Tube tester for a Predicta 21EAP4

[El Predicta]

I've tested/rejuvenated scores of Predicta tubes. Those can be delicate. I would never use anything but a Beltron on a Predicta CRT .

[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.

[rickfray]

I got my current meter and completed the current test on the 21EAP4 and as suspected from the previous CRT tests with the heater set to 2.3v, the current tracks to the range of 2.3v tube - 520mA. Attached are picture of the meter and CRT tests.
http://videokarma.org/picture.php?pi...016515&thumb=1
http://videokarma.org/picture.php?pi...016515&thumb=1
http://videokarma.org/picture.php?pi...016515&thumb=1

Rick

[charokeeroad]

I couldn't get the pictures but it looks like from the info you've gathered that you've got a 2.3V CRT and a good one at that. Try the "go advanced" button to download your pics, then "manage attachments".

[rickfray]

Originally Posted by charokeeroad

I couldn't get the pictures but it looks like from the info you've gathered that you've got a 2.3V CRT and a good one at that. Try the "go advanced" button to download your pics, then "manage attachments".

Yes, that's what I believe. Let's see if I get the pix displayed right this time.

current reading with filament set to 2.3V

emissions test

cutoff test - setup chart said 28-72 and it's right on 30.