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Old 12-28-2017, 09:33 PM
mbates14 mbates14 is offline
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Zenith 24NC31Z

I recently located my old Zenith 24NC31Z, I think I had a thread on here over a decade or so back when I was a teenager in collage, working on it.

Well it hasn't been powered on for over 13 years, and it was missing for 10 of those 13 years. The fact that I got it back is amazing in of itself. But I digress...

Anyways I had to purchase a new copy of the Photofact online as a PDF, due to the the original I obtained (maybe from here?) that many years ago is long gone along with all of my other papers from unfortunate events, etc..

The reason I am posting this thread is I want to do the composite video and audio mod, but looking at the schematic, its not as straight-forward. Any thoughts on this? Main 2 reasons is to eliminate the "buzz", and any alignment issues.

Also during the time it was missing, someone had man-handled the set and bashed in the back of the CRT and crushed the base, as well as the knob set being gone and tuner laying in the bottom of the unit.

Fortunately, I powered it up for the first time and the CRT still produces a raster, so I will need to locate a new base for the 21FJP22 and attempt to replace it. Anyone have one?

Last edited by mbates14; 12-28-2017 at 09:42 PM.
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Old 12-28-2017, 11:40 PM
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Great to hear you got it back. Just wondering, how exactly did you lose, and then find a television set?
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Old 12-29-2017, 08:27 AM
mbates14 mbates14 is offline
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Divorcing parents, house foreclosure, property theft when I was staying with others, etc.. I ended up losing nearly 80% of my property/personal belongings during that fiasco. I was staying with family after the divorce/foreclosure. Up until their serious Heroin habits drove me out as I wanted no part of that. All is well though as I ended up moving 8 hours south away from that mess, now living in North Carolina. Now that I mentioned Heroin, you can probably start to assimilate the picture on what happened to my possessions. Also during a large portion of that time period I was unemployed after collage, basically leaving me with not a whole lot of control over what was going on, and giving me no choice but to live with family at the time. So that made things that much worse.

My cousin had it at his house when he moved out, using it as a table! Family were clearing his house when he went to jail. Thats how I got it back. Otherwise, Really dont want to discuss more about it.

Anyways, I got tired of playing the victim so I ended up disowning/annexing my family and moving south as I mentioned. I now run a small start-up business with other shareholders, and I am the head electronics engineer/programmer. That past is now behind me, and so is my family unfortunately.

Last edited by mbates14; 12-29-2017 at 08:41 AM.
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Old 12-29-2017, 10:26 AM
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Wow thatís not what I was expecting. Iím looking forward to following your progress.
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Old 12-29-2017, 10:48 AM
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Wow thatís not what I was expecting. Iím looking forward to following your progress.
The 24NC31, the last Zenith roundie? If so, my favorite!
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Old 12-30-2017, 03:30 PM
mbates14 mbates14 is offline
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Yep I think so.

But, back to my more immediate concern, anyone know how to do the composite video mod on these? As I said looking at the schematic it doesnt seem as straight forward as the audio/sync is amplified and separated before it gets to the video detector diode.
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Old 01-02-2018, 12:48 PM
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Thanks for sharing the story. it must have been awful not knowing if you would get it back at all. Evil forces are constantly trying to separate us retro-tv folks from our stuff.

Recently, I traded a last-GE roundie for one of these last of Zenith 21" round color sets too, in an Early American Maple cabinet. Not the most desirable furniture but clean. After all, the insides are what sold most Zeniths.

Once I get mine from safely buried in the garage into the house and up to a third floor room, Ill have a look at the standard Zenith issues. Bad caps and faulty controls, plus the horiz eff coil which flies apart may have been changed from earlier chassis, Im not sure.

The CRT socket is not a big deal. I replaced a busted one on an RCA of mine here.http://www.videokarma.org/showthread.php?t=267995 the outcome was good using the flowable silicone. Use a vacuum desoldering tool and heat each pin to remove solder, bend pin a bit and it comes right off, but do not to move any of the 12 copper-plated steel wires right where they emerge from the CRT. If you need a good base, a plug-in CRT brightener is one source.

A Zenith may be more adaptable to S-video than to composite video, the sync in may need to be reduced in amplitude and possibly inverted before feeding to the 6BA11.
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Last edited by DavGoodlin; 01-02-2018 at 01:10 PM.
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Old 01-02-2018, 05:18 PM
mbates14 mbates14 is offline
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Yea it was pretty much at the point where I wrote everything off as long long gone. Even today 99% of what was gone, is still gone. Just amazed that this one returned. On the vintage computing side of things, I had a handful of Model M keyboards, and other PS/2 stuff that "disappeared" as well as a handful of radio equipment including a tube type CB.
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Old 01-02-2018, 09:28 PM
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I have not had time to compare schematics, but if the last Zenith roundy is the same as the first (which I did the mod on), you want to take composite video and feed it to the output end of the two detector diodes on the vidoe IF chain, disconnect the audio detector from the amp (to keep noise out of the sound) and inject audio there.
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Old 01-03-2018, 07:37 AM
mbates14 mbates14 is offline
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nope, completely different from how I gather you explaining it, and what I see in the schematics.


Last edited by mbates14; 01-03-2018 at 07:46 AM.
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Old 01-03-2018, 11:49 AM
dieseljeep dieseljeep is offline
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nope, completely different from how I gather you explaining it, and what I see in the schematics.

I never heard of that audio/detector tube and I worked on several of those Zenith sets.
How could Sams come up with 6JB8 from 6J10/6Z10?
When working on Zenith products, I always used the factory servicing information.
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Old 01-03-2018, 01:00 PM
mbates14 mbates14 is offline
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Well I dont have zenith factory anything, so....
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Old 01-03-2018, 01:03 PM
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Inject video after the video detector diode. The sound-synch amp V6A is really just part of the AGC circuit. Zenith tied the AGC and synch sep circuits together in one tube V7A, and they interact a bit, but the sync sep should work fine without the AGC system... The idea of tying the AGC to the sync in that manner was to throttle the IF gain and seperator threshold in such a way as to achieve best sync on snowy deep fringe reception...Injecting video makes all of the AGC system meaningless.
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Old 01-03-2018, 01:33 PM
mbates14 mbates14 is offline
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What about the audio? I noticed the audio circuit has a tube that serves both the detector and the pre-amp. So I assume I would inject the audio at the Grid of V8?
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Old 01-03-2018, 09:42 PM
mbates14 mbates14 is offline
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I went on ebay to look around to see if there were any knobs or anything for my set, I sat astonished on the prices that TVs are listed for. even for BPC stuff its ridiculous in my opinion. Starting to wonder if the American Pickers, antique roadshow, storagewars, pawn stars, or any of the "insert reality TV show here" have brainwashed the ebay sellers. "Hey, I found an old TV in my parents barn, I could get $500 for it!" Craziness. I almost thought about seeing if I could expand my collection a bit, but not from ebay. Sorry for the rant.
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