to 230v
I gave it a quick try by connecting it up with a lamp limiter, but the lamp was too bright for my liking, I removed the rectifier AZ1, still the same, removed all the valves, still the same, I then connected my DMM to check what the actual load was, which was 0.55amps, but also there was the start of bubbling from between the paper layers on the transformer windings.

Am I safe to assume that the mains transformer is ”cooked”?

Did not try anything else at the moment,as thought I may just cause further damage.

https://www.doctsf.com/grandlivre/fi...004&variante=1

https://www.radiomuseum.org/r/philips_a460aa_460.html

Any thoughts, suggestion or comments?
Many thanks

Philip

[Boater Sam]

You can only condemn the transformer if it still overheats with ALL the secondaries disconnected and all the unused input voltage taps disconnected too.
In other words ths correct mains voltage only connected to the correct primary connections and nothing else.
A short on any winding or the interwinding shield will cause overheating as will a single shorted turn in any winding.

[Wishiknewmore]

Thanks for your prompt reply

I perhaps naively thought that with the rectifier out, valves out and dial bulbs lit I had covered that,but I suppose this would not cover any wiring shorts

Would it be the wise thing to do to unsolder all the secondary connections at the transformer?

Philip

[Herald1360]

Makes sense. That way you're guaranteed to be checking just the transformer. Eliminates the possibility of condemning the transformer unnecessarily.

[Wishiknewmore]

I disconnected all the secondary's, still bright lamp limiter, any way connected up to full power and took voltages

Rectifier heater winding 4.16v
Valve and bulb heaters 6.51v
Main secondary 216v and 283v end to end 502v

But in as long as it took me to test the voltages, the transformer was bubleing
and measured 50 centigrade against the core.

I suspect this is not good news.......

Philip

[TonyDuell]

The thing that most worries me about those measurements is the difference between the 2 halves of the HT winding (216V and 283V). They should be very nearly equal.

I therefore suspect shorted turns in the '216V' part, which would account for the transformer overheating.

Time for a rewind I guess...

[TrevorG3VLF]

There was a thread with a similar problem recently. The faulty winding was the outer winding and this was chopped off and the good winding was reconnected with a couple of diodes to make a bridge rectifier.
I wonder how reliable the transformer would be even if this modification worked.

[Ed_Dinning]

Hi Philip, there is lots of advice about transformer testing on the forum, if you get stuck you can PM me and bring it over for testing.

Ed

[kevinaston1]

But in as long as it took me to test the voltages, the transformer was bubleing
and measured 50 centigrade against the core.?


In order to measure the core temperature, you would have to destroy the transformer by cutting it open (without generating heat). How did you come about the measurement of 50 degrees?

Kevin

[Wishiknewmore]

Perhaps core was the wrong word, I suppose I meant the windings part of the transformer rather than the laminations. I used the thermocouple that came with the multi meter.

Philip

[Chris55000]

Hi!

A substantial difference like that across the outer–ends of the H.T. secondary winding is enough to condemn the transformer, Quarrington in the fault–finding chapters of "Radio & Television", Vol. 3, makes it clear that the difference between the voltages at the winding–ends should not be more than a volt or two, (allowing for winding tolerances – look at a few Murphy Circuit Diagrams for examples) – me thinks you need to have a chat with Ed about rewinding I'm afraid!

Chris Williams

[julie_m]

If it's the outer part of your transformer windings that is faulty, you might just have dodged a bullet. The intact half of the winding can be used in a modified power supply arrangement using a bridge rectifier, formed out of the existing rectifier valve and two new silicon rectifier diodes such as 1N4007.

[Chris55000]

Hi!

If it's the outer part of your transformer windings that is faulty, you might just have dodged a bullet. The intact half of the winding can be used in a modified power supply arrangement using a bridge rectifier, formed out of the existing rectifier valve and two new silicon rectifier diodes such as 1N4007.

I have tried this myself, but it only works if the damaged half of the H.T. secondary is completely removed from the transformer - any of the shorted turns remaining, even if no load is connected to the faulty section, will still overheat and potentially damage the remainder of the transformer - Gordon King specifically refers to this in his 1970 "Radio & Audio Servicing Handbook."

It is much safer to either get another one that will physically fit the chassis, or have a chat to Ed Dinning about rewinding.

Chris Williams

[Skywave]

There is always the chance that someone here will have a suitable replacement. So a photo., dimensions and volts / current requirements will be needed.

HTH,

Al.

[Ed_Dinning]

Hi Philip, I'm now back from holiday. Send me a PM and you can bring it over.

Ed