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  #16  
Old 11-09-2013, 10:27 PM
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Tim, that TV had probably been sitting since the 70s, I got it from the estate of a deceased repairman-his nephew told me the man never owned a color tv (?) I sorta think he never did anything with it. I think the main thing wrong with it when I got it was a bad power switch. I seem to recall that I eventually replaced the filter caps because I noticed them running warm. I don't recall any vertical issues on that one.
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  #17  
Old 04-07-2014, 04:23 PM
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Zenith color 25MC30

Quote:
Originally Posted by dieseljeep View Post
I wonder if that is the yoke with the nylon fish line strings for the horizontal and vertical centering.
I used an RCA yoke for the last replacement. The centering was close enough.
I have this Zenith TV now and I'm trying to get it to work. I got a nos yoke for it but I don't know what the nylon strings are suppose to do. I also can't find the vertical height adjustment pot on the chassis.
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  #18  
Old 04-07-2014, 05:39 PM
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The nylon strings are centering controls.
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  #19  
Old 04-07-2014, 06:00 PM
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Quote:
Originally Posted by philtar0079 View Post
I have this Zenith TV now and I'm trying to get it to work. I got a nos yoke for it but I don't know what the nylon strings are suppose to do. I also can't find the vertical height adjustment pot on the chassis.
IIRC, the vertical height is on the front auxiliary control panel. It should be identified on the label. You have to remove the knob to adjust it.
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  #20  
Old 04-07-2014, 08:20 PM
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It could likely be the vertical output transformer. My set went through two of them. The convergence winding goes bad and it can take out the two resistors that provide B+ to the vertical stages. It is not a difficult part to replace but may be a little hard to find. If you already have a new yoke, I hope that fixes it. These sets really perform well when correctly set up.
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  #21  
Old 04-07-2014, 09:51 PM
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I'll take another look. Thanks for the help.
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  #22  
Old 04-11-2014, 08:07 PM
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Quote:
Originally Posted by Kamakiri View Post
It's in the correct position, that was my first thought. I'd be further inclined to test the switch, were it not for the very noticeable "ticking" noise.
I do have a changer in a mid 60's rca home entertainment center, I'll look and see what model it it.
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  #23  
Old 07-12-2014, 09:14 PM
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Well, this set is now back in my possession from a trade deal

So here's where I'm at.....

Now that it's got a new yoke (installed by Preston), it's got a picture, but it's rolled up at the bottom. Picture shows it with the vertical size absolutely maxed, and it still won't go to the bottom of the screen.

Now, before I go any further, he gave me the old yoke back in the box, along with this paperwork which was included in it. I'm thinking that this yoke has to be modified somehow to work in this set, but I'm not wholly sure anything was, or how to do it if it does need to be done.

Take a gander, I've scanned the paperwork in. Suggestions are appreciated
Attached Images
File Type: jpg 100_4444.jpg (45.2 KB, 92 views)
File Type: jpg zenith1.JPG (34.8 KB, 63 views)
File Type: jpg zenith2.JPG (47.5 KB, 53 views)
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  #24  
Old 07-12-2014, 10:43 PM
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Quote:
Originally Posted by dieseljeep View Post
When Kennedy and Cohen came to town, now defunct, they used an inexpensive B/W set as a leader. It was branded "Kenco", made by Sampo. of Taiwan. They sold them for $38.00, a lot of them!
I got several of them as freebees, as the customer thought, that it was so cheap, it wasn't worth repairing. I don't remember what went wrong with them, but was a simple cheap repair. They worked well and IIRC, I sold them for $35.00.

They also sold a Kenco branded Sylvania-built hybrid 19" color table model for $177.00. Unheard of at the time.
I recall Kennedy and Cohen being in the Delaware Valley (media speak for Philly-Jersey-Delaware). I also remember the cheapest set was a 1975 Sanyo 12" BW for $59.66 when all the others were $80+
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  #25  
Old 07-12-2014, 11:19 PM
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Quote:
Originally Posted by Kamakiri View Post
Take a gander, I've scanned the paperwork in. Suggestions are appreciated
Tim, from what I can see in the instructions, you need to look at the original yoke (if you have it) and/or the schematic for the Zenith set, and remove (or add?) parts to make the new yoke match what the old one has. The schematic of the new one is in the one page you posted. Other may have a better way to describe this. I have not worked on any Zenith color sets, though.
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  #26  
Old 07-13-2014, 11:22 AM
DaveWM DaveWM is offline
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I doubt its the yoke, I would take a look at the PP drive signal, and/or voltages in the vert circuit. This is another place the TV analyst comes into play, a good source for a Known vert drive.
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  #27  
Old 07-13-2014, 07:26 PM
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Yep, just got a text from Preston. He made the appropriate changes to the yoke before installing. Now, to get a Sams....
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  #28  
Old 07-13-2014, 08:38 PM
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if you don't have the scope or tv analyst, the 1st place to go is a sub of the vert out tube and if used a vertical osc, IIRC Zeniths used two tubes for vert sweep (unlike the multivibrator dual triodes used in RCA).

I would test by subbing tubes.

after that you would want to check the voltage from the height pot. Assuming it uses boost voltage in the osc circuit, there is often a high meg resistor in series with a height pot. check to see if the pot and or resistor has drifted high.

The other likely suspect would be any eletrolytic that is a cathode bypass of the vert out. If that cap opens it kills the gain and output of the tube.

If you have a scope the 1st thing to look at would be the PP at the grid of the vert out, this would quickly narrow down to either the drive signal (vert osc tube) or the output (vert out tube). from there you can focus more closely on where to look.
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  #29  
Old 07-14-2014, 07:21 AM
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Tim,
I checked my Zenith service manual and it confirms DaveWM on a resistor that feeds 870 volt boost to the V height pot.

Also noted that the .1 mf/600 V cap from boost to one leg of the horiz eff coil will cause the HV to be intermittent.
It notes this should be replaced with a 1000 volt cap.

BITD, I worked on maybe one or two of these sets, compared to RCA, Zenith roundies were not seen much.
Good luck
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  #30  
Old 07-14-2014, 08:46 AM
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For the problem pictured, I would go for the vertical integrator. Looks like a three legged brown ceramic disc but it has three wires coming out of it.
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