21FBP22 G1 Leakage Removal

[Kevin Kuehn]

I have this Zenith 21FBP22 that has great emission on all 3 guns, however it shows significant G1 leakage on red and green. Symptom with the set on is extreme smearing of the picture. No luck using the remove shorts function on my CR70. I'm not really sure which elements have the leakage between them, so was wondering if I dare use my Sprauge Tel-Ohmike to test for leakage between individual elements, then possibly blasting any pair that show leakage between them with a charged cap? Any suggestions on how to proceed by veteran TV repair folks would be greatly appreciated. I realize I don't want to blast anything in series with the heater element.

[old_coot88]

Is the smearing in the luma or the chroma? If luma, and since it goes in via the cathodes (and color via G1s), the leakage would likely be heater-cathode. Seems like anyhow.

[miniman82]

Try an isolator/brightner first in case it's H-K related, if no luck see if a capacitance tester will give you any readings between elements. If so you can bounce the values off the tube data sheet for interelectrode capacitance, and if something jumps out at you as odd then try zapping that pair of elements. I doubt you'll have much success if that's the case though, at typical cap voltages you're not likely to see anything change. I have a tube here that took a microwave oven transformer to clear out, it had a partial short/leakage between K and G1. One momentary blast at 1600v fixed it. I was worried it would destroy itself but it's still working fine, it's a metal cone 8" tube in my Arvin.

[Kevin Kuehn]

Don't have an isolation transformer for color CRT. I know I could rig something up, but I jumped to the next step.

Tried measuring inter-electrode capacitance but was not getting clear eye openings on red and green.

Next tried the Sprague insulation resistance test between cathode and grids -

Red gun: 350-400 meg between cathode and grid.
Green gun: Less than 150 meg between cathode and grid.
Blue gun: Infinity between cathode and grid.
Every other combination shows approaching infinity, so I'm convinced there's no significant heater to cathode leakage.

Measurements were with no heater voltage. I have a suspicion the leakage resistance will drop if I light the heater, but I guess that's beside the point now.

So this all coincides with what both my CRT testers are telling me. I suppose I'll try charging a 4.7uf cap to 600v(highest my Sprague will go) and see if it has any affect on the green reading.

[Kevin Kuehn]

Discharging a 600v 2uf cap across the green cathode-grid brought no joy, not even a little snap, although it did discharge the cap. So I decided to try it with the tube warmed up. That got a big arcy sparky at the clip lead when I connected, but several attempts would not open the short.

[jr_tech]

Was the grid positive with respect to the cathode? You probably drew a bunch of grid current.

jr

[dtvmcdonald]

There may be a way to live with the leakage. That is to install cathode or
emitter followers on the afflicted guns. Depending on the setup, which you
don't describe, you will like need them on both cathode and grid. DC coupled
of course.

[Kevin Kuehn]

The set uses a Zenith 26KC20 chassis. I don't know what I did, but amidst my fumbling the picture has cleared up and I have very good back and white tracking. I don't think I cleared any shorts because the green gun still pegs the meter for grid leakage test. I'm not aware that it will hurt anything to operate the TV this way? Maybe I imagined the whole thing.