7JP4 Picture Tube
 

Hi I found a spare 7JP4 picture tube for my Motorola VT-71 television. I tried it with my VT-71 television. It produces a good picture, but every now and then there's some arcing around the high voltage capacitors at various locations. When the arcing happens the screen will go to a straight line until the arcing stops, then the picture will be fine. The connector of the picture is a little loose and some of the pins are a little loose. I tried resoldering the pins on the picture tube and this did not correct the problem. Is this picture tube bad? Would this help if I removed the connector from the picture tube, clean wires, re-install connector and resolder pins? Thanks, any info would be greatly appreciated.

What happens if you put the other CRT back in the TV?

Phil Nelson

HI Phil When I put the other picture tube back in the tv, there's no arcing, so I'm thinking that there's something wrong with my spare picture tube. What do you think? Thanks for your fast response.

If the emission is a little lower in the spare tube, the HV in the TV might be a little higher. Those VT-71s are marginal in terms of insulation and arcing. It's a good idea to put some insulating goop on the terminals where the high voltage ends of the capacitors are connected (back of the centering controls). Also put some on any other connections at high voltage that have any sharp features (which tend to be where arcs come from).

Are you saying it's arcing in the base of the tube or under the chassis at the capacitors?

I had a brand new 7JP4 that had one of the wires in the base bent over when the bakelite base was installed, that is probably why it was never used.
It behaved very much like yours but I could see the arcing through the glass.

In that case I was able to remove the base and fix the problem, it was a nerve wracking job however trying to get all 12 wires back in without breaking any.

There's an easier way to get those wires back into the cap. Stop me if you already know this trick. Attach (solder) a long very fine gagewire to the end of each of the CRT leads so the neck looks like Madusa. Clear the cap of any solder and feed the fine wires throught the cap holes pulling each one until the cap fits snub on the CRT. Resolder.

I watched Tom Albrecht do this and the CRT worked great afterwards. I tried it myself once and although the technique worked I over heated one of the leads during the soldering process. The CRT went to air. by by..

I've sucessfully implemented the process cherokeeroad described on a 52' philco, and with all the difficulty I had trying the other method, I'd never try anything other than the "madusa" method CR suggests.

Tom C.

sounds like there is a internal short in the tube

I had not thought of that particular name -- the "Medusa" method -- but it's a good description!

It worked for me when I had a damaged base :)
http://www.bobandersen.com/images/7V...eaned%20up.jpg http://www.bobandersen.com/images/7V...20threaded.jpg http://www.bobandersen.com/images/7V...%20success.JPG

HI Thanks to all for responding to my question pertaining to the picture tube. Sorry for the late response, but I was out of town for a couple of days and just got home about an hour ago. To Eric H, It looks like the arcing is coming from the HV capacitors and not the picture tube itself. I checked the arcing when it was still light outside, with all the lights out in the basement and with the metal covers on. Maybe I should check it with all the lights out, when it's dark outside and remove the metal covers. Also, Eric H, when you removed the picture tube base, did you do anything to loosen up the old glue, as I don't wanna cause any further damage when trying to remove it. Thanks again to all for you input on this matter.

Thought I'd add my Madusa pic just for the fun of it. In the process of trying to get the extension wires soldered on I broke off one of the other leads. instead of just soldering on the extension wire directly onto the stem I decided to try and reattach the broken off lead thinking that the difference in gage of wire might make a difference. I use the tape to hold the broken-off lead in place while I tied a fine wire around the base then I soldered it at the knot then trimed off the excess wire. One can imagine that all this mucking around is what finally caused the CRT to go to air.

Y'know, just thinkin' out loud here, wonder how many 7JP4 sets there are still out there ? I'd think they oughta be a relatively easy CRT to duplicate-Wonder if some Chinese tube Mfger could be coaxed into making a run of 1500-2K or so ? I'd buy one, just to "squirrel away"-I'd guess a lot of us here would, too-If they weren't TOO expensive.

Yes, I would check again in total darkness. If you see arcing at the HV capacitor solder joints, etc., you can try insulating them as Tom suggested. I have used "Corona Dope" by GC Electronics for such purposes. It's available from http://www.tubesandmore.com/ and other suppliers.

You also want to make sure there are no sharp points -- clipped lead ends, solder tails, etc. -- on such joints.

If the arcing source is not immediately obvious, a plastic or rubber listening tube held to your ear may help locate it.

It's possible the CRT itself has a problem, but I wouldn't mess with its base until you had eliminated other possibilities.

Don't run the TV for long periods when it is arcing.

Phil Nelson

Me too!

-J