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#76
05-11-2019, 12:45 AM
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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?
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#77
05-11-2019, 10:33 AM
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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. Last edited by dobulee; 05-11-2019 at 10:37 AM.
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#78
05-11-2019, 10:09 PM
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Take 2
So...hopefully you didn't dig into that circuit too much
 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.
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#79
05-11-2019, 11:52 PM
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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.
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#80
05-12-2019, 12:35 AM
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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.
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#81
05-13-2019, 08:11 AM
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Just one additional thought:
The 12LP4 datasheet for your CRT shows that the Heater Negative with respect to cathode shouldn't exceed 140V. Without the sams, its not immediately obvious what the plate supply voltage is on the video amp. But, if you implement this, make sure the average heater to cathode voltage doesn't exceed the limit.
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#82
05-13-2019, 03:07 PM
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dobulee, for the love of god don't take this conversation offline please! I'm working my way through a couple of TV engineering books right now and this discussion was super, super helpful to me. Thanks!
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#83
05-13-2019, 05:23 PM
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Most of the DC restore circuits are more complicated than this, and a lot of them use the same triode for synch separation and DC restore. It makes some sense, since the triode grid gives diode action when driven positive.
Im interested if the above circuit functions decently or not. Im sure the original set designers (and others here!) are aware of more practical limitations and tradeoffs than I am. screen shots and/or scope captures would make for further discussion! that little simulation doesnt model the video pentode output impedance or anything...so...mileage may vary!
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#84
05-14-2019, 02:56 AM
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Quote:
I have ordered some diodes. Will keep you posted.
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#85
05-19-2019, 08:42 PM
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Alright, so I assembled the circuit as described, and it causes some pretty wild image instability :/
Some tearing near the top always, with image/scene changes the horizontal and vertical stabilities change, sometimes the image jumps halfway down the screen, other times its starts to roll. Contrast control used to affect image stability, but now contrast, brightness, and focus have drastic effects on it. And at all times, no matter what, the image is never perfect. At best it's skewed and jittery, and ripple-y. That all having been said, I *think* it helps the DC restoration, but it's kind of hard to tell when the image is getting as screwed up as it does. So I'm thinking. This shouldn't affect anything but the brightness and contrast, yeah? Is there some way of isolating it (more) from the rest of the workings? Or wait. The path to the cathode used to be a capacitor and resistor, so that would mean it's capacitively coupled, but with a limited straight connection. Now it's just capacitive only. Would a resistor over the first cap help? EDIT: I also tried a 1k resistor for R5, it barely made a difference. Last edited by MadMan; 05-19-2019 at 08:57 PM.
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#86
05-20-2019, 06:26 AM
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if you put a resistor over the cap, you wont have DC restoration anymore
the resistive path to common is now through the 2 series 470k resistors.unfortunately I do not have a cathode-video set to experiment with, however, i suspect the tearing problems are due to the circuit interfering with the synch separation, which also comes off the video amp in your set. anyhow, thats the basic idea: a diode to clamp the black level capacitively isolate the CRT grid from the DC level of the video amp set the DC cathode level with a resistor divider the details are the complicated part! go even to 10k at r5. without a similar set, I cant do much other than consult :/ do you by chance have a scope to post some waveforms? as a last resort, i can try to simulate your entire video amp circuit, but i wouldnt have a lot of faith in the tube models up in the Mhz range, if the models even exist. for what its worth, the 48 RCA 6t45 chassis has dc restoration, and even it doesnt work thr greatest. I took some captures of thr cathode waveforms with all black and all white screens -- i will post them over in the 6t75 thread. But the front porch "black" level on the CRT grid drifts by about 10 volts from "all black" to "all white", causing all black to look gray. That set uses a positively biased triode grid to get diode action from grid current - and it probably doesnt make for the "tightest" clamping. if anybody else has ideas, id love to discuss. seems like an interesting little project.
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#87
05-27-2019, 10:01 PM
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Alright I experimented a bit today. First thing I tried was to change R5 to 10k. It made a big improvement in stability. yay! In fact, about 99% of the problems could be adjusted out, and did not come back with changing images. Now, I made a set of test images and use them in a slideshow for testing. I have a regular test pattern, all-white, all-black, then 4 sets of half black on top, bottom, right and left. They don't really approximate rapid scene and lighting changes, but they work well enough for basics.
First thing I noticed was that on the all-black and top-half-is-black* images, the black level starts out at about dark gray, then fades to medium gray over the course of 1-2 seconds. However this happens in reverse when going from all-black to side-half-is-black, where the black area starts (darker than it is with all-black) dark gray and darkens down to a good black. Which, I'm guessing, is what you intended to happen. Why it happens in reverse on all black is beyond me. *top-half-is-black improved with later modifications. Then I tried lowering C3. This brought back some of the stability problems and generally made it worse. I then raised it to .2uf and it made things better. Better stability and better evenness of black areas, but not really better black levels. All-black was still the same medium gray. It also seemed to help a bit with retrace lines. I added .1uf more to it, which improved it only a tiny bit more, it seems it's reached the point of diminishing returns at .3uf total. I then tried doubling C5, which didn't do much besides raise the brightness momentarily while it charged up. I played with the value of R6, but that didn't seem to do much besides alter the overall brightness. Also, I just noticed when I turn the set off a spot remains on the screen, which it never did before. But only for maybe 5 seconds, and it's about the size of a quarter, so I don't think it'll hurt anything. So then I put it back to factory spec, and examined the black levels. Interestingly with the test images I have, they all exhibit perfect black levels until you get to all-black, which is so light gray that it's nearly white. So at least the all-black has some improvement with the new circuit. However when watching actual widescreen video with black bars, it is noticeably worse than it is with the new circuit, that is to say - the changes in black level are more prominent. And also, because of that fade in time with the new circuit, it sort of takes the edge off of rapid light-to-dark and dark-to-light scene changes, by allowing it to slowly fade down or up, so it's not as jarring. So that's good. Which capacitor manages that, though? I changed the values of both of them and it didn't seem to make much of a difference in the fade in time. Also I measured the crt cathode voltage to ground: Sam's says: 115 Factory spec: 150 Current circuit: 145 idk, make of that what you will. However I do not have a VTVM, I'm just using a cheap digital multimeter. Below is attached a schematic of how I have it currently. I understand that it'll never be perfect, but I feel like it could still be a bit better. If you have any more suggestions, I'm listening. And thanks again for all the help! Oh also, if I should hook up a scope, would I connect to the cathode? And what would I be looking for? Last edited by MadMan; 05-27-2019 at 10:08 PM.
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#88
12-03-2019, 02:46 AM
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After reading the Rider's TV Manufacturers' cure for retrace suppression, I was kind of disappointed because the resistor numbers do not match the numbers on the Sams schematic. Frustrated, I had put the issue aside. Recently I decided to give it a go again, and through very careful reading and educated guesses, figured out where the connection points were.
Quote:
I'm pleased to say this retrace cure works like a charm. It's perfect, there are NO retrace lines, period. Whereas before it was installed, glaringly obvious retrace lines were onscreen about 90% of the time. I used a .047uf capacitor. I had a .056 ready to try, and even if I put them both in parallel it made no difference. So it's not very picky about that capacitance. Also the old 3.9k resistor tested at 4.5k... makes me wonder how many other resistors have drifted. >_> I then finalized the DC restoration. I made a tidy little prototyping pcb. Sacrilege, I know. I think it's a much neater solution than perhaps sticking a lug strip somewhere. Plus the fact that it's obviously modern will be less likely to confuse anyone servicing this unit in the future. The schematic posted in the previous post is the current iteration - big thanks to Dobulee for that! As stated before, it's not perfect dc restoration, but it is an improvement. And it does this thing where a dark scene will start dark and fade to brighter (in about 1 sec). But at least that takes the shock away from the transition. Besides, even without that, it does actually make an improvement in keeping dark levels. However slight an improvement, it was pretty bad without it, so I'll take it. Next step is putting the chassis back in the case. I've already fixed a couple broken bosses in the case - for the screws that hold the back cover on. And retapped the screw holes on the chassis. Oh yeah, how would I direct inject video into this? I tried putting raw video right into test point B (I think) which is right before the video amp. It came through but was very faded and gray. Maybe it needs a transistor to amplify the signal? The test point being right before the video amp makes me think that's what it's for. Last edited by MadMan; 12-03-2019 at 03:08 AM.
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#89
12-03-2019, 08:37 AM
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Quote:
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
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#90
12-29-2019, 06:45 PM
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I had this set out on Christmas day running a fireplace and playing xmas music. It performed well, only had to adjust it once a couple hours in, but I was running the contrast right near the point where it's about to make the image unstable, anyhow. I think it's just fine. And the sound is really crisp and clear, even with a couple of weak tubes in the audio section. Something I noticed though, is that it no longer has a spot when you turn it off - the screen just goes black instantly. Which is what it does from the factory, but after adding Doublee's DC restoration mod, it would leave a spot the size of a quarter when turned off, for about a minute.
It just now dawned on me that the retrace suppression circuit was the only change made between then and now. So I guess it kills the spot too. Just thought I'd throw that out there.
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Linear Mode
