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  #1  
Old 12-06-2010, 11:57 AM
DaveWM DaveWM is offline
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Zenith roundie

some pics

CRT test good after brief wake up.
Attached Images
File Type: jpg back off.JPG (91.3 KB, 164 views)
File Type: jpg Front.JPG (98.0 KB, 178 views)
File Type: jpg Top[.JPG (100.3 KB, 70 views)
File Type: jpg control panel.JPG (95.7 KB, 114 views)
File Type: jpg tuner knobs.JPG (94.4 KB, 98 views)
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  #2  
Old 12-06-2010, 12:02 PM
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zenithfan1 zenithfan1 is offline
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Is that a 24MC32? I'm working on one right now and my chassis looks just like yours. Mine's a 1966. I see that it's the "gel" PVA bonding that gets the green halo instead of the "mold" looking cataracts of RCA style PVA. My tube is a gel one too. Nice looking set, glad she's saved.

Edit: I just noticed the knobs broken off, I'll see how many spares I have left, I think I already gave them all away.
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Old 12-06-2010, 01:09 PM
DaveWM DaveWM is offline
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Great if you have any thanks
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Old 12-06-2010, 01:43 PM
DaveWM DaveWM is offline
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yes thats the chassis number on the back.
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Old 12-06-2010, 01:46 PM
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Mark, I think the one you have is in slightly better chassis condition.
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Old 12-06-2010, 02:13 PM
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the whole CRT seems to have a slight greenish tint in addition to a slight green halo.
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Old 12-06-2010, 07:14 PM
DaveWM DaveWM is offline
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progress...

the color demodulators seem to be working, I have a loose focus lead (not sure if I knocked it out or if it was already loose, I had very poor focus until I pushed it back into the socket. The CRT socket is pretty loose on the tube so I have to be careful (it was loose when I got it). sound is good, sync is good. have to work the vert scan, prob a cap (there is a paper cap in there).

HV seemed a little low, about 21-22kv full on. Will have to look into that, maybe a low B+ maybe shunt tube (I checked the HOT current, about 220, but did not check th shunt 6BK4). the HV adj does work, just lacks head room. It looks completly orig, I see no evidence of any work being done.

I think I will try a new vert tube, and check that electro cap. then need to find the B+ and check it. This set uses a doubler so I guess if one or both of the of the doubler caps was a little weak the B+ could be down.
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Old 12-06-2010, 07:42 PM
DaveWM DaveWM is offline
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yep looks like the cap I was checking out is the one in the vert out cathode bypass. I will be replacing that automatically.

I will check the can caps with my cap tester, so far most of the zenith tvs I have worked on had good can caps.


there is also a "brightness range" adj pot called out on the schematic, I did not see one of those. on the chassis.

There are some of those white caps 1000v that I will prob get rid of, and one of the Black with red stripe, I will check it.

Not sure what to do about the focus pin lead. Any ideas there? just clean it up well and epoxy it in?

The fly looked good, no dripping wax or any of that nonsense.

I see the shunt tube has test points across a 1k resistor in the cathode circuit. I will assume a 1v drop is normal (assuming its still 1k). there is a 1.5meg resistor in the HV adj pot circuit, goes to ground. If that were to drift, I think it could cause the shunt to work hard, I will check that if the shunt voltage drop across the 1k it too high.

The finish on this thing is hard, like a photo finish, I don't think I am going to be able to make it look too pretty.

Oh I changed the video cathode follower which seemed to help the video some (the video amp did not). I am hoping if I can get the HV up the pic will be a bit brighter, its not very bright right now, of course the tube was a bit weak until it heated up, so its prob tired anyway.
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Old 12-07-2010, 09:47 AM
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found the test point for the shunt reg (right behind the damper tube). so I got the set on its side, service saver cover off, HV probe attached,HOT catode on the mA meter, and now a volt meter on the shunt test points. I will be firiing if of later after today to see where it is. I will prob just jump that vert out bypass cap for a quick test to see if that resolves the lack of vert sweep. I was thinking it could be a boost issue as well, since the vert uses boost voltage, so if the jumped cap does not work I will check the boost.
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Old 12-07-2010, 12:03 PM
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1.75v drop across that resisitor, equates to a 1.75ma, on other 6bk4 equiped set (the Packard Bell) they want that to be around 1ma, so I am going to assume this should be the same. So now I will poke around the shunt regulator and look for a problem. Not a lot there so I hope it will be easy. Maybe just try a new tube 1st? I guess it could be gassy.
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Old 12-07-2010, 01:32 PM
ctc17 ctc17 is offline
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what did the hv go up to with the shunt tube unplugged? Should be max 1 volt at minimum brightness, if its 1.75 at max brightness its way off. That like 30 watts dissipation through the shunt. That should turn that tube black fast.
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Old 12-07-2010, 01:35 PM
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did not pull the shunt tube yet. will do later.
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Old 12-07-2010, 02:21 PM
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I don't know about Zenith, but RCA lists shunt dissipation at 18 watts for most CTC-X(X) receivers. 6BK4 data sheet says 30 watts is the max.
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Old 12-07-2010, 04:37 PM
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disconnected the shunt tube, KV shot to 30kv so I have to assume there is a problem in the shunt. Tried a new tube, no change in Vdrop across the test points (1.7v) so I will focus on the few componets around the shunt, there is not much there so hopefully it will be an easy fix. 1st check will be that 1.5 meg resistor.
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Old 12-07-2010, 05:33 PM
DaveWM DaveWM is offline
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1.5 meg reads 1.7 so that is a bit out (5% resistor) but that does not seem far enough out.

I am going to see if I can get a direct read of the grid and cath voltage, I am thinking maybe the cathode is low, it goes to the 350v so I will start there.

It would seem that if the over all B+ is low the boost would be low also, but maybe the relationship between the two could be upset? The boost is the source (thru a resistance series and the HV pot) for the shunt grid bias.
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