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#1
11-05-2020, 10:48 AM
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Zenith ch 24NC31...Z
The Z prefix after the chassis number is an extended run of this last Zenith round 21" CRT color set. I am restoring this for a friend who is trying to match furniture and decor. Since he is also an appliance collector and has little room to add TVs, I have been saving this fully-appropriate match among my treasures. I was trying for a 20Y1C48 in a legged EA cabinet, which didn't materialize.
On a constant hunt for this same TV in a plain compact walnut with Scandinavian-style legs, Some late relatives had one like this, lasting some 12 years of brutal daily use. This loss leader had the best color design to date, with a retro CRT! Zenith was using the 25" rectangular for two years already in 25MC36 and 25NC38 versions. I wonder if they did not always figure on the reverse-use of a newer and narrower chassis for a budget 21" model, not rectangular either. Just like Philco's last 21" round chassis (17MT80 and 19MT79) which offered a hybrid design using all-transistor tuner, IF and video driver, THIS Zenith was what the budget-minded bought for their upgrade to color, after doing all the homework. See also RCA CTC-20 and GE CB-21, having their very own similar entry-into-color affordable sets  . While we may see the Z suffix and not think much would be different, this chassis had an obvious change in the audio tube to 6J10, Not obvious is the dozen more changes listed in Sams 803, folder 4 update in the Set 945 "production change bulletin". A must-have document if you encounter issues in one of these fine examples of the "service saver chassis". The issue pictured below is obviously loss of sync. My broadcast mod's channel 3 in the house is coming from the garage. Having the only conformed schematic for this set, a supplemental in Zenith CM-109 covering the aforementioned 25NC37/38. The resistor feeding the 6BA11's sync-side plate is suspect, stay tuned election results still unclear here.  Zenith 21X4122M-1967.jpg Zenith 24NC31Z-1967.jpg
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"When resistors increase in value, they're worthless" -Dave G Last edited by DavGoodlin; 11-05-2020 at 10:55 AM. 
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#2
11-06-2020, 01:01 PM
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Great to know all this good stuff about the "Last of the Mohicans" Zenith round model. Interesting that they used their best chassis of the time in their budget model... cool, and a very Zenith kind of thing to do... and still all handwired too! ;-)
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Happy Motoring!!
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#3
11-10-2020, 11:23 AM
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Success Saving Service
I checked video detector output with the oscilloscope, the video looked good at 6.0 v p-p but separate sound/sync detector was only 0.2 volts p-p when it needs to be 1.8 volts peak-peak. Oddly, Sams 803-4 does not show waveforms here but the Zenith schematic in a 24NC31Z supplement to CM-108 does show them. Factory schematic advantage here, but not always. I'm thankful for HW Sams regardless!
With essentially nothing going into the 6KT8 sound sync amp grid , swapping tubes did vary AGC response. I found an NIB Zenith branded 6KT8 with a 1961 date code, Zenith EIA code 343 in RED ink, not the usual yellow or white. I found a suitable suspect right there under the chassis, under an additional shield over the 3rd IF transformer and 6EJ7. Two glass diodes (1N60) common germanium can be seen tightly packed in. The video detector tested OK f-b, but the sound/sync read .3 ohms, a short. Finding just one such diode measuring .3 volts drop (not .6v like the Si diodes) in my junk drawer without robbing a parts chassis was a nice treat indeed. It has been a long time since I had a bad one, plus the detector was not in a list of case histories of trouble in the Tab Books service manual. Robert L Goodman must have thought that one just too elementary to include  keeping us retro-techs on our toes.After replacing the detector and the shield above it, see step forward with solid V + H sync locking-in nicely. there may be other caps obviously needed but most seem to be the orange and maroon drops. Electrolytics are staying cool and ESR was low on all that I tested. Each small group of caps I replace will be followed by a power up on the Sencore VA-48 to confirm any improvement. The convergence board alone has several suspect caps on it along with two convergence rectifiers, with dynamic so far out of whack, I can start there - see pic. Zenith convergence.jpg Even better than a simple diode was finding this set's model 21X4122M as " the Harvard" listed in a 1967 Zenith sales step-up guide / brochure I got from the 'bay, long before I got this set. One great thing about THIS chassis along with 25NC37/38 is no failure-prone efficiency coil and no VDRs in the HV section! I also have an empty cabinet of the same model in my garage, also from Tim in Buffalo. A big thanks to Tim and Justin (OSx) for both a complete set and one gutted, I'm still on a hunt for a chassis and appurtenances. Its just too good not to rebuild it! Thanks for reading my posts. VK need to members to know how easy these 60s Zenith are!!! Had this been one of my post-CTC-11 RCA color sets, much soldering on a PWB would be needed even before getting to this point!!!
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"When resistors increase in value, they're worthless" -Dave G Last edited by DavGoodlin; 11-10-2020 at 11:53 AM.
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#4
11-13-2020, 02:47 PM
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Just saw this post, encouraging! This chassis seems to be a good one in of the last of the Zenith (or any) roundies! Haven't been checking in here lately as we've been doing a lot of get-ready for winter, it won't be long before it's here, unfortunately for those of us warm weather people. Winter is a great time though for all those electronics projects!
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Happy Motoring!!
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#5
11-16-2020, 10:09 AM
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Quote:
I also have the 15" roundie Dumont B/W with a good CRT.
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#6
11-16-2020, 04:58 PM
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Quote:
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"When resistors increase in value, they're worthless" -Dave G
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#7
11-17-2020, 09:25 AM
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Quote:
It's the cabinet that seems to be the most common. I'll look at the model number and report back.
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#8
11-17-2020, 10:10 AM
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Quote:
It was 3 years of no UHF (exc 39-PBS in Bethlehem) and no rotor because the 5-wire Alliance T-45 rotor never did work because it was mis-wired via one of those wall-outlet plugs. Then Dad called the dealer for help at last. Coming to install an Alliance U100 rotor, B-T voyager preamp and simple 4-bay UHF antenna, I got off the bus that day and was more than impressed. After that, we got every one of the 23 channels listed in the newspaper! The e-skip brought in stations from Washington DC and NYC, solidifying my opinion of the brand. The JFD's "UHF section" using trapezoid log periodic element and round discs was a silly gimmick according to the Antenna man. Just 100 feet through the woods on the same ridge as our house, my Aunt & Uncle had that "Alton" Zenith roundie and it displayed a clear picture on all three (!) of those channels  with only a 4-bay bowtie and Finco VHF on the roof. every time I visited there (quite frequently) that '66 Zenith was on all day every day. It was on the entire time of holiday parties, birthdays, etc. They finally traded it on a '77 Chromacolor 25V with a slider varactor tuner, after our mutual Zenith repairman/dealer told them "I can replace tubes to improve the tired performance but the surge of increased voltages will risk your picture tube (?) and transformers." Of course, I found out after the fact. I wanted that old set in the worst way and maybe I did not speak up, so I was devastated when it slipped thru my hands. Every Zenith roundie I got my hands in since was never given-up on and donated like an RCA, GE or Magnavox, it was in my place for repairs.sigh:
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"When resistors increase in value, they're worthless" -Dave G Last edited by DavGoodlin; 11-17-2020 at 10:24 AM. Reason: add facts
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