Zenith 25MC33 HV Trouble

[Electronic M]

Originally Posted by compucat

I disconnected the hv rect plate cap and it did arc but swapping rectifiers produced the same result, red plating. I took the cap off of the shunt regulator but that made no change either. I wonder if there is a problem with the fly back high voltage winding. When it failed, there was this loud hissing sound like I have heard before from failed flybacks and that telltale smell of burning windings. I suppose 53 years is a good run to get out of an original flyback. When my wife finds out it is not working she will be after me to get rid of it. She does not share my passion for vintage electronics, especially big items like a round tube console tv.

I can practically guarantee that the flyback is fine if you are still able to draw a healthy arc off the HV rect plate cap lead.

There is going to be some kind of short or excessive loading on the cathode end of the HV rectifier.
Open the cage up and examine things carefully. Remove the rect socket from the cup and the insulator cup from the cage, clean and examine the cup for arc through damage (I've seen insulator cups fail) follow the wiring to the HV rect socket around and look for pinholes where it could have arced through.
One of the more likely culprits is the HV rect filament winding. The filament winding typically consists of the same type of wire that the HV lead is made of wrapped loosely 1-3 turns around the frame of the flyback....Get some fresh (40KV rated) HV wire and replace it if it looks bad.

What you were hearing is probably arcing/corona from the short. What you smelled was probably burning HV wire insulation or HV cup...Electrical arcing consists of a tiny path of superheated plasma...Arcing is hot enough to melt metal and easily hot enough to burn plastic which is probably what you are smelling.


Do NOT allow your wife to throw it out! If the CRT is good that alone is a $100-300 part that is becoming scarse, the rest of the set also has value and I suspect if it were on my bench with the problems you describe I could have it working again in under an hour.