Zenith 20Y1C38

[DaveWM]

Going over the schematic, a few questions

there are some transistors I am just wanting to make sure I know what I may need, but the cross to the zenith part number comes up with various possible choices. Looks like a NTE 107 would do for all if I am reading that right. or is a rat shack equivalant unit available? its a NPN silicon, used in UHF tuner, video amp and the AFC buffer amp. the UHF tuner is jammed up, so I dont know if it is working, the AFC seems a little flaky but that may be the slide slide switch, and the video seems ok but I need to check what I think is a decoupling cap (C3 50uf) on the collector. getting some kind of interference in the video.

[drh4683]

I assume you used the NTE cross reference thats on the web? The 121-587 which I believe is a video driver in that chassis is replaced with an NTE-123AP. In fact, I think the 123AP replaces many of the different types all together. Not sure what you would use from radio hack though.
Intermittent AFC is most always caused by the switch. Give it a spritz with some contact cleaner and troubles should be over.
All tubes are tested and good? Intermittent video problems/interference? Alot of times those IF tubes have intermittent contact in the sockets and can cause flashing and odd syptoms on the screen.
Cathode current should be under 200mA. Once you find the lowest setting, turn the core into the coil 3-4mA above that point.

[DaveWM]

I will try the cleaner on the switch. It's also kinda stiff, so I am guessing some corrosion in there.

I did use the transistor sub from NTE, using the sams listed GE-12 part number. I did not even think to look at the actual transistor, duh.

[drh4683]

Stiff tuner is a problem in early production concentric tuners. The 1968 models used an all plastic VHF channel drum, which has a tapered plastic shaft as part of the mold in the center. The UHF drum is also plastic and fits into the VHF drum. The plastic basically deforms, either the UHF drum shrinks, or the VHF drum shaft expands causing the two to lock up. Lubrication will not cure it. You will need to take it apart and use emrey paper to sand the tapered shaft on the VHF tuner. You are only removing a few thousandths of an inch of plastic, just enough to make the drums able to turn freely again. You want no stress on the dial, it should spin free before you put it back together. You will also notice that you will need to add an additional washer to gain clearance between the VHF and UHF drum as the spring tension on the top of the UHF tuner caused enough stress in the center of the VHF drum which ends up deforming the VHF drum. This then caused the tuner to no longer contact the thrust washer in the center and the outside edge of the UHF drum would then come in contact with the VHF drum, causing additional drag (and wear).
Its certainly not the best tuner design around, Zenith could have done much better. I've taken many of them apart, so I am very critical of the design. However, By 1969, Zenith must have realized the design problem (considering an improvement was made only 1 year later, most likely caused by heat from the tubes causing deformation to the plastic components) as they incorporated a brass sleeve insert for inside the UHF drum. This solved the stuck tuner drum problem, but the spring tension on top still caused the VHF drum to felx enough to still contact the OD of the UHF drum. That alone still caused enough drag to make the tuner difficult to operate.
It might sound more complex than it really is, you will see it all when you take it apart.

[zenithfan1]

You nailed it Doug, your trick worked wonders on the set we talked about last year. It works like a charm still It's funny how that shaft seems to swell up and seize, or the drum? Either way it's weird, maybe the plastic starts to break down over time and warp. I've seen old plastic do funny things LOL.

[DaveWM]

roger all that Doug. I have kind of put it on the back burner for a bit, got to get the shop picked up, get ready for holidays etc...

I will prob send in the tuner for a rebuild after I get it all out and deal with the stuck UHF.

I still need to replace the caps and take a shot at the purity/convergence (the convergence is off). And of course I still plan to add that HOT cathode fuse.

Anyway back to the shop....