Oh, No, Purple Glow!
 

Another long-winded story from Bryan...last year I was given a Zenith Flash-Matic. The crt is all used up. 24 hours at 8 volts barely moved the needle. Into the corner it went. The set has a 21ALP4 and while there are a lot of subs listed in the different books, I didn't have any of them. Well, the other day I was checking out my old DuMont 21" set, about the same age, and found that the Sams lists it as using a 21ATP4, one of the subs. But when I checked it out I found the original had been replaced with a Silverama 21CBP4, not on the sub list...but I have a NOS Channel Master 21CBP4 up in the attic! If it will work for the DuMont it should work for the Zenith. Well, when I saw this I couldn't wait to make the swap. (I had been saving that tube for a Motorola I have that is watchable but weak)

So, I pulled the chassis (thats a big crt to be chassis mounted!) and did the switch, no big deal. The neck is a lot shorter on the CBP4 but it fits. (inside the crt box is a substitution guide listing this as the appropriate swap) While it was out I swapped the caps in the vertical circuit, since the set had the typical height/lin problems. I put things back together, plugged her in and threw the switch. And then...

THE PURPLE GLOW OF DEATH :sigh:

Yes, that Channel Master, waiting in a box since 1969, has apparently gone gassy. I guess. I get a fantastic purple glow in the neck. So I tried the crt tester and the filament lights fine but no emission. I crank it to 8 and let it sit, when I come back the emission reads great but there is a faint purple glow. I wish the tester was better, it's pretty basic. I've manually looked for shorts and found none. I've tried running the heater off an isolated power supply, no change. I don't guess there are any other ideas out there? Yeah, I also used my handy-dandy crimping tool on the pins in case there was a bad connection. This crt being a dud isn't the end of the world, but I just don't want to toss it prematurely.

It is good to know that a CBP4 will fit here...I have a couple sets I could part out and get usable crts, if I could bring myself to do it. The DuMont, for one, would make a good donor. One last try on my "Spring Cleaning" thread, I guess. I'll have a pic on there soon.

There for a moment I thought you were watching a Prince music video! ;)

Bryan, you say after running it at 8 volts the emissions now check great?
but it still doesn't work?

Correct...oddly, I tried again the next day, starting it off at 6v and no real emission showing so after a minute or so I turned it to the "color" position (which I think is close to 8) and left the room. When I came back, sure enough, the needle was high in the good range and there was a noticeable glow in the neck. When I turn the control from emission down to "test for shorts" one of the lamps glows dimly for a few seconds and then goes out, and I can repeat that. My initial thought was a short, perhaps only showing up after warming, but I can't find one. I did resistance tests with just the filament hooked up and no shorts between any elements. When the tube tester is showing the purple glow and I switch off the grid voltages the glow dissappears. My guess is that the tube has enough vacuum so that the filament will light but is too gassy to work properly. I did not immediatly blame the crt since I saw on a CTC-31 a purple neck glow caused by a defective video amp. That crt was fine. I can't blame the chassis here, though, since I can duplicate the problem with the tester.

That really sucks. Best of luck finding another one.

John

A final expriment would be to put the suspect CRT in the chassis and operate with a CRT britener, so the filament is at 8V, and see if you get any display and or purple glow.

Good luck!

To update, I powered the filaments off of the crt tester at 8volts and ran the rest off the set, but the glow remained. I let it run for a few minutes, tapping the neck and playing with the filament voltages, to no avail. Also tried holding a magnet up to the neck, just for the heck of it. Nope. As a last effort I pulled off the socket. Of course, 2 of the wires had to break despite my being careful. One of the grid wires, which left enough of a nub, but naturally the focus wire broke and did so flush with the glass. I was able to get a temporary connection for testing-I thought perhaps one/both of the wires was already broken and that was the problem. No, still the same. So, I am condemning this tube. I hooked the old crt to the tester again and it really is used up. The needle barely moves after hours at 8volts. I tried rejuvanation, and the filaments were lighting up like 12volts but as soon as it goes back down so does the needle. So, 2 bad 21 inchers...which bodes poorly for that DuMont across the room. Its good 21CBP4 could end up in the Zenith real soon!