Pet 3032 Vertical Collapse

[Hugo Holden]

Just another update, I was fiddling with it again a bit more today and I think I now have it working! I thought the brightness pot was all god, but when I was adjusting it, the image disappeared. If the pot was wiggled though the image would come back. So I removed the pot, gave it a good cleaning and put it back, and so far the image seems to stay on!

That makes sense.

If the wiper of the brightness pot went open, the g1 grid of the CRT acquires electrons and charges up to a negative voltage value (charging up the capacitor there too) cutting off the CRT beam, as the grid becomes so negative it repels the electrons from the cathode. The same thing happens with any type of tube where its g1 grid floats in an open circuit condition.

In fact one way to bias a tube for audio work, is to use an extremely high value g1 resistor like 10 Meg Ohms. The small grid current via the resistor develops a negative voltage on the grid, but in this case not enough to cut the tube off.