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.
Attachment 201773
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!!!