philips: B3D03A (B 3 D 03 A); Gemma 303 Problem

Dear Radio Friends,

 While going over this set and replacing several of the filter caps, I ran into a problem.  The set blew the fuse after about 5 minutes of operations with a small bit of smoke coming from the power transformer.  I suspect a resistor or two way out of tolerance.  It was working wonderfully on all three frequency selections (UKW, LW and MW) up to this point.  Does anyone have some suggestions as to the likely suspects?

Thank you,

Bob

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

Dear Bob,

Usually a moderate overload does not cause smoke from the power transformer. A leak in the grid coupling capacitor from the EABC80 to the EL84 can cause the saturation of the power tube with an overheating. In this case you should note a distorted and low sound and voltages should drop to very low values.

The smoke from the power transformer could indicate a poor insulation between windings or from windings to the metal shell or the core. If the insulation is punctured, the carbonized paper offers a good path to leakage currents. Unfortunately in this case it is not easy to fix the problem. You should disconnect the wires from the transformer ad check the insulation of each winding to other windings and  to core with a 500V insulation meter. And you should pray that the leakage is not between the turns of the same winding.

Regards, Emilio

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

Look at the filter capacitors.
These can unsolder for the experiment.
I am sure that the insulation of the capacitor is bad.

Regards WB.

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

Looking at the diagram, I fully agree with the diagnosis of Wolfgang. Unfortunately smoke is harmful for transformers as well as for humans. If smoke came out from the transformer, some paper layers are now carbonized and lost their insulating properties. So, even if the radio works again after replacing capacitors, the tranformer is unsafe and should be replaced or winded again.

Regards, Emilio

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

Thank you so much Emilio and Wolfgang,

  The problem was indeed the two capacitors suggested by Wolfgang.  I replaced them with two new 6800 pF capacitors and checked the temperature of the transformer off and on over a 15 minute period of playing the radio.  There was no significant warmth to the touch, so perhaps I caught the initial problem before irreparable damage occurred.  Thank you both again for your thoughtful and knowledgeable advice.

Sincerely,

Bob

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