Hi all,

I have a 21FBP21 that tests weak, but may wake up (it's a grade B rebuild). The green gun had what appeared to be a fairly high resistance G1 short. I used the "remove G1 short" function on my Sencore CR-70 several times, and once I did see a bright flash from the neck which the manual says is the short being "blown away." But each time I would retest, it still indicated a short. The short gets worse (lower resistance) the longer it runs. I verified the short on a second CRT tester (a CR-31A).

The CR-70 manual has a procedure for removing stubborn G1 shorts. They have you disconnect G2 and use the "manual restore 2" function for 20 seconds and then quickly switch to "remove G1 short" and blast it there. I did this twice, and the short remains. However, now the green gun tests excellent for cutoff and emission. Could it possibly be usable like this? I thought there would be no cutoff control with a G1 short. I assume that the emissions went way up because of the restore function.

Any other G1 short removal tricks to try or is this thing toast?

Thanks,

Clark

Can you measure the short with an ohmmeter? Perhaps its not inside the tube.

That's a good suggestion. I just tried it with the tube cold, and it reads infinite resistance between G1 and the cathode or G2. I'll try it with the filament hot. Maybe the Sencore is being fooled somehow.

Thanks,

Clark

Interesting. The short is only there when the heater is on. I watched the reading go from infinite to a low of about 600 Ohms as the tube warmed up. When I switch off the filament, the reading slowly rises back to infinity. It's obviously not a metal-on-metal dead short, but if it's a piece of debris between the cathode and G1 why doesn't it show when the tube is cold? I understand that the elements expand when they're hot, but would that cause the reading to change so gradually? I would think that it would suddenly go from infinity to the shorted reading when things expanded enough to make contact. I'm not sure what to make of this. Meanwhile, the cutoff and emission still test good.

Do you measure the same hot resistance with the meter leads reversed?

jr

Wow, when I reverse the leads it shows infinite resistance! The "short" or whatever it is acts like a diode when hot. What does this mean? Dissimilar metals creating diode? I wonder if this means I could blow the short away if I reversed the polarity of the "remove G1 short" function.

Thanks for the insight, jr!

-Clark

The hot cathode emits electrons... if the grid is positive (voltage supplied by your ohm meter) current can flow, and the meter indicates some resistance. When you reverse the meter leads the grid will be negative with respect to the cathode and no current flows... open circuit. Usually on ohm meters the red lead is positive.
You have indeed have a diode.

jr

ps: I don't know how the remove shorts function on these checkers works... do you actually measure a short between cath and g1 or g2 and g1 with an ohm meter (cath hot or cold) that is not caused by diode action.

.

In case the previous post wasn't clear - this is normal now, no need to fix anything any more. Your ohmmeter was giving a normal indication for an hot, emissive cathode.

JR and Old TV Nut,

Thanks very much for the help. I guess I should have realized what was going on (basic vacuum tube theory). But what I don't understand is why the CR-70 indicates a short in this instance. I believe that it just functions as an Ohm meter measuring the resistance between the cathode and G1 in the "G1 shorts" function. You'd think that this method would yield a lot of false positives because of the diode action. Maybe the cathode and G1 are unusually close together in this case. Anyway, I'm glad that it doesn't make the tube unusable. Now if I can just get the emissions up on all three guns...

-Clark