I'm thinking of buying this Admiral

[old_coot88]

Good sleuthing! There was a run of RCAs of about that same vintage afflicted with bad horiz linearity that Nobody was able to fix, and finally had to pass it off as the 'nature of the beast'. Gotta wonder now if it coulda been the horiz drive trimmer all along.

[kvflyer]

That looks very, very good. Thanks for the update.

[MadMan]

Ok, so...

*sigh*

Since it's been over a year since I've last run actual video through this thing, I seem to have forgotten its other problem. If the video being displayed is too dark (seemingly an average of the whole picture), the image becomes bright and washed out, and the retrace lines show up. With the test pattern, the contrast knob is right about in the middle, and it displays perfectly. With any other image, yeah pretty much ANY other image, if it's not mostly brightly colored or white, this happens. The darker the image, the worse it gets. With any letterbox video (because of the black bars) it's terrible, and no amount of contrast adjustment makes it better.

Imagine you're watching a tv show. With every scene change, the contrast of the picture gets changed.

I feel I should also mention that depending on the video, again, the darker it is, the more buzz that's audible in the audio. This problem isn't really an issue when you're watching the tv - it's not noticeable. But I thought I should mention it as it might be related.

Thoughts?

[Electronic M]

The set probably lacks DC restoration in the video...Many sets back then were like that and those that didn't grow up with it think it is a defect.
It is possible to add DC restoration and there are articles on it in period TV repair magazines.

There probably is a retrace line suppression circuit in that (if not adding one ain't hard) should be from the vertical output to the video...play with the RC time constraints and see if you can improve things.

[MadMan]

Originally Posted by Electronic M

The set probably lacks DC restoration in the video...Many sets back then were like that and those that didn't grow up with it think it is a defect.
It is possible to add DC restoration and there are articles on it in period TV repair magazines.

There probably is a retrace line suppression circuit in that (if not adding one ain't hard) should be from the vertical output to the video...play with the RC time constraints and see if you can improve things.

I understood about half of that. What are RC time constraints? What does DC restoration do, exactly? Is that only for hiding the retrace? Because I could care less about the retrace. The contrast is not stable, that's the real problem.

[Electronic M]

RC = Resistor Capacitor there should be such a circuit coupling the vertical to the video to eliminate retrace.

DC restoration has nothing to do with retrace. When video amplifiers are AC coupled (which is the case most sets) the DC level of the video at the detector is normally lost...It manifests it's self in many ways such a scene with one large black and one large white section side by side smearing when fine detail looks good, the brightness immediately after a scene change either being lower or higher than average and sinking or rising despite image content being fairly constant...Things like that.
Some RCAs, Dumonts, etc dedicated a diode in the video output to the task of restoring DC level to the video. I forget the exact electrical description of their operation. They became standard with color TV, but in the monochrome era they usually were only in the more expensive sets.

[kramden66]

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

[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.

[jr_tech]

Definitely cathode drive, the video signal is connected to the cathde (pin 11) of the CRT. The grid (pin 2) only gets a variable DC voltage to set brightness. I looked briefly for a DC restore circuit that can be used with cathode drive video and have not found one yet. Will look some more tomorrow.

jr

[MadMan]

Thanks jr. I had hooked the O-scope up to the cathode and saw video there, I just wasn't sure because I don't know enough of the theory.

I suppose the issue I'm experiencing is likely not even a AC/DC problem. Because the cathode is negative, and more negative voltage would be more white on the screen. So what I'm having is the zero point not being clamped to 0v, but fluctuating. (Right?)

Now that I think about it, then, a diode would be entirely useless. I probably need some much more complicated circuit. Unless I go before the video amp and correct the issue there, or further back. Hmm...

[bandersen]

This chassis does not have a retrace suppression circuit but it can be added easily as described in Riders Receiver Troubles and Cures vol 1 page 1.

https://www.earlytelevision.org/rider_cures.html

[dobulee]

some years ago, I posted a resto of an RCA 6T75 chassis with bad horizontal linearity much as youve shown. Now im excited to dig back in and see if I can improve it with the drive trimmer! thanks for the motivation. incidentally that set has shadowing similar to yours, and ive never solved it.

im on my phone and couldnt get a great look at the video section of your schematic...but rhe set does appear to lack DC restoration as has been said.

skmewhere I have a 1950 breitman service book with dozens of late 40s schematics...Ill also look for a similar cathode drive circuit with a dc restoration circuit.

can you pass along your full schematic? or at least pass me the chassis number? Im a little late to the party...

thr RCA has a phono input with a switch that blanks the video...i repurposed thr switch to also cut over to an composite video input, which in thr RCA is relatively painless to add.

BTW, i find a strange array of sets Have/dont have dc restoration. by 1950 even Muntz sets sported DC restoration at thr lowest possible price point. honestly, i suspect that with low contrast "live" images of the late 40s, youd never notice the difference. With high contrast content now, the difference is fairly bothersome.

[MadMan]

Here's the schematic.

I'm glad I might've actually helped someone else with their problems, hopefully. I suppose the thing that gave it away on my set was that I messed with the drive trimmer and it made the linearity worse, so I figured I could make it better with more capacitance.

Thanks for your interest in my case. If you ask me, it's very important to keep these old things actually useful. I'm getting the vibe that many here like to restore tv sets to the way they were, and even use them that way, but that way is already long gone. If we want to preserve these pieces of history, I feel like it's paramount that they can actually be USED for some functional, present-day purpose (even if I personally don't plan to watch a whole bunch of tv on this set). You know what I mean?

Let me get down from my soap box here.

Anyhow, um, as for the shadowing, I don't think there is much to do about it. I'm trying to picture it my head, you'd have to move the beam over. Wait, is that even possible? Like, you can bend the beam, but can you just nudge it over? I suppose maybe... a different ion trap? Perhaps if you mess with the deflection outputs... Say, add a resistor and a diode in parallel with each other, but in series with on one side of the horizontal output. That would force the horizontal to favor one side, I think. idk, I'm just spitballing.

I was thinking, for the DC restoration, when the video signal goes higher (negative-er), it could power a transistor base, and that transistor could be used to pull the video drive down closer to zero. It would probably take an immense amount of fine tuning. It would need a specific capacitor to average out the video signal, and it would probably need a diode to prevent it from having a feedback loop (or whatever you'd call it, when the transistor lowers the voltage going into its base and then unlowers it, etc).

My other thought was to bypass the whole video section and use solid state electronics to do the whole job, but that seems like a bit of a cop-out. Not to mention difficult.