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 Post subject: Zenith 23G22 Chassis Puzzle
PostPosted: Oct Fri 06, 2017 2:36 pm 
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Joined: Jan Sat 16, 2016 10:03 pm
Posts: 316
Hello everyone,

I'm still working on this 23G22 Zenith chassis and I have a puzzle that I just can't seem to figure out. First, I've doubled checked all the tubes and replaced the vertical output tube with a NOS even though it checked out good, rechecked all capacitors (except Micas), rechecked/replaced as needed all resistors, replaced the entire yoke assembly with a known good one.

The unit has great horizontal, the problem arises when I sub a 12LP4 CRT, the vertical is lacking and when I adjust the vertical linearity to obtain a full screen the picture begins to roll and the vertical hold pot adjustments cannot stop the rolling. Adjustment to obtain a stable picture results in a 2-3 inches of blank screen even after centering adjustment.

However, when I power the unit up with a 12UP4 CRT I obtain a full, bright and stable round picture. The 12LP4 is a known good sub for this unit so I'm puzzled by the problem. I also tried a NOS 12KP4 with the same vertical problem. The manual centering adjustment which must be adjusted for each CRT will not resolve the lack of vertical with the 12LP4 as well.

The only components I have not changed out is the original 1N64 diode which checks out good. I know the micas were under a lot of stress in the high voltage circuit but I have great horizontal.

The vertical output tube voltage checks out good as well.

I'm unable to check the high voltage at the anode, because my dog bumped into the folding table causing my heath kit high voltage probe to fall and break the glass insulator. But I got high voltage nevertheless, I just don't know if its lacking a bit.

Any thoughts out their. Thanks for your help.


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 Post subject: Re: Zenith 23G22 Chassis Puzzle
PostPosted: Oct Sat 07, 2017 1:36 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 157
Location: Warren, OH
Did you ground the dag on the 12LP4?

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 Post subject: Re: Zenith 23G22 Chassis Puzzle
PostPosted: Oct Sat 07, 2017 4:44 am 
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Joined: Mar Sun 01, 2009 10:27 pm
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Location: Ann Arbor, MI
Perhaps there is some difference in the shape of the envelope of the CRTs that prevents the yoke from going far enough forward on the ones that do not fill the screen.

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 Post subject: Re: Zenith 23G22 Chassis Puzzle
PostPosted: Oct Sat 07, 2017 6:27 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Redlands CA
Both are a 54 degree deflection tube so no difference there.

I was going to suggest grounding the Dag too but I don't see how that would make a difference unless the capacitance of the tube is somehow causing an issue with the deflection. I would think that might affect it in both directions and not just the vertical.


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 Post subject: Re: Zenith 23G22 Chassis Puzzle
PostPosted: Oct Sat 07, 2017 4:36 pm 
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Joined: Jan Sat 16, 2016 10:03 pm
Posts: 316
Thank you for your responses.

I did ground the Dag with no results. I did use a small piece of masking tape to attach the wire to the dag until I can outfit the unit with a proper grounding spring.

I have other Zenith portholes that uses the exact yoke assembly with the 12LP4 sub crt that has great deflection and illumination. So I'm going to mark that off my list of a possible concern.

I'm thinking of checking the micas or just replace them all and mark that off my list. Any thoughts on this?

One thing I forgot to mention, is the 12UP4 crt was a bit dimmer than the 12LP4 however its been my experience the 12LP4 is a brighter tube.

I'll post pics of each tube illumination so you all can check out the difference, but I need to re-glue the socket on the 12UP4 first, its a little loose and I don't want to damage the pins because these tubes are nearly impossible to find.


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 Post subject: Re: Zenith 23G22 Chassis Puzzle
PostPosted: Oct Mon 09, 2017 1:08 am 
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Joined: Jan Sat 16, 2016 10:03 pm
Posts: 316
After spending much time trying to obtain a full screen picture on this Zenith 23G22 Porthole using a 12LP4 sub, I finally found the mistake I had made and I wanted to share it in case others encounter the same problem.

As previously reported I was substituting a 12LP4 crt for a 12UP4 CRT and the 12LP4 crt would not fully illuminate at the bottom even though it was an approved sub crt for the unit. I exhausted all measures.

The mistake I made was I used the ion trap that came with the 12LP4 (dual magnet) and after reading the Zenith Service manual once again, I notice a small notation of an ion trap specifications (or as the called it a Beam Bender). So I removed the dual magnet trap and installed the original trap that came with the unit (dual ring type), adjusted the rings as the service manual specified and with little adjustment effort the screen fully illuminated.

I thought an ion trap was engineered for the specific crt but I was incorrect and learned something new.


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 Post subject: Re: Zenith 23G22 Chassis Puzzle
PostPosted: Oct Mon 09, 2017 9:07 pm 
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Joined: May Fri 29, 2009 4:35 am
Posts: 1965
Location: Chicago, IL USA
An ion trap magnet is designed for a specific CRT type though many early B&W types are very similar if not identical. Look at a datasheet and you see single or dual magnets specific along with the strength in Gauss units.

I suspect you either had it on backwards or it has lost some magnetism over the years.


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