I'm thinking of buying this Admiral

[MadMan]

Originally Posted by kramden66

if the retrace lines are not there with brightness turned down to a dim picture its a weak crt

Well, it's not the strongest crt. And come to think of it, I have had the brightness all the way up. At max, it's very watchable in a bright room. Of course, I understand it wasn't necessarily meant to be watched in a bright room. I tried lowering it, and it does help greatly with the retrace, they still show up with a very dark video image being fed through. I think the CRT is slightly weak, but I have other problems.

So here's what I have so far. I'm not seeing any problems underneath. There is one big resistor (1M) that seems to have drifted a little high, but that's about it. Of course, I haven't checked everything, just what I've seen so far. It's R38 and it goes from the grid of the sound IF amp tube to ground. The grid is also coupled to the video output via a 2pf capacitor. I was looking in that area, because of the buzz in the audio.

If you feed in a white image, it increases the buzz in the audio. The same is true if you turn up the contrast. On a test pattern, if you turn up the contrast past 80% or so, the picture smears. But as most video images are a lot less white (on average) the picture will not smear.

I did a little watching of old tv commercials, etc, all stuff that was 4:3 (so no black bars), and it was reasonably watchable. Retrace was rarely on screen, and the huge swings in contrast between bright and dark images wasn't really an issue because most video from back in those days was brightly lit stage recordings. So perhaps it wasn't an issue in 1948, but given that we are living in the next millennium...

The diagram shows a direct connection from one side of the primary winding of the transformer that feeds the vertical of the yoke, that goes to a grid in the CRT. Would that be the retrace suppression? I'd imagine when the input goes positive, it would blank the screen. Or perhaps that's the old fashioned way of doing it, and not terribly effective.

Still, I'm more interested in this DC restoration. If I can rig it to keep the contrast constant, the retrace lines will not be a problem at all. Because if you have a constant picture on the screen, adjusting contrast and brightness to hide the retrace is easy. I'm looking up old threads on the topic, but if someone would care to chime in, that would be great.

[jr_tech]

“The diagram shows a direct connection from one side of the primary winding of the transformer that feeds the vertical of the yoke, that goes to a grid in the CRT. Would that be the retrace suppression? ”

The line between pin 10 (grid2/anode1) of the crt and the red wire of the vertical output transformer merely supplies boosted B+ from the damper to both points. As far as I can tell, the tv does not have a retrace blanking circuit.

jr

[MadMan]

Originally Posted by jr_tech

The line between pin 10 (grid2/anode1) of the crt and the red wire of the vertical output transformer merely supplies boosted B+ from the damper to both points. As far as I can tell, the tv does not have a retrace blanking circuit.

jr

Yeah, that's starting to make sense.

I really couldn't find a whole lot about DC restoration... other than unhelpful theoreticals, and this.

I have attached the video output portion of the schematic. Can someone tell me if this is grid or cathode driven video?

I *think* it's cathode drive. But it seems I don't know nearly enough about the actual theory of operation to know what I'm doing. I tried putting a silicon diode and 200k resistor in series with the cathode wire, basically, and it made no difference. The resistor alone just made the image very soft and blurry. If it was cathode drive, wouldn't it dim the image?? I suppose maybe the diode I have leaks too much reverse current at that voltage? Then I tried putting the diode across R35, and it arced rather loudly (oops), tried it the other way around and it made no difference at all. I did also try adding a ~4uf capacitor to ground, also no difference.

I'd really appreciate a suggestion.