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 Post subject: Rejuvination of a 7JP4
PostPosted: Sep Mon 14, 2009 12:54 am 
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Location: San Jose, CA USA
I managed to purchase a 7JP4 in unknown state from a vintage electronics sale here yesterday for just $5. It was mixed in with a bunch of radar and oscilloscope tubes, which were obviously selling very cheaply.

Testing the 7JP4 on my National Union Videotron CRT tester, it registered no emission at first, but after a few hours with filament voltage on, it showed a small amount of emission, in the "bad" region of the tester scale. The Videotron is a very simple neon-bulb tester, which does not exactly inspire confidence, but it is the only tester I have which is designed to test early CRTs including those with electrostatic deflection. It applies a higher G2 voltage on electrostatic tubes.

Testing the tube on a Sentinel 400TV television, the tube showed a somewhat dim picture, although not drastically dimmer than the "good" testing 7JP4 in that set (casting some question on the ability of the Videotron tester to really evaluate a 7JP4 very well).

Having little invested, I thought I'd try rejuvination with my Sencore CR-70 CRT analyzer/restorer. Using its universal adaptor (nothing but a bunch of alligator clips to be connected to the appropriate pins of a test CRT), I connected the filament, cathode, G1 and G2 wires in the usual way. On the Sencore, which is not designed to test electrostatic tubes, I could get it to read "10%" emission with the "cutoff" adjustment set to allow maximum current.

I then tried the automatic restore function of the Sencore, and it went through its automatic three cycles as it should. Some sparking was visible inside the electron gun, just as would occur on a more modern CRT.

After this procedure, the 7JP4 read "20%" on the Sencore, and read well into the "good" region on the Videotron. On the Sentinel TV, it did look noticeably brighter -- just as good as the CRT that was in the set. Picture was as sharp as any of the 7JP4s I have (not as good as a magnetically deflected CRT, though).

I have no idea how long the improvement will last, but if you have a weak 7JP4, it could be worth a try. Don't worry that your regular CRT tester will show pretty low emission on a 7JP4. Try some rejuvination, and see if you can get the emission to increase.

If you need to make an adaptor, the connections are:

electrode............12-pin base.....14-pin base (7JP4)
----------------------------------------------------------------
filament 1...................1.............................1
filament 2..................12...........................14
cathode.....................11............................2
grid #1.......................2.............................3
grid #2......................10............................9

Would be interesting to hear whether anyone else has some success with this, especially with a tube that has much lower emission than the one I tried today.

Info on the Videotron tube tester can be found here: http://www.antiqueradios.com/forums/vie ... c&start=10 . A few posts further down there is some mention by another person of success rejuvinating and testing 7JP4s with a Beltron unit.

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Last edited by Tom Albrecht on Sep Mon 14, 2009 4:12 am, edited 1 time in total.

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 Post subject:
PostPosted: Sep Mon 14, 2009 2:07 am 
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Location: Detroit, MI USA
Years ago I made up an adapter to go directly from the 7JP4 base to the more common 12-pin type, so that it could be plugged into any CRT tester-rejuvenator.

I used it on several B&K units which I had in the shop and at home, and also some other assorted types ranging from the vintage Cathode Beamer from the 50's on up.

Every 7JP4 I tested was nearly dead as far as emission, leading me to believe none of the testers had an appropriate bias setting for that tube even when wide open. While it was possible to get arcing in the gun when attempting rejuvenation, it usually didn't make a big increase in the emission reading, nor did the picture get as bright as one would have wanted it to be.

My conclusion is that you would need to know the proper voltages that need to be applied to a 7JP4 in order to test it. I am thinking that none of the commonly used testers are capable of doing that. The 7JP4 "G2" is actually part of the anode, and runs at 4-5KV in a set. A tester putting even 300-400 volts on that element is not going to be capable of getting a meaningful reading. This is why the bias needs to be all the way to the end to get any reading.

I always wanted to build a dedicated tester for the 7JP4 capable of applying at least 1 or 2 KV to the "G2" in order to more accurately measure emission.

However, just for testing functionality I find it much easier to plug the tube into a working chassis, and looking at the picture.

BTW you would have to see a brand new 7JP4 to realize just how good the picture on one can actually be. I was lucky enough to find one in an old dealer's attic about 20 years ago, and only used it one time for a few minutes as a test to see what it really should look like. I was very surprised, and have seen very few that come closer than about 20% to that level of performance.

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 Post subject:
PostPosted: Sep Mon 14, 2009 2:13 am 
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Location: Cascadia
Tom,

Thanks for the report. I have a stack of 7JP4 tubes sitting on my shelves and I don't have a tester for them either. But I do have a Beltron and a Sencore CR-31. I think I'll give your approach a try.

I'd been waiting to buy or build a tester for electrostatic tubes, but they just don't show up...


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 Post subject:
PostPosted: Sep Mon 14, 2009 4:27 am 
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Following up on Dennis's comments a bit, I thought I'd measure the voltage that the little Videotron tester applies. For regular magnetic deflection tubes, it puts 250 VAC (349 V peak) on G2. For electrostatic tubes, it puts 500 VAC (707 V peak) on G2.

I have not checked what my Sencore applies.

I wonder if perhaps the rejuvination might still work, since I believe the rejuvinator likely puts a high voltage on G1 as well as G2.

Nonetheless, it doesn't sound like the rejuvinated 7JP4 is as good as the new one Dennis mentions he has worked with.

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 Post subject:
PostPosted: Sep Mon 14, 2009 2:49 pm 
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Location: Detroit, MI USA
Someday when I have time I'll have to find that new 7JP4 and take some photos of it running in a restored chassis. I was really amazed, while not as bright as an aluminized tube you could have easily watched it with the set in a brightly lit room. And there was plenty of contrast, without washing out the highlights. Very similar to what you would see on a strong non-aluminized 10BP4 for a comparison.

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 Post subject:
PostPosted: Sep Mon 14, 2009 5:15 pm 
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Location: S. Dartmouth MA 02748-1225 USA
I too, have rejuvenated a 7JP4. I used an Eico crt tester and clip leaded it to one of the crt adapters that permitted access to the circuits.

Results were good but short lived. The crt proved to be gassy. Lasted about 15 minutes and faded down to what it was before the process.

Best,

Chas

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Last edited by Chas on Dec Tue 01, 2009 8:50 pm, edited 1 time in total.

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 Post subject:
PostPosted: Dec Tue 01, 2009 5:29 pm 
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Joined: Nov Wed 04, 2009 8:41 pm
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Location: Plano, Texas USA
Has anyone tried to rejuvenate a 3KP4 from a Pilot TV? Mine is very very weak.


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 Post subject:
PostPosted: Dec Tue 01, 2009 8:52 pm 
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If you have a pinout of the crt, operating parameters and a picture tube rejuvenator that can do B & W tubes, there is no reason that it cannot be tried..

Chas

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 Post subject:
PostPosted: Dec Tue 01, 2009 9:56 pm 
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Location: Woodinville, WA USA
Here's a 3KP4 data sheet.

http://scottbecker.net/tube/sheets/049/3/3KP4.pdf

I have been thinking it might be fun to try testing mine with the CR-70's universal adapter. One of many little science experiments I haven't gotten around to yet.

Phil Nelson


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 Post subject:
PostPosted: Dec Tue 01, 2009 11:17 pm 
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The problem with those picture tubes, both types, is the cathode has very little comparative coating on it. They may not survive a rejuvenation.
Add to that a weak filament in 3KP4, and, you are asking for double trouble.
Bill Cahill

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