Two 15GP22s At TheBay

[Robert Grant]

Question about testing a CRT in unknown condition:

If a CRT has gone to air, but nobody has powered the filament since before the vacuum was lost, could such a CRT be restored to operation by re-evacuating the tube, without have to do a total rebuild (assuming any leak would be extremely slow or could be found and closed)?

I say this, because I wonder if a test of an unknown CRT should start with a very low voltage continuity test of the filament, then, if there is any conductivity, doing some type of test for any presence of air in the tube.

Rather than just hooking up the tester, which would destroy the gun of the tube if in fact the tube had gone to air?

[Eric H]

Originally Posted by Robert Grant

Question about testing a CRT in unknown condition:

If a CRT has gone to air, but nobody has powered the filament since before the vacuum was lost, could such a CRT be restored to operation by re-evacuating the tube, without have to do a total rebuild (assuming any leak would be extremely slow or could be found and closed)?

My understanding of it is no, once the cathode has been activated exposure to air destroys it.

I'm sure someone who really knows the answer to this will chime in.

[Phil Nelson]

Testing an unknown CRT is certainly a common scenario. Perhaps one of the sages could tell us all what the potential risks might be.

Phil Nelson

[Phil Nelson]

Originally Posted by Phil Nelson

Testing an unknown CRT is certainly a common scenario. Perhaps one of the sages could tell us all what the potential risks might be.

So, what is the layman's answer to this question? If you fire up an unknown CRT on a tester, are you risking anything for a future rebuild? Or do you just go ahead on the theory that the damage (if any) has already been done?

Phil Nelson

[andy]

---

[Phil Nelson]

Originally Posted by andy

simply applying HV to the CRT while the socket pins are grounded will tell you if it's very gassy.

What would you look for to tell if it's gassy? Glowing fog or other light show in the neck? Sorry to be so dense, but I haven't tried any of these experiments -- or had a gassy CRT, for that matter.

Phil

[andy]

---

[jr_tech]

Originally Posted by Phil Nelson

So, what is the layman's answer to this question? If you fire up an unknown CRT on a tester, are you risking anything for a future rebuild? Or do you just go ahead on the theory that the damage (if any) has already been done?

Phil Nelson

If you fire up a gassy CRT at full heater voltage, it is very likely that the heater will quickly burn out. Since the heater and cathode are replaced during a rebuild, it is unlikely that a future rebuild would be adversely affected by doing so.

However, once the heater is burned out, a rebuild would be the ONLY option. No possibility of a rework (however slim) would exist. I guess that I would prefer to leave as many options open for as long as possible. Perhaps an attempt to seal the leaks followed by a getter re-flash and cathode re-activation could still rescue the tube, as long as the heaters remain intact. However, this is very unlikely and perhaps a waste of time and money, but at least the option remains open. If I had access to an RF generator and a getter "wand", I would try a getter re-flash in a New York Nanosecond... not much to loose!

jr

[jr_tech]

Originally Posted by Robert Grant

I say this, because I wonder if a test of an unknown CRT should start with a very low voltage continuity test of the filament, then, if there is any conductivity, doing some type of test for any presence of air in the tube.

Rather than just hooking up the tester, which would destroy the gun of the tube if in fact the tube had gone to air?

When I first powered up my 22EP22 after sitting for 15+ years, I did just that, and brought the heater voltage up slowly. I did NOT want to see a flash from the heater that would indicate that I had destroyed the tube!

I was fortunate, the heaters appeared to power up normally. Had they not, I really had no game plan, and I doubt that the tube could have been saved anyway... Perhaps long term low voltage on the heaters would "pump" the tube somewhat... perhaps the getters could be "re-flashed"... but at least the heaters would be intact for further experiments.

jr

[jeyurkon]

During the activation of the cathode BaCO3 is dissociated to BaO. Subsequent heating at a higher temperature then reduces some of the BaO to Ba with the help of a reducing agent like Si. When the tube goes up to air the Ba oxidizes back to BaO or BaO2. Mostly BaO I believe. There are other alkali's involved too, but Barium is the major one.

You would need to reactivate it, but the reducing agent should have been mostly used up during the first activation.

There would a significant expense in trying it, with no way of knowing the status of the cathode before hand. I don't think it would be worth it. If I spent that much I'd want to know that I had a fresh gun in the end.

John