1949 DuMont - issues when turned on cold

[bigaudioal]

When I turn on my DuMont RA-108 and it is cold, it takes several minutes to come up to full brightness. When it starts out, raster is barely visible. But then slowly, after a few minutes it is full brightness and totally fine. Any idea what might cause this? Once it is up to full brightness, set plays for hours with no issues. If I turn it off and then back on, it comes right back to full brightness. Unless I let it sit for a while and it cools down. Then condition returns. Any ideas?

[MadMan]

Bad vacuum tube?

[Gleb]

What happens if you try to force brightness while the TV is 'cold'? If the picture degrades, looses focus, turns into a negative, then the CRT seems to be weak. If the picture fades and grows in size, it points at the weak HV rectifier tube. If nothing happens, i.e. the control just doesn't increase brightness as it supposed to, then the problem is somewhere else (the brightness control itself, the video amplifier, power issues, etc). Good luck!

[bigaudioal]

Originally Posted by Gleb

What happens if you try to force brightness while the TV is 'cold'? If the picture degrades, looses focus, turns into a negative, then the CRT seems to be weak. If the picture fades and grows in size, it points at the weak HV rectifier tube. If nothing happens, i.e. the control just doesn't increase brightness as it supposed to, then the problem is somewhere else (the brightness control itself, the video amplifier, power issues, etc). Good luck!

The brightness control functions properly, even when the set is first turned on and you can barely see the picture. Along with the brightness slowly coming up to full when set is turned on cold, the vertical takes time to fill out as well.

This set has dual HV rectifier tubes. I will try swapping them tonight and see if condition changes.

Thx.

[Electronic M]

It sounds like it might be a B+ boost issue. Many sets drive part of the vertical and the G2 of the CRT off of boost....If boost is lousy, or coming up slowly that could cause both issues.

[Gleb]

Originally Posted by bigaudioal

Along with the brightness slowly coming up to full when set is turned on cold, the vertical takes time to fill out as well

It looks like B+ is coming up slowly. The brightness control and the vertical sweep section seem to get power from the same line. Can you measure it while the TV is warming up?

[bigaudioal]

Originally Posted by Gleb

It looks like B+ is coming up slowly. The brightness control and the vertical sweep section seem to get power from the same line. Can you measure it while the TV is warming up?

I will give the schematic a look at this circuit to try and see where the brightness control and vertical sweep section get their B+ from and see if I can ID any issues from that common point back.

Thanks for these clues.