how to test 7JP4? - Videokarma.org TV - Video

old_tv_nut · #1 08-08-2010, 02:53 PM

Nick is over here with his newly acquired 7JP4, and we're wondering what the commections should be to test it with the RCA CR-III CRT tester. (7JP4 is not listed in the booklet).

Appears to take the same socket as a color tube, but I suspect we have to use the clip leads instead.

David Roper · #2 08-08-2010, 03:01 PM

Heater is on pins 1 and 14, cathode is pin 2, g1 is pin 3.

Hope that helps.

Phil Nelson · #3 08-08-2010, 03:07 PM

This page has the 7JP4 settings for my Sencore CR70, if that's useful.

http://antiqueradio.org/art/temp/7JP4TestData.jpg

Phil

old_tv_nut · #4 08-08-2010, 03:38 PM

thanks

Edit - we just went through the procedure and cleared a K-G1 short, but see no emission (kind of expected) - but we are checking for loose pin connections now.

miniman82 · #5 08-08-2010, 05:53 PM

Base is completely removed now, and connections are direct to the leads. Still nothing, it's dead. Oh well, I half expected it.

bandersen · #6 08-08-2010, 06:26 PM

It's not necessarily dead. Are you sure your tester can handle it ? When I rigged up my B&K 440 to test 7JP4s, the needle just barely moved above 0 for a good CRT.

Another possibility is that the cathode is open after you blasted out the short. It might be possible to weld it back on.

If you can get it over to me, I'd be glad to check it out on my Sencore CR70.

miniman82 · #7 08-08-2010, 06:36 PM

Honestly, I think the 'grid/cathode' short was in the base. We read it out with a DMM, and it gave a 500k reading. It was really loose, with stuff rattling around in it anyway. Where in Chi Town are you?

bandersen · #8 08-08-2010, 11:42 PM

Quote:

Originally Posted by miniman82

Honestly, I think the 'grid/cathode' short was in the base. We read it out with a DMM, and it gave a 500k reading. It was really loose, with stuff rattling around in it anyway. Where in Chi Town are you?

PM sent.

Tom Albrecht · #9 08-10-2010, 12:39 AM

You really need a tester that is designed for it. The Sencore CR70, which I have, is designed to test both TV CRTS and scope tubes. 7JP4 gets treated as a scope tube. If your tester doesn't do scope tubes, it probably won't test a 7JP4 properly. Sounds like you're going to pay a visit to banderson and his CR70. That should clarify the situation!

Steve McVoy · #10 08-08-2010, 06:44 PM

My experience is that emission testing of 7JP4s doesn't really tell if the tube is good. I've had several that tested OK, but produced very washed out pictures. The only way to tell for sure is to put them in a working set.

cbenham · #11 08-11-2010, 02:10 AM

FYI, the Eico 625 tube tester has an CRT Testing accessory kit with a socket for 7JP4s. It includes a list of CRTs with test switch settings for them.

Cliff

miniman82 · #12 08-15-2010, 11:51 AM

Great news: I had a brain storm!

I remembered that my brother is building me a new o-scope built from parts of an old B&K, so it dawned on me that the chassis he built (which is just a power supply at the moment) could possibly power the 7JP4. The scope has a 5DEP1 electrostatic tube, which runs a 2kv anode. We soldered wires to the 7JP4 and ran them to the socket on the scope chassis- it worked! We were able to get a nice bright white spot, even though the anode voltage is a lot lower than the 7JP4 requires. I am now confident the tube I have will at least show a picture, so I'm ordering caps for the chassis right away. Still need a new socket for the tube, though.

bandersen · #13 08-15-2010, 01:27 PM

Congrats! That's a great idea

On the flip side, I use a 7VP1 scope CRT while working on TVs to avoid damaging a7JP4.

miniman82 · #14 08-15-2010, 01:47 PM

Any idea where to get a new base, since I broke the old one apart? Ebay only has sockets, and I need something with pins...

bandersen · #15 08-15-2010, 02:53 PM

Maybe scavenge one from a picture tube brightener ?