[VintagePC]

Progress. Having found no funny components in the area, I realized I don't need direct contact to observe the HV waveforms. Hooked a small loop of wire around the HOT plate lead and to my probe... ta-da, there is the flyback pulse for easy viewing, and at a safe input level for the scope.

It occurred to me the symptoms above are very similar to the issue I had with a dud HOT I bought; it would work on startup but fail shortly after. Did some tube swapping and now have some slightly burnt fingers (Laboratory lesson #1: Hot glassware and cold glassware look identical until you pick them up. Then the hot glass is broken on the floor) and observations to show for it.

Here's what I saw:
1. Known "dud (?)" 6AV5: Oscillator comes up, along with HV pulse. There's some chitter on startup that I get in all tube combos. Seems to be caused by the HOT waveform getting out of sync with the Horizontal drive...?
Once the picture comes up, the height of the HOT pulse wavers a bit, and then it all dies (H. Oscillator shuts down too). Killed power.

2. Original 6AV5: HV, oscillator come up. Also chitters a few times as they sync up. HV pulse wavers quite a bit in sync with all the picture funny business I've been trying to troubleshoot. Sure enough, *plink* and the HV pulse disappears. H. Osc. stays running this time.

3. New 6AV5: Comes up as expected, pulses stablilize. Doesn't go out, but still funny business with the picture flickering and pulse jumping around. Shut it off and I notice the spot stays on the screen a LOT longer now. Maybe picture is a bit brighter and more in focus too, so I almost certainly had a weak HOT.

4. Swapped the damper. Lo and behold, my wavery HV issues seem to be gone. Nary a flicker after a minute or two whereas before it would have been acting funny already. The arc from discharging the HV (5meg resistor so it's a bleed-off, not a bang) also seems much stronger now.

Looks like we can chalk it up to a flaky damper diode as well. I'll swap my original 6AV5 back in to see if it's still funny, perhaps it was the damper all along.

Weird I didn't see this previously when I swapped some tubes, but at the same time, not surprising if it was convoluted with some flyback issues and a dirty rectifier socket.

I'm questioning if that 6AV5 I thought was a dud really is... the HV pulses on it were quite strong compared to the other two tubes. Calling it a night for now, but will do some more swapping tomorrow to find out once and for all.

One final observation is I'm now hearing a soft ticking coming from somewhere. Best I can pinpoint by ear is either the HV anode lead or yoke, the former being far more likely. I'll give it a good cleaning tomorrow, could just be some crud causing the odd arc