Quote:
Originally Posted by Electronic M
I'd try changing the contact point. I f the dag is missing (even just a whisker) under the point of contact or making poor electrical contact with the spring then you will get HV arcing (and spark gap transmitter action) at that contact.
|
Okay, so pulled away the metal contact and cleaned it up a bit and then bent it to another place on the tube, and looks to be making good contact. I don't see any "islands" or places where the dag is separated from the rest. The coating looks pretty complete... just a little scratched off where the metal has been resting for the last 60 years.
Now here's the thing... before that, after playing a bit with the suggestion of grounding out the 3rd IF with a cap, and working with DC on the AGC circuit, the noise seems to have gone away... along with decent picture contrast. It's possible that it could have been poor contact with that rubbed-off dag and in working on the chassis the contact was intermittent.
So without a clear explanation, I seem to have a more or less clean but poorly contrasted image. As with before, it looks brighter in the video than in person.
https://youtu.be/xWOoI252aZY
There is definitely some wobble to the picture as well, which you can see with the DTV but can see it much better with the test pattern. I'm adjusting first the fine tuning then the brightness then the contrast and you can see there's not really a sweet spot.
https://youtu.be/XO5ciFJxHiM
Since the CR70 rejuv, the tube now produces a lot of light with brightness all the way up. I re-checked it again and still has responsive cutoff and gets up to about a 10.5 on the CR70 emissions when it's warm.
Still nothing on the AGC control, but injecting negative or positive DC doesn't make any large improvement to the picture's contrast so maybe that's just not going to be a big help.
I would say contrast and vertical hold tends to get worse as the set warms up. The sound (as can be heard in the test pattern video above) get very soft when the channel is fine tuned in for best picture. I know this is typical with old B&W sets and cheap DTV boxes though.
Just got the HV probe and it's measuring 15kV, while the schematic calls for 17.5k. I'll try a few different horiz damper and output tubes and see if that makes any difference.
FK