#46
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As stated above, this is due to the video output transistor in that color saturating, that is, driven to the point where the collector-emitter voltage goes to zero or minimum possible. There may be a video drive limiting adjustment somewhere or a bias or G2 adjustment that will improve it. It can occur if the particular CRT (due to either original tolerances or age) requires cutoff voltages that then make the video outputs be run at low collector voltages. Referring to service info that shows the scope traces at the cathodes can tell if the outputs are biased lower than usual even though the video B+ is normal.
[Edit: oops - I just saw your post about G1 being low. This will definitely cause this problem, because the cathodes now must be lower also so that the CRT is not cut off.] Last edited by old_tv_nut; 09-02-2016 at 10:55 AM. |
#47
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As an aside I find it interesting the video amplifiers are just wide open transistor stages with no negative feedback to control gain or improve linearity. This really is an early model solid state design. |
#48
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TVs ain't HiFi audio amps....Feedback is rare (ignoring AGC, and sweep oscilators).
__________________
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 |
#49
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Never be run open loop. I ought to study some schematics of some newer stuff just for education purposes. This Moto is my first foray into solid state TV repair, everything TV related I've done prior has been tube based. |
#50
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1978 is about my cutoff on how new a set I'll fix. Can't tell ya if and how the newer SS stuff works.
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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 |
Audiokarma |
#51
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I think you are more likely to find CRT drivers with feedback in computer monitor designs. Usual TV drivers were class A resistive loads, with large power dissipation. Like tube video outputs, they were run with the maximum gain possible to reduce the need for multiple earlier stages. Transistor sets, especially once video ICs were introduced, still commonly used class A outputs, only now there were three for R,G, and B. Push-pull outputs were also developed ( I remember them in European sets)- much less power dissipation, and less hefty transistors could be used, but the CRT arc protection had to be better to avoid frying the smaller die. The class A outputs would get quite non-linear at high frequencies and large signals due to the capacitive loading of the CRT, but it didn't make much difference for a 3 or 4 MHz bandwidth video signal - just put in emitter peaking capacitance to get enough amplitude and don't worry about the shape of a 4 MHz sine wave. Computer monitors needed more bandwidth, and hence more complex design; also compromises like even more output current and dissipation, less output swing, etc., etc.
The above is my partial knowledge and memory of the status of things in the mid 80s, so I am sure there is more to learn. |
#52
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So new resistor installed and the problem is 90% fixed. g1 is at 95 volts as per spec m. Still a bit of bleeding on very saturated colours, like fine block letters on a black background, but the picture is genrally superb. Way better than my RCA CTC38.
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#53
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This TV never got a lot of use after I restored it, I couldn't find a good spot for it. Recently though, it has become useful. It has a strange characteristic, the picture begins quite dim and green, and takes about ten minutes to achieve full brightness and good colour balance. At that point though, the picture looks great, and it looks fantastic as long as you want to use it, except for red which is a bit saturated. Is this just how it goes with a weak CRT?
I, believe red is weak, but certainly not so bad I can't live with it. It's sure nice how there's no cataract on this tv! |
#54
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Cataracts especially the green ones are easy to fix... certainly less hastle than recapping a Dumont, or refinishing a wood cabinet.
__________________
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 |
#55
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It does balance out well when it's warm, except red smears a bit because it's pinned all the way up. The Motorola chassis makes a wonderful clean picture, very little geometric distortion and great colour. Sounds good too. I have an RCA tv which has a nos testing 25XP22 in it, which would likely look better with the motorola chassis than it does with the CTC38 it's in. Some day maybe will make the swap - that one does have an RCA style cataract needing repair still though. |
Audiokarma |
#56
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I will update - the two colour controls (tint and hue) make it hard to get it looking right, but once it does, I can say without any hesitation that this is the best CRT TV I've ever had.. The scan lines are razor sharp, there's no noise in the picture, no buzz in the sound. The audio is powerful, the cabinet looks great. It actually has bass, and the slider controls work well. And the geometry is really nice - squares are square everywhere on the screen, and nothing bends or looks funny. I actually find it hard to believe they made such a good TV in 1968, I've seen worse pictures on sets 20 years newer.
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#57
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Having been in the TV repair business in the 70's and 80's I only occasionally saw one of these sets. Probably because I was in southern Indiana and don't remember their even being a Motorola dealer in the area. I did however enjoy working on them when I did run across one. Motorola had a TV advertisement at the time that said " Works In A Drawer " with a jingle played in the background. It was sort of a catchy phrase as they used it in the commercial. While making a service call on one of these works in the drawer sets we found it had a connection problem. If I removed the drawer from the set and sat it on the floor the set would play. When I put it back in the set it would stop working. I turned to the repair tech with me and whispered " Works on the Floor " in the style of the TV advertisement. We both got tickled with what I had said and started laughing. I don't think the customer ever did understand what we were laughing at even though I tried to explain it. I hope a few of you do remember the Motorola commercials with the phrase " Works in the Drawer, by Motorola " and " Quasar, by Motorola " and get a little chuckle out of this story.
Dennis |
#58
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Thanks Max - for this is a great thread for encouraging early SS TV repair. I think I fumbled 30+ years ago by NOT salvaging every Zenith tension-band CRT I came across, these did not get cataracts. I kept just a 23V from a 1978 CCII, and another VK member drove from Wisconsin to pick it up for a 1968 Zenith. An RCA 25XP22 might even improve things but you put a lot of time into the restoring that original CRT.
I have acquired another Motorola WID (very recently and free in the Phila suburbs!) that was built in 1970, A later run of the same TS915 chassis yours has along with the 25ZP22 in similar condition with yellow cataract. The cabinet is French provincial walnut and in need of a stripping also, I use Soy-gel for that (beanie-doo) because it pulls off finish nicely but leaves much of the grain filler alone. The TS-919 was a very rare flat-chassis version of the TS915. Never saw one in person. As far as commercials, I recall "Qua-sar" being sung by a female, the "by Motorola" in a deep male voice. I think the WID design sold quite a few sets to Engineer and Mechanic types alike!
__________________
"When resistors increase in value, they're worthless" -Dave G Last edited by DavGoodlin; 05-06-2019 at 12:28 PM. |
#59
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Its called secondary emission. It means the crt is weak.
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#60
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Audiokarma |
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