Philips radio B310A- O/P tarfo

Hello Forum,
I am restoring a Philips radio type BX310A from 1951/-52. It is generally in quite good shape but has one painful fault; namely internal arcing (= random short circuit) between primary/secondary winding -S23- S24 in the audio output transformer. Trafo's Philips code is A3 152 18.0, and iron core (E+I) size is 40*37*16mm3. Output tube is EL 41 and O/P power 2-3W? The trafo is compact and impregnated with " black tar " so very difficult to open and rewind.
This fault had very interesting symptoms and not so easy to discover. When I smoothly increased AC input voltage by Variac, so around 150V became random "crackle" audible. At the same time one or more arcs happened on tube base socket from anode pin to ground (about 5mm) also a few arcs were visible inside of the tube, especially with higher AC input. These phenomena continued with random rate. Following the anode voltage with oscilloscope showed that the voltage dropped sharply during "crackles". I changed several components and also EL41 + socket at O/P stage without success.
So my final conclusion was a high resistant short circuit between the primary and secondary windings inside of the trafo. One leg of secondary is grounded. Attached drawing shows how I think this phenomenon happens. I also tested the trafo separately with DC voltage and the "crackle " repeated. Normal ohmmeter did not indicate leaking between windings.

Does the arcing happen due to "Spark Coil" effect just like in cars??

Can somebody to advice me are there any means to repair the trafo? Due to trafo construction, rewinding of it look very difficult because of the primary section seems to be top layer. What about availability of adequate replacement trafo (new or surplus)

Best regards
Ake Nyholm
Attachments:

• Ph310_audiotrafo-small (149 KB)
• EL41 anodipiiri (18 KB)

To thank the Author because you find the post helpful or well done.

Dear Ake
Just by chance - looking to the model - I saw your request which has "gone under" without reply. You are probably right with the effect of a sparc coil.

If the defect in reality is in the transformer you better change it because there will be many of this type in old chassis where the case is too bad to restore the set.

With this short answer the thread will come up again and I have asked Konrad Birkner if he can analyse a bit more because I'm in a hurry. 

To thank the Author because you find the post helpful or well done.

Hi Ake,

Well, You did already a good job finding the culprit: output transformer.


Arcing happens of course like in a spark coil: a current built up a magnetic field. If the current is cut off by whatsoever means (open or short), the changing magnetic field will induce a voltage opposed to the previous one. The steeper the slope, the higher the voltage (differential function).

Testing the Xformer separately: was the secondary still connected to the iron core?
It might probably be an  insulation fault to the core=ground. If so, one might try to insulate the core from chassis and only ground the secondary as a makeshift remedy.
If the arcing happens between primary and secondary there is no way out.
I think there is no way to repair it. You could only verify the findings by an insulation test, e.g. using the ISOTEST instrument (which is my best recommendation for a problem like Yours).

Good luck!

KoBi

To thank the Author because you find the post helpful or well done.

Hello KoBi,

Thank you of your comments and advices. I was lucky to find a suitable replacement O/P trafo from one of my colleque's surplus shelf. After change of o/P the B310A works well. I'll still like to check the condition of tubes and may be also IF /HF tuning before final "polishing".

IsoTest6 is very nice instument. I ordered one kit from Mr. Heigel and buit it.
It was a good hint to test the old o/P trafo with it too.

Thank you for your support in solving this extraordinary fault.

Best regards
Ake

To thank the Author because you find the post helpful or well done.

Congratulations, Ake!

I am only curious whether the internal insulation breakdown happened to the secondary or to the iron core. Is it possible to find out?

Kind regards
KoBi

To thank the Author because you find the post helpful or well done.

Hello,
Breakdown happend between the primary and sencondary windings. Because of one end of secondary is conneted to ground, there was discharge path available. I haven't break down the faulty trafo yet but i'll do it just for curiosity.

Besr regards
Ake

To thank the Author because you find the post helpful or well done.