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Old 12-19-2007, 12:32 PM
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1966 Zenith HV /focus issue

Hi Everybody,

I have been running a 66' Zenith roundie origional capacitor daily watcher for over 3 years now. Only had to replace a damper tube.

My question. As the TV is turned on and the HV powers up the picture gets perfect focus and convergance and then continues past that point to not perfect focus or convergance. This happens on the inital HV power up, and happens super fast.

I have already replaced the focus resistors and the focus control has good range. There is a point in the 3/4 range of adjustment where the picture appears in good focus. But not great like on the initial power up.

Is this a case of exessive high voltage? I based my assumptions on the fact that during the HV power up that the voltage doesn't instantly begin at 23 to 24 thousand. I know the voltage starts low and rises(duh) and therefore if the the picture gets in focus before the picture stablizes at full HV than I am running over voltage.

I have a HV probe with a meter but I don't know where to put the tip. Under the rubber HV connector on the CRT, or somehow under the cap of the regulator?

Does adjusting the HV affect the cathode current to the Horizontal output tube? I just want to make sure that if I lower the HV it won't cause my 6JS6 to draw excessive current. Its expensive and I also don't want to replace the flyback.

I know I wrote too much but thats how I am.
Thank you for all your help,
Matt Davala
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Old 12-19-2007, 12:47 PM
MRX37 MRX37 is offline
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"original capacitor" caught my eye.

Those original caps might be failing.
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Old 12-19-2007, 01:22 PM
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Matt, What you are explaning in where you say the crt has perfect focus then blooms out upon startup. This is usually normal for roundies, my rca does the same thing. Do you have any crawling in the pic, shrunken raster, humming going on? If you turn the contrast down, try to pull in good fucus, turn the contrast back up, check your focus tube, even if it reads good, replace it. Also you could have something going on with a bad cap, like MRX37 said, if this set is that old still running on it's original filters, I would start there! Alot of issues can be fixed when replacing these old can caps! You might even have a better picture than you ever had before! The HV probe goes under the suction cup on the anode lead on the crt. You should have roughly 25kv. BE CAREFUL!!! What I like to do is affix the tip of the probe under the suction cup and then find something to hold it there, power up the set then see what you got, do this after replacing the filter caps..
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Old 12-19-2007, 02:33 PM
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Last edited by andy; 12-07-2021 at 02:02 PM.
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Old 12-19-2007, 03:10 PM
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Right.

I will normally wait at least 15 minutes before adjusting focus, size, convergence, etc. That way I know the set has reached its operating temperature and there's less likely to be components drifting making changes to my adjustments.
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Old 12-19-2007, 06:05 PM
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Be sure to adjust high voltage with the brightness all the way down and adjust for 25KV. Its important that the brightness be turned down as it puts full load on the shunt regulator which is necessary for proper adjustment. Not to argue with anyone, but I have found many 1960's color TV sets to work excellent with their original filter caps. As long as the ESR is low and the set works fine, its good, assuming the low ESR isnt a false indication due to excessive leakage. I always feel the cap after a few minutes, if its cool to the touch on top of low ESR, then I leave it alone. I put many hours of use on my 20" zenith 20X1C38 upstairs for the past 4 years and its been going great on its original filters. My everyday basement zenith is the same way. I understand the precaution everyone takes, but I dont think its a "must" for every TV. As long as you use the set, the caps generally stay in good shape. Id be concerned about the newer cheap caps that we end up putting in the sets when we need to. I dont think those will last as long as the originals did, unless you use sprague brand, but those take up so much space under the chassis. I end up having to use the cheap xicon caps as they are small and fit well under the chassis for the most part.
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Old 12-19-2007, 09:10 PM
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I agree with Doug on this. I have ran into bad can caps, of course. However, I've ran in to a great deal that were good. Since most of them are a pain in the butt to change, I usually leave them alone unless they are defective. I see far more bad cardboard tubular electrolytics than I do the can type. In fact, most of the multi-section carboard caps I find in tube radios are shot. I have seen a lot of bad caps in newer equipment. I think a lot of that is due to the fact that they are cheap parts to begin with. I will say that if you run into a can cap with one bad section, replcace all the sections. I can remember two solid state sets that I fixed where only one section was bad and a week or two later, the other section died. That's when I started changing the entire can even if one section was bad. Also, it's not a good idea to leave the old can in circuit.

Now, the old paper caps get changed in radios I fix regardless. I had a friend that was so tight that he would not change a paper cap in a radio unless it was totally shot. His theory was that if the radio worked; then, why bother. Really, those caps are cheap enough and it's worth it to me just knowing that the safety factor of the radio will be better and knowing that it will be more reliable.
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Old 12-20-2007, 11:31 AM
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Wink

These guys are right Matt, they have been into this longer than I have, and if you have a way of measuring ESR of a cap, thats good, the sets I have worked onn have not been used in 20 years. Since yours is a daily watcher, you could just have a cap bad or two, see which one it could be and go from there, replacing good caps is not really neccesary unless it was my condition like I said my sets have been out of use for 20 something years or so. These guys here are some of the best! They have helped me so much and I have learned alot since I have been a member on this site!

Quote:
Originally Posted by freakaftr8 View Post
Matt, What you are explaning in where you say the crt has perfect focus then blooms out upon startup. This is usually normal for roundies, my rca does the same thing. Do you have any crawling in the pic, shrunken raster, humming going on? If you turn the contrast down, try to pull in good fucus, turn the contrast back up, check your focus tube, even if it reads good, replace it. Also you could have something going on with a bad cap, like MRX37 said, if this set is that old still running on it's original filters, I would start there! Alot of issues can be fixed when replacing these old can caps! You might even have a better picture than you ever had before! The HV probe goes under the suction cup on the anode lead on the crt. You should have roughly 25kv. BE CAREFUL!!! What I like to do is affix the tip of the probe under the suction cup and then find something to hold it there, power up the set then see what you got, do this after replacing the filter caps..
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Old 12-20-2007, 12:05 PM
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I've also had good luck with 60s can caps. Most still have life left in them. Now, I had a Zenith which would gradually lose focus the longer it was on and after lots of tinkering I found it was a defect in the HO tube.
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Old 12-20-2007, 08:11 PM
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Thanks everybody! Hope you read my reply, because I do write long. Good luck!

This group is great. I'm always here reading every day, even though I don't post every day. Always great help.

Just to give an update. I just replaced every filter cap in the set, and three resistors in the vertical section. These TV's are annoying to work on. I've worked on a few B/W Tv's and its relativly easy to remove the chassis. This 66 Zenith requires removal of the VHF tuner, UHF tuner, disconnection of convergance cables and cables to the yoke. There is however, plenty of room under the chassis for new electrolytics(since I do not stuff the old cans)

After replacing the electro's and vert caps I was just about ready to install the chassis. Two power resistors on the top of the chassis looked a bit odd. I touched one of them, and it basically crumbled. I was quite upset! I had the TV in the middle of our bedroom, chassis out behind it and parts and stuff all over the floor. My girlfriend wasn't home yet to see the unexpected surprise awaiting. She was OK with it . . . . . man was I lucky!

I bought two new resistors to install this next morning. Got those in and fired up the TV using the lightbulbs in series trick to make sure my electrolytic replacments were ok and no accidental shorts happened. I've got three bulbs that I can turn on and off in paralell. a 60 watt, 100, and 200 watt bulbs. Tried lowest to highest and no short. Yay! Plugged it in direct and she was Ok.

Used my HV meter for the first time. Brightness and contrast to min, and I was 3 or 4 thousand volts short of 25Kv. Adjusted Ok. Next was my convergance. . . . . . . . .

I have a DVD of test patterns that I used for convergance. I spent quite awhile going over and over the alignements. Not quite perfect. Some of the adjustments are on the full range just to get it "ok". I know in the future I will have to replace the rectifier and capacitors that are on the convergance board itself. But better than it ever was before.

My degaussing system is cooked. Something on top of the CRT looks like a ceramic cap broke in two, and something that looks like a super big ceriamic cap right next to it looks a little special . . . . I figure its the thermister and resitor. Am I correct? Where the heck do I find these thinggys to replace?

My old filter caps were ok. I didn't need to have them replaced. Keep in mind this is a 66 Zenith 24NC31Z that has been used daily for 3 years. But now that they are replaced I have a weight thats removed from my chest. You just never know when it will or would have failed. Or what would have happened if it did! I was super paranoid of installing the electrolyics for shorts and improper installation. I did a very thourough check of my work before firing up the set. Always double check your work.

I'll take some photos of the set in operation. I know everyone always enjoys photos. My avatar photo is of LOTR's 2 years ago. Its in 16:9 so thats why there are black bars top and bottom.

Happy Holidays everyone!
Matt Davala
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  #11  
Old 12-22-2007, 01:09 AM
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Just a day after replacing the lytics and adjusting the HV to 25Kv I had a tube fail. THe Hv suddently died. I did not immeditaly suspect the horz out tube.

I removed the tube and reinstalled it and I could see the tube arcing inside. Not by the grids. It was arcing from those silver rings that are attached to the plate of the tube to the silver getter on the inside of the glass. The TV worked though even with this small arc. How or why it was arcing to the silver getter is besides me. The tube could be getting gassy.

I had a new 6JS6 and this tube is doing fine with no arcs. The tv had been run for 3 years daily on the old Horizontal output tube. Pretty good since these tubes are super expensive. AES wants 56 dollars for one 6JS6C!

Happy holidays everyone!
Matt D
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Old 12-22-2007, 11:29 AM
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Yes, it's unreal what they want for those tubes. I guess you can blame the ham radio community for that, as these tubes are used in RF amplifiers. I was flipping through the current AES catalog the other day and I believe they wanted $75 for a 20LF6! I believe that's the HOT that was used in the Zenith 4-tube hybrid's. I thought only the 6 volt tubes were the only really "hot" ones for ham radio use. I wonder what this 20LF6 is used in to create such a demand to drive the price up that much. If I ever find one of those Zenith hybrid's, I hope the HOT is good because at $75, I may just have to let it die. Maybe one day they'll start making sweep tubes again like they did audio tubes.
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Old 12-22-2007, 10:42 PM
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Ive got a few nos HOT tubes luckily. I myself have no idea what the demand is on the 20LF6 either. I dont see any ham equipment that uses this tube. You can also find the 20LF6 in the series string zenith portables from the early 70's. Luckiy, its a rare occurance to have to replace the hot, or else id be spending a lot of money in tubes. I needed a 6JS6 last year for a set and got a nice NOS rca on ebay for a $30 bin, which seemed decent.
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