Quote:
Originally Posted by Kevin Kuehn
It's the .2v AC on the cathode of your sync/DC restorer tube that worries me. It stands to reason that same AC is then present at pin 2(grid) of the CRT? When it's doing it's hula, what if any AC do you measure on pin 3(cathode) of the CRT? As an experiment you could hang a few uf's cap alternately from pin 2 or 3 of the CRT to ground to see if that clears up the hula. Observe polarity of whatever DC you measure on those pins to ground. Of course you won't get any video with the cap on pin 3 of the CRT, but at least you'll find out if there's AC modulating the signal there. Somethings got to give.
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Yes, that assumption was right. I did measure .7AC /-96DC off the CRT, pin 2.
I also measured 2.4AC/-22DC off pin 3. Both measured to ground. These values jump around a little compared to all the other points I've tested, I guess due to video signal.
That's the highest measurement of AC on a cathode in the entire circuit that I've tested so far.
Raw line current goes straight into the CRT of course (which is why I always bring up on a variac
)
Wondering about putting one of those TVS diodes across the CRT heaters to limit current and maybe it will take care of this problem as well.
I feel like I need to take resistance measurements again in these areas between the sync, B- and CRT.
I've looked at all the underside tube pin terminals for any shorts or crud that could be creating a short with a magnifier and wiped the undersides with contact cleaner.
NOTE: many of the metal surfaces were coated with a white powder when I got the set, which I have cleaned off what was visible, but there could be some that got into an area and is conducting - who knows
Haven't tried hanging a cap from the CRT pins 2,3 to ground yet (I assume the - terminal will be on the CRT side, right?)