Tube tester for a Predicta 21EAP4

[Tom Albrecht]

I'm working on a 17" Predicta right now. The customer mentioned that the CRT was known to be weak, so I tested it when it arrived and found it to be nearly dead in terms of emission. Increasing the filament voltage to about 4 volts brought the emission up into the good range, and I wondered whether it might be a 6 volt replacement tube. However, visual inspection confirmed it was an original 17DRP4 with a 2.68 volt filament. I gave it a shot of "auto restore" on my CR-70, but the emission didn't improve much.

Testing in the set, however, surprised me. A nice bright picture! This set had been "minimally restored" (just replace a few problematic components) by a previous tech, so I started working on the full recap, etc. After a few days of minimal usage of the CRT, I noticed it went suddenly quite dark, and now looks like the weak CRT it always appeared to be on the tester.

After further investigation, I found that the previous tech had moved the filament wire for the CRT from the 2.68 volt tap on the transformer filament winding to the 6 volt tap. That's why the tube looked so good for a short while! The filament didn't burn out even after a few hours, but the cathode emission did not hold up over time.

Now I know this CRT is really dead.

By the way, the CRT filament did not look unusually bright at 6 volts. These Predicta CRT filaments have a rather dim glow at their proper voltage. Don't be fooled by that into thinking you have a 6 volt tube when you actually have a 2 volt tube.