Should I auto-restore 10FP4 with Sencore CR70?
 

I'm nearly done restoring a GE 810 tabletop, and it looks like my 10FP4 CRT isn't as good as I originally thought. I have played the set several hours in the course of restoration, so the CRT has already done as much "waking up" as it ever will.

On my Sencore CR70 tester, this 10FP4 CRT fails the Cutoff test: when the Function switch is set to Cutoff, no adjustment of the Cutoff-Set control will bring the needle into the meter's Cutoff box. When I move the Function switch to Emission, I can get the needle to move up into Good territory -- but only by cranking the Cutoff switch clockwise.

https://antiqueradio.org/art/GE810Te...nCRTTest04.jpg

When used in the chassis, the CRT has mediocre contrast and brightness. The picture is sort-of watchable, but no combination of the TV's Contrast and Brightness controls produces a strong, crisp picture.

The CR70 tester suggests that a CRT with bad cutoff and good emission is a candidate for using the Auto Restore function.

My question: has anyone successfully revived a CRT with bad cutoff using this Auto Restore function? Or should I write this CRT off and look for a better one?

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html

If it is unusable then what do you have to lose?

I forget which rejuve setting was used but some years ago Mark Mason and Nick Williams used a CR-70 to coax life from a stone dead 16GP4...If I ever find a CR-70 that is in my budget I'd buy it as a main rejuvenation rig.

I'd call the CRT marginal, not totally unusable. What I have to lose is that I'll have a dead TV set if there aren't any replacement 10FP4s available and I kill this one by randomly playing with rejuve functions.

So, I'm wondering:

1. If anyone has successfully revived a tube with this particular defect on a CR70.

2. If so, whether they used this particular function.

Thanks,

Phil Nelson

I have had several 7 jp4 crts although different then what you have which had many issues the contrast was lousy as well as the brightness and using a bk440 I used the shorts setting and just pressed the shorts button for a second and seen all the whisker sparks in the gun then I tried it and it was like having a new tube. Although the 440 cannot be used on electrostatic tubes I can , I made an adapter and established a good tube point on the meter and the other functions work except the cutoff because of the lack of proper voltage but it served the purpose. So any old tube could very well have the same status good emission but lousy cutoff due to whiskers that present a slight short within the gun just enough to take from the integrity of the tube. So if you have a shorts remove function try it shouldn't hurt.

I've tried auto restore on a few CRTs including 10BP4s, but not a 10FP4. I can't honestly say that I've ever seen a big improvement. Generally, the emissions will improve for a while but gradually decline. Never saw much improvement in cutoff.

Any chance you could try it in a another set ? Is the cutoff more responsive if you change the bias range on the CR-70 ? If so, perhaps you could tweak the bias in the set to shift the 10FP4 into it's "sweet spot".

I went ahead and tried the auto restore. It didn't improve the cutoff reading on the tester at all. The emission reading on the tester improved a bit, but I don't see any real improvement when playing the TV.

I'll peek around and see if I have another TV with an aluminized 10" CRT that I can borrow. I don't think you could sub a non-aluminized CRT; the 810's focus coil and fiber gasket doesn't leave enough room on the CRT neck for an ion trap magnet.

Phil Nelson

I believe the 10BP4 has a longer neck to accommodate the trap, it might interfere with the back though.

My GE tube manual shows that both 10BP4 and 10FP4 are the same length (17-5/8 inches)... would there be room for at least a single magnet trap?

Do you have a spare 10BP4 to try?

jr

I took this photo when I first got the set, to record how things were arranged on the neck:

https://antiqueradio.org/art/GE810Te...ckOriginal.jpg

The fiber gasket with big notches is used to jam the focus coil in place. (It's between 5 and 6 on the ruler.) That's in the neck zone where you'd often see an ion trap magnet.

The focus coil is between 6 and 7 on the ruler. It is a strong permanent magnet, so I don' t know what would happen if an ion trap magnet needs to slide close to it. The trap magnets are much weaker in my experience.

In this photo, the focus coil is too far forward (CRT-face-ward). In practice, I have moved it back toward the tube base -- maybe another 1/2 inch -- to get normal focus.

Between 7 and 8 on the ruler are the centering rings, also permanent magnets. The phenolic sleeve that lies face-ward from the rings is attached to the yoke.

I can probably come up with a 10BP4 and a magnet to try. I guess I could leave off the fiber gasket temporarily; otherwise the magnet might not be loose enough to slide over it.

Directly under the CRT neck is an adjuster in an aluminum can with a hole on the top. There's only about 1/4 inch of clearance between that can and the CRT neck, so the magnet hardware might collide with the can if it nears the tube base.

Phil Nelson

I decided to measure some voltages and do an eyeball comparison between the 10FP4 and my 5AXP4 test tube. The HV is somewhat low: 8KV versus 9.2KV given in the manual. Voltages on the 12AU7 video amp are normal. The two CRTs look like this, 5AXP4 first:

https://antiqueradio.org/art/GE810Te...oTesting01.jpg

https://antiqueradio.org/art/GE810Te...oTesting02.jpg

Not very scientific, but I was able to get a respectable picture from the 10FP4. Maybe I was being too critical before -- or I'm still learning how to adjust the neck gadgets.

I need to do some more poking around. There is some intermittent crashing (audio + video) whose source I haven't found. Maybe a leaky HV filter doorknob cap, or a tuner that needs a second cleaning? Anyway, I'm not giving on this 10FP4 just yet.

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html

I'm guessing that the fiber sleeve should be further foreword, almost totally under the metal collar. :scratch2:

jr

edit add... yes very respectable.:thmbsp:

Well, that gasket doesn't slide under the focus coil as far as you might guess. I took this photo during the Pepsi challenge.

https://antiqueradio.org/art/GE810Te...oTesting03.jpg

I don't think you could get it any farther under the focus coil without hammering. But the position of the whole business (gasket + focus coil + centering rings) is significantly closer now than in that old photo. The phenolic sleeve coming back from the yoke is nearly invisible.

Phil Nelson

I think the gasket was is in a similar position in mine. Goofy arrangement. That picture is looking pretty good to me

How do you adjust the focus with a permanent magnet? Slide it up and down the neck until you get good focus and then jam the fiber gasket under the focus coil?

Yes. A cheesy setup, although it does work.

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html