OK. Update time.

I got a new thingy. I followed the whole big horizontal adjustment guide on sams to a tee. The picture is not bad, but it's not really any better than before I did the whole procedure. The only noteworthy thing about the procedure was that I was unable to get the picture to slowly scroll horizontally, in the one step that asked for that. The picture either flies by all skewed, or the closest I got was the picture far to one side with a bit of a ghost image next to it. Also, I think it asked to see the retrace as a bar on the side, and I couldn't find that either.

It's hard to tell from the photos, but the picture is pretty blurry. I could be mistaken, but it seems like it gets a bit worse as the set warms up. The focus is turned all the way up, and it does get worse if I turn it down. Maybe one of the resistors associated with the focus is bad? There was a capacitor I mentioned earlier on that was double value on the same circuit, I replaced it with the specified one, and it did jack.

There's also a shadow of everything, if you look closely, especially noticeable in the darker image.

Turning the brightness down improves the clarity of the image, but even with the lights off in the room, the image would be too dark (and have too poor a contrast) at a level of brightness that produces a clear enough image. On the other hand, with the brightness all the way up, though the image is blurry, the screen is pretty damn bright. So... the crt is good, I guess?

The thought occurred to me at one point that the ion trap might be out of whack. Before doing anything to this set, the FIRST thing I did was mark where it was, so that's where it stayed. But I decided to mess with it, and while I don't know ANYTHING about adjusting an ion trap, the picture is at its brightest where it was rotationally, but I found that backing it up about 1/4" made it even brighter. Is that... ok? The photo below shows where I moved it to.

Also, the horizontal linearity slug doesn't seem to do much at all. All the way in or out, it has VERY little effect. And no matter what I do, the left side of the image is a bit stretched out.

With Ion traps you adjust rotation then fore and aft for max brightness...If neck shadows or centering issues occur occur try to fix it with positional adjustment of the focus coil first and only try to fix them with the ion trap as a last resort.

Double magnet traps like yours only have one 'front' side (be sure not to reverse it). But single magnet traps can use either side as front and sometimes work best if you swap front and back (the rotational sweet spot will shift 180 degrees when you flip it). I mention single magnet as they are more common and if you own enough sets you will run into them eventually.

...So what you're saying is... I did it right without even knowing what I was doing? Score!

Also I have no way of knowing which direction the magnet is supposed to be, other than the way that I found it. It has no arrow.

"Maybe one of the resistors associated with the focus is bad? "

The resistors would be the first thing that I would check. Are you getting too much or not enough current to the focus coil?

jr

Black to the base - it's on backwards. Also the linearity and width slugs have a very subtle effect and are meant for really fine tuning the picture.

MadMan,
After you reverse the ion trap and reset it, you may find an improvement in the focus. Otherwise everything looks good :thmbsp:. Some of the horizontal linearity issues and slight phase shift you mention seem to be inherently common with these Admiral sets. The last three Admiral sets I repaired had some of the same issues.
I just got an Admiral 12X12 this past weekend, won't get to it till next year.
Did your set come with a metal rear cover and metal underside shield ?
The rear cover that came with mine looks it may fit more than one model.
Ed

Yup, I has cover ^_^ but ty.

Ah. Thanks. Well. It was like that, so not my fault. Actually now it makes sense - blue for electrons, black for the base.

I flipped it around and it seems to have made no difference in the clarity of the image. Perhaps I should readjust horizontal. However, I had to shove it all the way forward on the neck, touching the focus coil. Which I might move it back a bit so the focus can be adjusted, but hey on the plus side, the focus coil gets to be a lot more straight now.

Idk, how awful would it be to put it on backwards? I'm concerned that in the future if the focus coil adjuster gets bumped, it'll snap the crt neck, what with the magnet butting up against the coil.

Can always loosen it to the point where it won't spin or move easily, but will move with a little bit of force.

"Turning the brightness down improves the clarity of the image, but even with the lights off in the room, the image would be too dark (and have too poor a contrast) at a level of brightness that produces a clear enough image. On the other hand, with the brightness all the way up, though the image is blurry, the screen is pretty damn bright. So... the crt is good, I guess?"

Pretty good indication of a weak CRT... yes you can turn up the grid drive and get a fairly bright image but not good focus, as the "sweet spot" (center) of the cathode is depleted... when you turn up the drive, you are drawing more electrons from off center areas, this is not conducive to good focus. Hopefully, the cathode will improve as you run it longer, or perhaps try to re-activate by running the heater at increased voltage (like 7 to 8 volts) for a few hours.

jr

Well... big surprise, right? I had it running the other day and it seemed better, but like I said, it really seems to get worse as the set warms up, but I'm not 100% sure about that. I'm still going to inspect the focus circuit anyway. Plus I still got that slight shadow, but maybe that's not related.

Ok. So.

I was thinking (yeah I know, that's my problem right there) if I could increase the voltage going through the focus coil it might focus the picture better. To do this, I used my decade box to put a resistor across the focus pot, and it worked. The picture was razor sharp. Though, come to think of it, I'm not entirely sure if the resistance I added increased or decreased the voltage through the coil. :/

I only had it on there for a minute max, but when I removed it, the picture was blurry as hell. I let the set cool off and it seems to be almost back to its normal level of blurriness.

Did I break something?

Things to consider: Current through the coil often matters more than voltage. Most decade boxes use half watt resistors. The average focus coil can handle most or all the B+ current through them so the focus pot often is/needs to be a 10-25 watt resistor to handle the power...If you put a half watt resistor big enough to significantly effect the control in parallel with it, then odds are that resistor is going to see several times it's rating and burn open......
Some time ago I found a sencore 'Big 20' which is a decade box made with 20 watt resistors...It is situations like this and power supply work where that box really shines.

Check your decade box is in spec on the setting(s) you used, and if the setting you were using is out of tolerance/open replace the one in the box, and get a 20W unit the same resistance to use in the TV.

lol my decade box uses wire wound resistors. idk what wattage but I'm certain they can handle it. Besides, does care about the box, I'm worried about my tv.

Just to nit pick. It's voltage across the coil and current through the coil. Anyway. you definitely do not want to burn out that focus coil. If B+ in the set is close to what the service info specifies, I'd look elsewhere for a solution.

For instance, the entire focus coil/yoke assembly can move back and forth on the chassis. There are two large screws down at the base that hold it to the chassis and there are slots in the support. It was designed so that you can loosen those crews and move it around. It might not be in the "sweet spot" for this particular CRT.

Alright I found the focus problem. I spent a good amount of time today going through all the voltage checks - and many of them (~25%) are WAY out of whack. So I sat back and did some thinking, and it occurred to me that I should just do what I originally wanted to do - investigate the focus circuit.

I disconnected the focus pot and big resistor that comes off of it. The resistor is in spec, but I found the pot was open from one end to the other. I took the pot off the chassis, and cracked it open and immediately could smell it was fried. It's still stinking up my room, my god, it's so pungent. If you look at the pic, you can actually see the bit of wire that broke and is now rubbing on the... turn-y thingy.

So there's that. Apparently, my adding a resistor across the pot was actually completing a previously open circuit, and that's why the focus would work properly then.

New thingy is on its way from fleabay already.

This just leaves me with the H linearity problem and hopefully this new pot will bring all the voltages in line, because some of them are really far off.

On wire wound pots (cough rheostats cough) you can often bridge the open and get the pot usable again.

In that rheostats case, you may need to add series resistance (given a lot of turns look bad) and reverse the ends of the pot so the wiper/rotor does not rest in the bad zone when focus is adjusted. Necessity (or cheapness and poor parts availability) is the mother of invention (but does it have Zappa too?).

My first thought as well, but it's pretty screwed up. You can see a large section of turns aren't damaged, but the ring they're on is wrinkled inwards.

2 years later update!

This thing's been sitting on its side in the middle of my bedroom for the past 2 years. I've been struggling with this stupid horizontal linearity problem. The left side of the picture was stretched out, and no matter what I did, it just didn't work out. But today I decided to take a fresh look at it, I tried a bunch of things, poked a bunch of stuff, and it was a little frustrating, but then something clicked. I'd been focusing on the linearity area. Mind you, I've never repaired or adjusted a tv like this before. But I decided to start messing with other stuff related to horizontal.

Wouldn't you know it, the horizontal drive trimmer was the culprit this whole time. It was about 99% tight, and if I loosened it, it worsened the stretching on the left side. So I figured that if I add a capacitor in parallel, it should either make it worse or better, either way, that was the culprit. I dug up a 22pF disc cap, put it on, and it happened to be just about right. I could now adjust the H drive to get the picture almost 100% linear.

It's still not 100%, but you know, 99% is pretty good for a 70 year old tv. And I'm quite proud of myself. :3

I'm imagining a 1kv ceramic cap would be acceptable for that area? I was considering replacing the actual trimmer, but it still works, I think maybe it's simply drifted to a lower capacity? Idk. Also, there's a bit of a neck shadow in the lower right corner, but I've given up on that. Doesn't bother me much, anyhow. You can see the stretched out test pattern here.

Now onto another project, I'd like to add RCA inputs. Any good threads on the topic?

Nice!

Looks good. The drive should be in the grid circuit of the output so 1KV will work and give you a large safety margin.

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.

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

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?

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.

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.

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.

“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

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.

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

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...

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

DC resto and linearity
 

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.

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.

Have you tried adjusting the centering control lever? It pivots the focus magnet around the neck. You might also make sure the yoke is pressed up firmly against the bell of the CRT. Also can you make the shadow go away by moving the on trap magnet around?

DC Restore
 

Don't get discouraged! I don't know what your background with regard to electronics is, but you've made this set look pretty good in my opinion.

Even as a practicing engineer, I look at a lot of 40's schematics and say either 'what the heck is that', or 'how in the world did they come up with THAT!'

So, interestingly enough, the 1950 Breitman manual shows a ton of schematics, more than half of them have DC restoration. (interestingly none of the cathode driven sets seem to have it, but I think thats coincidence) The 1951 Breitman manual shows the next years sets, and only a handful have DC restoration. Even sets from GE lack it. I'm going to say that with content of the era, average video brightness didn't change much, and manufacturers found they could save one tube with minimal impact.

If you're interested, we can talk more about DC restoration either offline (or in the forum). It's not unique to TV. The basic circuit only takes 2 or 3 parts (1 diode, 1 capacitor, 1 resistor). Its used any time that you have to AC couple a signal for amplification or other reasons, but then want to 'restore' the DC information that was stripped off the signal when you AC coupled it.

As you know, the average video voltage is low for an all black screen, and high for an all white screen. The DC information is ALWAYs lost in broadcast reception, and even if it weren't it would be lost in the amplification stages. (and actually for your set, it's reversed since you have cathode drive which reverses the polarities) If you fed your set an all black screen, it would eventually settle in to '50% brightness). If you then immediately fed it an all white image, it would flash white then fade to 50% brightness again once the DC image level ceased to pass the amplifier.
TVs use the level of the synch tips as the DC reference level for the picture - however the absolute voltage of the synch tips also changes over time with picture brightness. So, we create a circuit that looks at the 'local' height of the synch tips and recreates the picture information relative to that. the time constant of the circuit allows our reference level to change over time as brightness level changes.

I have no experience picking the time constant required, but we can look at other sets for ideas. What I've attached seems typical.

I came up with the attached schematic. and ran a quick simulation with some video waveforms. I *think* it should work well enough. Though, you'll probably have to tweak the values. The diode can be any low capacitance signal/detector diode. You'd be wise to go with 400V or greater. Play with R5 some - my guess is that small values (or zero) will make dc restoration better but may cut down video bandwidth. larger values will not restore dc as well, but will have less effect on video. This circuit also adds an additional cap into the video chain - it may or may not have an effect on video bandwidth. try it and see. Once you find a place for the circuit, you can play with the values.

I attached a picture of my 1948 RCA 6T45 test pattern, as well as a live image. I dare say that you have a much better picture.

Take 2
 

So...hopefully you didn't dig into that circuit too much :D

It should have been obvious, but that circuit will also lose the DC level over time.

Cathode driven DC restoration is indeed a little bit more difficult since the set requries the cathode voltage to be about midway between the video amp plate and GND.

Anyhow, I gave it some more thought. We need a way to clamp the DC level to a reasonable voltage that is stiff. What I came up with uses the same 2 470k resistors to set the cathode bias, but rather than come off the video amp output, the 'top' 470k connects to C40 -- the same place the brightness pot taps off of. Those resistors give you a voltage at approximately half the video amp screen voltage, which is then stiffened by a 5uF (or so) 250V or better electrolytic.

D1 clamps the black level relative to the new 250V point. C2 and R6 set the time constant of the DC restoration (it's set similar to other sets of this era). The comments about R5 still stand - bigger R5 gives worse DC restoration, but might give better video and/or avoid synch problems. Give it s spin and see what works.

I've attached the new schematic.

I attached some simulated waveforms of what you might see at the original cathode and at the cathode with this circuit. The DC restoration isn't perfect, but it should be better than what you have.

I created some simulated composite video with positive synch (like you should have at the output of your video amp). The simulation has a bunch of lines of high brightness followed by a completely black section of video, where there is nothing above the synch pulses.

The original composite video is at top, the DC restored cathode signal in the middle, and the original circuit's cathode at bottom.

I'm not sure the AC coupled composite simulation is entirely accurate...but at least it shows how the set's original video amp causes black to fade to gray, while the new circuit eventually finds the right black level after an abrupt brightness change.

Nothing's perfect, hopefully this is something that others can common on and improve upon. After looking I was unable to find any actual schematics of cathode driven sets with DC restoration.

Wow thanks! That's super helpful!

How does THIS diode look? It's low capacitance, 400v, and fast recovery time. But all the capacitance ratings are at like 4v, so idk.

Also, which values do you think I'll have to tweak? All of them lol? And lastly, would wire wound resistors work in a pinch? That's about all I have on hand, if they'll work for testing, I'll order carbon later once I have the values picked.

diode
 

Ideally youd use a shottky, but the voltages are too high. id get something categorized as an ultrafast diode. stth1r06 is my favorite.
https://www.digikey.com/product-deta...43-1-ND/654613

the biggest thing to tweak will be C2. i would think values between .01 and .1uf are reasonable. i thought .022 was a good place to start.

if the brigtness ends up on one end of the dial instead of centered, you can adjust one of the 470ks to tweak thr bias.

if it looks okay, and still synchs etc, youre good to go. if it interferes with synch..try to increase r5 to about 1k.

you could use WW on thr 470ks, but i wouldnt use them in the video path.

no guarantee this will work, but its cheap, and it looks good on paper.