Westinghouse H886K21

[Electronic M]

Given the rate of wobble I'd reckon that the power supply filtering is insufficient, your AC outlet voltage is fluctuating, or some component is shorting/leaking/arcing intermittently at a regular interval.

I've seen problems with AC utility voltage fluctuations causing picture size to change 1 a minute to ~3 times a sec. There is a business park about 1 mile away with manufacturing operations there, it was worst for the first 2 years I lived there, but still happens occasionally....I knew it was the power line since I could have 2-3 working tube era sets and one new set fed the same signal source on at once, the 3 tube sets would 'wobble' identically, but the new set would not wobble (due to having actively regulated B+ rails)...The cure was to run the tube sets through a SOLA VRT (AC voltage regulating line isolation transformer).

As for contrast check the video amplitude with a scope and compare to the schematic. If it is low there are a number of things that could be affecting it.

Your AGC may be tied to the low contrast issue....For instance the tube IF grids should be biased negative enough relative to the cathodes to prevent conducting more than their max rated plate current....If the IF tubes are weak and not amplifying enough then the AGC could be be pushing the grids as positive as the biasing design wall allows...When it hits the wall it can't make any more gain, but still it trying to.

It might be interesting to connect that external bias supply back to the AGC line, see how much more contrast you get and how the IF and tuner voltages (especially the AGC controlled ones) compare to the schematic with the external AGC voltage.

May also be good to check the sync amplitude entering the vert mult/osc system. Bad vert hold can be weak sync or osc off freq.
If you can get the vertical to roll both ways (up and then down) slowly, even 'float' without rolling but not fully synced for a while, then the problem is in the sync pulse level/cleanliness and or the osc circuits handling of it.
If vertical only rolls one way then it is off freq and the vert osc/mult/output stages are to be suspected (and sync signal can be ignored till float/dual direction roll is achievable).

[FrankieKat]

I've gone through and checked through all scope traces and voltages several times and everything is generally within spec, except for a few places.

The biggest anomaly is that the grid on the sound IF / sync amp should be, according to the schematic, -1.8V however I measure between -0.3 and -0.5V. The plate voltage should be 60V but I see about 54V when the set is cold, and it falls down to about 45V. When first powered on cold, I get actually watchable contrast and fairly strong sound, however... after it warms up for about 5 minutes the contrast, sound and to a lesser extent sync gets worse (as that plate voltage goes down).

I have tried no less than six different 6AN8 tubes and all the good ones are the same. Have verified and/or replaced all nearby resistors (nearly of the originals were still within 20% spec) and replaced all but the mica and ceramic caps. And still absolutely nothing from the AGC control. The DC voltages on the IF stages are pretty much right on.

What could be the temperature sensitive components that would cause that?

FK