frsj8112 wrote: ↑April 12th, 2020, 12:37 pm
Could the 7853 be bad?
It wouldn't explain the 130v B+. Is it still like that after resoldering the flyback pin? Check the high voltage caps on the HOT collector/H yoke. Do you have any voltage on R315 on the neckboard (HOT in place and chassis connected to tube)? Check with multimeter leads already attached to it when you turn it on.
Resoldered the flyback pin 2.
HOT soldered in place
Chassis connected to tube.
B+ still rising up to 130VDC.
Checked voltage at R315 on neckboard (one of the large ceramic resistors) and there's no voltage there during startup.
Interesting!
I pulled out my digital oscilloscope and measured the signal at pin 13 of the 7853 and there is activity on that pin.
And also I got tube crackling, you know the normal crackling sound when the tube is energized with HV.
But after a few seconds I can hear a little click sound and the chassis shutdowns and HV is no more.
Shutting of the power supply and turning it back on again have no effect, chassis is still dead.
But if I wait a few minutes, then I can turn it on and get tube crackling again!
AND I JUST REMEMBERED
I did take the bad 7837 (vertical collapse fault) and put it in this chassis just to see if could get some voltages. This might be the issue now.
Because B+ is stable at 75VDC until it shutdowns and the chassis that I repaired with a donor 7837 acted the same way
OK so we have some progress
I replaced the 7837 with another one and now the monitor starts and looks great!
BUT
It shut downs after a while and it is not consistant. It can be after 1 minute or after 7 minutes.
Here's a video with the multimeter probing the B+.
It is unsure if it's shutting down because the anode voltage rises above normal level. You would need a HV probe to see that. I would expect the B+ to stay at 75V after HV shutdown. A less safe method would be to suck solder off pin 14 of the 7853 and see if it doesn't shut down. It wouldn't be a bad idea to replace the polypropylene caps on the H yoke/Hot collector anyway.