I dug into my 25MC33 Zenith roundie set this weekend to try to figure out why the horizontal output stage is not working. I found the efficiency coil was broken. the plastic coil form was broken, the ferrite slug was stuck at an angle and the plastic appeared to be melted where the slug goes in. The horizontal output tube was glowing at the plate when I got the set going initially. It had a shorted power supply electrolytic cap.

The question is, can these coils have a meltdown like this? Does the coil itself go bad or is it the excessive current that kills it? The wire on the coil was not melted, burned or charred at all. It looks like the plastic coil form gave out and maybe the slug shorted it out. This is my first color roundie restoration so I am curious as to others thoughts and experience. I am looking now for a new coil because I know I'm at least going to need that.

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The plastic is too brittle to glue. The coil is ruined. I found a replacement from Moyers so I won't have to try and salvage it. So far, everything else under the chassis looks good. I won't know for sure until I take the chassis out of the cabinet. I need to get it on the table and do some serious work on it.

this is typical problem with zenith sets that used the convergence coil spool for the horizontal efficiancy coil windings. Ive found a good number of my roundies, the spool became very brittle. When attempting to adjust, it just crumbles apart. The coil itself still worked OK, even though it came apart. Your set should still work even if the coil is a bit out of order. Ive had them with the melt down symptom as well. I find that the flyback tends to run rather warm on the 25MC33.

I have a bunch of parts convergence coils. If I have defective eff. coil, I simply unwind wire off a good spool from a convergence coil and carefully rewind the efficiancy coil onto it. Good as new and snaps back into the rear apron of the chassis.

I suspect the failed coil is more from the aging of the coil form. I'm going to look at horizontal oscillator tube voltages next to see if the output stage is even getting any drive. This will probably be the most fun and challenging restoration since I did my Motorola 9VT1 several years ago.