Admiral 12x12

[rbc7snc]

Hi All,

I'm a fairly experienced radio restoration guy attempting his first TV restoration.

The set is an Admiral 12x12 that a friend of mine bought at a local antique store. It's the same model that he had as a kid, and supposedly produced some type of raster prior to purchase. I don't know exactly how it appeared prior to our restoration attempt as I prefer not to power up unrestored electronics from the late '40s.

We finished recapping the chassis, but are running into some oddball problems.

The audio section seems to be working, and the horizontal section seems to be working. The problem is with the vertical section.

When I first power on the set, I can display a test pattern without too much difficulty except that the pattern only takes up the top 2/3 of the screen (picture attached) Waveforms in the vertical section initially look fine. After about 2 minutes resistor R77, which is in the plate supply of the vertical output tube, overheats. When this happens, the picture scrambles and the waveforms at the grid on the vertical output tube (2nd section of a 12AU7) become very distorted.

I've checked my wiring several times, and haven't found any issues (at least not yet).

Someone suggested checking the domino mica caps, which I did (2 were probably actually wax/paper caps, and were replaced during the recap procedure). They all are within tolerance of their value and have leakage current in the microamp range at 300V.

The DC resistance on the vertical oscillator transformer is within spec, the DC resistance of the vertical output transformer is within spec, and the DC resistance of the yoke is within spec.

I'm starting to wonder if there is a problem with the vertical section of the yoke, but I'm open to other thoughts and suggestions. This set is complicated by the fact that there are damping resistors across the yoke sections, which makes using a yoke tester a little tricky.

This is my first foray into TV restoration, so please forgive me if I've missed something obvious here. I'm happy to post a copy of the schematic, but the images I have exceed the file size limit for the forum.

Any thoughts or suggestions would be greatly appreciated.