7jp4 HK short
 

Got a 7jp4 with a HK short yet I don't get any continuity between the heater either pin to any other pins whatsoever. So where is the short.

Some H-K shorts are only present when the heater is on and has expanded enough to make contact.

As the heater was hot I then pulled the plug and still no continuity

Maybe it's voltage related, like how sometimes a capacitor will only show shorted when it's full voltage is applied. Maybe your HK short isn't evident till the tube tester (or the tv your running it in, whichever it is that's giving you the belief that it's shorted) puts more voltage to it than your meter does ?

If I use this tube with a separate filiment trans then I would I think have to use a resistor in place of it being cut out of the circuit. I would use it in the Motorola ts4 or the ts18 sets so I don't remember if they are series wired if so then I need a resistor but the value I have no clue yet.

Yes they are series wired. Use a 10 ohm resistor rated 7 watts or better

If you mean you pulled the socket off and quickly measured the continuity, then the tube still could have cooled enough to unshort. It doesn't take long.

You can wire up a carefully set heater power supply directly to the tube and have your ohm meter connected at the same time. Make sure not to go over the heater voltage rating by more than a tenth or so.

Are you seeing any symptom on the screen or is the H-K short showing up only on a CRT tester?

John

Short shows on the tester

Ok so this is very unusual this 7jp4 still shows a short with a separate heater voltage source and it shows a short on the shorts setting. I had wired my bk440 to test these tubes and has never failed me but now I am thinking that the tube plug cap may have been wired wrong because the 2 neon lights that come on are the 2 that tell me if the tube is good only its showing on the shorts setting and not the emissions setting. Is this at all possible being wired wrong. This tube I just acquired and it was in the box and showed no marks on the pins suggesting it was not in a set also has a good getter. Or maybe the wires under the cap are touching being they don't have insulation on them and maybe upon putting it on they are touching. Or in some way 2 wires are reversed . Got to find a safe way to get the neck socket off.

Ok update, I tried the 3 wires g1 g2 and a2 and move them around to different positions and that didn't fix it. I have 7 Motorola tvs with 7jp4 tubes in them and tested all of them with this bk440 only this one did not test right and I think I know why. The tube I have is new as it don't have tell tale signs of having been used including scrapes from the plug being put on and or pulled off. This tube is from 1956 from RCA original box and compared to another tube I have the gun is clearly different then the other I have so I'm thinking maybe the design was changed slightly from the earlier versions because I plugged it into a working set and this tube is infact nos if it had an HK short there would be lines and likely the brightness would not turn down. Perfect pic, focus , brightness super bright. If a change did happen in 1956 then it's the first 7jp4 that did not test on my tester all my others did.

Something to consider is a short test doesn't necessarily mean a dead short. Some testers are very sensitive. The resistance between elements could be 1M or more and show as a short.

Glad it's working for you. A NOS 7JP4 is a rare thing.

Bottom line on a 7JP4 is how it works in a set, testers can't be relied on 100%.

I have a NOS 7JP4 that had a short in the base, in that case it was one one of the pins carrying high voltage, it would arc and you could see it through the glass inside the tube. It was a nightmare removing the base and getting it back on, but I did save the tube. A wire had folded over when the base was put on.

How can I find out a date code on a 7jp4 tube, it shows 7jp4 and under that it has the letters PEA. On the socket it has 604.

I presume you mean on the base. Probably the 4th week of 1956. By 1966 they were using 2 digit years in the EIA code.

Yes the 604 is on the base the pea is on the glass below the 7jp4.