7JP4 restore ?

[Tony F]

Looking for advice, as to what to try for a 7JP4 I have in an Admiral set. Sencore CR-70 shows the following: H-K shorts (way down in the RED between 2.5 and 3), G1 is excellent, almost off the GREEN end. Cut-off is showing good response back and forth. Emissions is way down in the RED, just off .5 but not quite 1. How would you proceed with this tester (or not) before I give up on the tube and call it toast ? Were 7JP4's meant to be "blasted" or cooked ? Thank-You everyone,.. and stay safe.
Tony

[bandersen]

I would start by clearing that short. Use the procedure outlines in the CR70 manual.
I have had limited success with rejuvinating them and have heard the same from others.

[Polaraligned]

I don't think H-K shorts are clearable with the CR-70 or any other tester. Your emissions are toast on top of it. Time for a new 7JP4.

[Notimetolooz]

There were TV CRT brighteners that contained an isolation transformer on the filament for tubes that had a H-K short. You might be able to find one and adapt the sockets. However, you would still have the emission problem, unless the emission test was being thrown off by the short.

[bandersen]

Your are both correct. Here is the relevant section from the CR70 manual.

[Yamamaya42]

Originally Posted by Polaraligned

I don't think H-K shorts are clearable with the CR-70 or any other tester. Your emissions are toast on top of it. Time for a new 7JP4.

This is correct for the most part, where as some testers DO HAVE a procedure to try to remove them ( mine induced ), it does not really fix the underlying problem that caused it in the first place, which is a breakdown oh the insulation of the heater wire, and where as trying to burn it free may work, it may also do more harm than good, and there is no telling if it will short again later, because the problem is never really gone.

So the BEST over all option is not to even try, as mentioned above, and try going with an iso transformer.

[Tony F]

Well, It turns out the tube is ok after all. I have 2 Admirals that have the 7JP4 tubes. I have only ever had a B&K 466 tester that has worked well on all the other tubes I've have. I came across a Sencore CR-70 that looks like it has never even been used. Knowing it is one of the few testers that can test a 7JP4, I hooked it up. Both tubes showed the exact same symptoms, HK shorts and almost no emission. Not knowing any better, I thought they were in fact both bad. I got side tracked with other things, then I thought I'd give it another shot. I retest some known good tubes, first a 12LP4 then a 10BP4. These both showed strong on my B&K, but they show the same symptoms: HK shorts and low emissions. So now bells go off in my head, hey it has to be the tester. I did the CR-70 Field Test with my Fluke 189 and everything is fine. Hmm, I removed the screws and pulled the "guts" of the Sencore out. Looking for obvious signs of internal trauma and pushing down on ribbon cables to re-seat them. Then I spy one ribbon cable way in the back with about 6 wires on it. It had a slight "cock" to it. Pushing it down to re-seat it and reinstalling the "guts" back into the case. I retested the tubes, and all is well now. No, HK shorts and the emissions are way up in the GREEN. The one 7JP4 tests strong on all settings.The other one tests good on all settings except the cutoff which only came up to the 1 mark. I tried the Auto Restore function twice with no improvement in cutoff. It has lots of "meter swing" dropping the Bias one setting from -68 volts to -52 volts. So hopefully it can be improved. I didn't want to hit it any harder without trying it first. The 12LP4 and 10BP4 also test good. I didn't think my new Sencore "toy" would be faulty when I just acquired it. Maybe that's why it looks and smells new.
Tony

[JohnCT]

Originally Posted by Notimetolooz

There were TV CRT brighteners that contained an isolation transformer on the filament for tubes that had a H-K short. You might be able to find one and adapt the sockets. However, you would still have the emission problem, unless the emission test was being thrown off by the short.

In the 1990s, H-K shorts were common with Zenith picture tubes. Since these filaments are sourced from a secondary flyback winding, a special toroid isolator could be bought and installed.

What I used to do is place 2.5 turns of regular insulated wire around the flyback core, then run the wires to the CRT socket and attache them after cutting the traces on the CRT board in order to isolate the filament pins from ground. I then slid the winding up or down the core to get the filament as close to (low side) of 6.3VAC using a TRMS meter. I sometimes added a 5W resistor to the arrangement if needed to keep it at 6.3 or just below.

The same could be done with these old tubes if an isolation brightner can't be found cheaply with the same socket arrangement.

John

[Yamamaya42]

from what I have seen, on RCA color sets anyway, the CRT heater windings are already isolated, having their own separate power source on the main transformer, but ARE tied to a pair of resistors to the main bias and ground, so, if a H-K short causes feedback through that or the main power x-former, does sticking a 2nd isolation transformer give better blocking to any HV that may try to bleed through?

[Electronic M]

This is a good illustration of the principal 'always test your test gear'.

A good sanity check on any instrument is important to do when first acquired and periodically there after.

[old_coot88]

Originally Posted by Yamamaya42

from what I have seen, on RCA color sets anyway, the CRT heater windings are already isolated, having their own separate power source on the main transformer, but ARE tied to a pair of resistors to the main bias and ground, so, if a H-K short causes feedback through that or the main power x-former, does sticking a 2nd isolation transformer give better blocking to any HV that may try to bleed through?

It will provide DC blocking, but the added capacitance-to-ground will cause smearing-out of fine detail in the pic (since the luma signal is fed in via the cathodes). Been there, done that.