7JP4 save - Videokarma.org TV - Video

Tony F · #1 05-28-2021, 11:47 AM

I managed to pickup this Admiral off of FB marketplace cheap. It does have a cabinet although not in the best of shape and no outer knobs. The fine tuning control is there, but the channel knob is broken in the usual spot. It is missing the speaker. I felt sorry for it and thought it might be good for parts. Upon testing the 7JP4 it showed nothing, I carefully removed the end cap and three of the pins soldered on to the CRT crumbled away. Pins 8,9 and 10. Amazingly the wires stayed with the CRT and somehow broke loose from the solder joint. Closer inspection revealed why. About half of the pins were green with corrosion. Pricing a new socket with shipping would be about $50 (no thanks). I proceeded to remove all of the pins from the bakelite cap and install new ones from a few 8 pin tubes that tested bad. (yes, they are the same size). Using a dremel, some small drill bits and some needle-nose pliers I was able to reinstall the used pins in the old socket. I soldered some wire-wrap wire to the ends of the CRT wires after I removed the oxidation. I used some windshield and water leak silicone around the neck of the tube to reattach the cap. The problem I realized was there is more of a gap between the glass and the cap than there seems to be with regular tubes, so there is still some "play" after it has set. I should have left the old epoxy inside the cap and not scraped it out. Although it is "catch -22" as it was kind of in the way from being able to reinstall the new tube pins. I ended up using a couple of turns of black electrical tape around the cap and glass. Anyway after all this messing around, it now seems I have a good tube, although the cutoff doesn't seem to be the best.
Tony

Polaraligned · #2 05-28-2021, 04:47 PM

I have removed good sockets from bad tubes and transplanted them a number of times. I have found the Permatex black gasket maker to be excellent for attaching sockets. It dries with a bit of flexibility and I have taken my D550 solder gun to it and it laughs at the heat.

dieseljeep · #3 05-28-2021, 05:31 PM

Quote:

Originally Posted by Polaraligned

I have removed good sockets from bad tubes and transplanted them a number of times. I have found the Permatex black gasket maker to be excellent for attaching sockets. It dries with a bit of flexibility and I have taken my D550 solder gun to it and it laughs at the heat.

I have two 7JP4s that have that lone broken off. I unsoldered the pin so the lead wire doesn't break off.
It seems I can fit a pin from a spent octal tube and possibly use Epoxy to hold it in place.

John Marinello · #4 05-30-2021, 08:37 AM

Drill/bore a small hole in the end of your socket & pump it full of silicone.

edison64 · #5 05-31-2021, 08:23 AM

To everyone that is saying use silicone, be very careful to use the correct silicone, do not use RTV or room temperature Vulcanizing or, the stuff that smells like acid, not only will it eat through those small wires but after a long time and contact will actually eat through them even though it has cured. I've had pretty good success by using J-B Weld and a few drops of gel super glue around the outer edge of the socket to Glass if need be.

Tony F · #6 05-31-2021, 09:31 AM

I used the silicone recommended by Bob A. (Windshield & Glass Sealant) I think the problem with the cap still being somewhat loose against the glass neck is the gap it is filling. I should have left the original epoxy in the cap and just used a small amount of this type of silicone to get back the expected bond it had with the glass neck when new. I didn't want to go extreme like JB Weld because if one of my solder joints on the pins didn't "take" I would have had a heck of a time trying to remove it. Oh well, lesson learned. I know what to do next time this problem arises. I will just have to be careful removing the outer plug.
tony

Dj3928 · #7 06-10-2021, 07:00 AM

I found new bases for the 7JP4 tubes on eBay. Item number: 263944993774

Here’s my YouTube video on replacing one

7JP4 7” CRT REPAIR , Installing a NEW BASE on a 1949 Picture Tube! https://youtu.be/h1ydeYzwNd4

Tony F · #8 06-12-2021, 08:25 AM

I had seen the listing on e-Bay for a new base. All well and good IF you happen to live in the U.S.A. But being here in Canada, with the difference in the money exchange and the increase in shipping costs. One has to think long and hard about ordering anything. The extra costs can easily triple or quadruple by the time something arrives. It sure takes the fun out of a seemingly "inexpensive" hobby.
Tony

Electronic M · #9 06-12-2021, 06:41 PM

Quote:

Originally Posted by Tony F

I had seen the listing on e-Bay for a new base. All well and good IF you happen to live in the U.S.A. But being here in Canada, with the difference in the money exchange and the increase in shipping costs. One has to think long and hard about ordering anything. The extra costs can easily triple or quadruple by the time something arrives. It sure takes the fun out of a seemingly "inexpensive" hobby.
Tony

A lot of scopes in the 40s and 50s used tubes with the same base... careful desoldering and a few hours bathing in ammonia will liberate a usable replacement base.