I wanted to test a 10MP4, which is a triode CRT, and is about as difficult to test as an electrostatic CRT.

I came up with a method to test it and would like opinions on whether what I did makes any sense.

I felt that the G1 cutoff voltage would be the most informative test. So, I set off to measure the voltage needed to visually extinguish the focused undeflected beam.

I set up a B&K 465 and added a HV supply to the anode. I set the HV to 7.5KV. I first tried it on a known to be good 5AXP4. It worked like a charm, but the 5AXP4 is a tetrode CRT.

When I tried it on the 10MP4 I didn't get a beam spot at any setting of G1.

I did notice that the HV supply started to have current drawn when G1 was set to -29 volts or lower. At zero volts it drew 195uA. That's not a great emission value and I still had no beam spot.

Triodes still have an acceleration electrode, but it's tied to the anode. The current I measure was either through that electrode or the beam hitting the dag instead of the screen.

I left the CRT in the chassis. The ion-magnet and the focusing magnet were left in place and undisturbed. I'm assuming that they were not dramatically out of adjustment.

The set was also turned off so there was no current through the horizontal deflection coil. There is usually a small offset current to center the beam. Is it safe to assume that the beam should be fairly well centered without this?

If this test had turned out positive I would have been happy and fairly sure the CRT is good. Since it turned out negative, I still have to worry about whether my method makes sense.

I was hoping to test the CRT before I finish restoring the chassis so I'll know not to blame the CRT if I don't get a raster.

Do you think my CRT is dead or is my test flawed?

John

1. Where did you get screen voltage for the 5AXP4 or did it work without it?
2. If the 10MP4 shows absolutely no beam, but draws current, I first suspect the ion trap is off. Is the open end toward the anode connector? I would try adjusting it at high brightness setting to see if you can get anything on the screen. If not, rotate it 180 degrees and try again.

Thanks for the suggestions Don.

I got the screen bias from the B&K465. I had it set at 300V for the 5AXP4.

During the test the CRT isn't hooked up to the set. It's just physically sitting in it. The brightness is essentially what I'm adjusting by adjusting the G1 bias.

I'll play with the ion-trap and perhaps raise the filament by 10%. If that doesn't give me anything I'll put a 10BP4 in that should work and see if it's a problem with the beam optics due to the ion-trap or focusing magnet. The focusing magnet is adjusted differently than the manual suggests. It says to keep the screw shunts adjusted the same as you adjust the focus. There are two of them. Whoever adjusted this one, could be the factory, has one at 1/16" and the other at 3/4" gap.

When I played with the 5AXP4 I just had it laying on the bench since it doesn't care about the focus or ion-trap.

John

Well, playing with the ion-trap didn't help, neither did the focusing magnet. In a later supplement they refer to the focusing magnet as a focalizer. :)

I'm trying to get the 10MP4 out of the chassis. The rubber mounts under the front of the CRT are really stuck. The manual suggests using a putty knife between the steel mount and the rubber, but the old rubber and glue is very hard now.

I'll have to remove the front panel with the CRT. Not too difficult except that I need to get a higher wattage iron to remove some leads soldered to it. Once I get it off, I should be able to remove the panel from the CRT.

The ion-trap seems weak, but I don't have much experience with them. I do know that it's supposed to be about 35 gauss. I can check it tomorrow at work with a Hall probe.

John

I believe the 12WP4 used in some 1951 Philco sets is a Triode as well, I tried testing it with a B&K & got nothing at all, I probably didn't have it set up right but in any case nada.

I finally just replaced the filters in the chassis so I could fire it up and see what happened, turned out it was OK not great but good enough.
I would suggest recapping your chassis just enough to get some HV and see what happens.

Usually just replacing the filters and the caps on the horiz oscillator will do the trick.

I would be worried about damaging the tube trying experiments on it, also I wouldn't rejuvenate it until I saw it working in the chassis no matter what the tester said.

I'm just kind of worried about Hawkeye going out of business before I determine the status of this CRT.

I used the 5AXP4 to experiment on to make sure I wasn't going to destroy the 10MP4.

It's a bit more than just getting the horizontal section working in this set since it doesn't use a flyback. I need to make sure the horizontal is working as well as the RF HV. If the horizontal doesn't work when the HV comes on I risk damaging the CRT. The RF transformer in the HV could be considered being made of unobtainium. I might be able to rewind it, but would prefer that I didn't have to try. I could use a modern HV supply, but I want to keep it original.

That means I want to be extremely careful about everything, and that'll take time.

I don't plan on trying to restore the CRT in any event. If it doesn't work, or is very weak, I'm sending it to Scotty if he'll take it.

John

I checked the ion-trap. It has a field of 38 Gauss, slightly more than the specified 35.

I'm a bit puzzled about the physics of a double field trap if anyone has light to shed on them. I was expecting "a double field." Instead it seems that the narrower pole pieces clamp the fringe field so that it doesn't extend beyond the wide pole on the bulb side.

Although the marks on the CRT indicate that the trap hasn't been moved in many decades it appears that it might have been maladjusted. It open end wasn't pointing directly towards the anode. It was also much farther back towards the base of the tube. This would make sense if the magnet was weak, but since it's a little stronger than spec and the CRT is run at 7.5KV instead of the 9KV that the 35 Gauss was specified for, I would expect it to need to be closer to the bulb. But, reality doesn't always seem to listen to theory.

John

Well, it appears that I have a quite healthy 10MP4. I moved the ion-trap to the position that the service manual recommended, even though there was no indication that it had ever been in that position.

That gave me a weak beam spot and with a little adjustment I was able to maximize it.

I was able to measure the G1 cutoff and it came out to be -30V. That's within the good range. The focus was excellent.

I think the CRT was replaced at some time in the life of this set. The Sam's and the factory service manual are a little misleading and it's possible the service tech readjusted the HV to 9.5KV instead of 7.5KV. If you just follow the instructions you'll end up doing this. The only place 7.5KV is noted for the 10MP4 is on the schematic. This would explain why the ion-trap was positioned where it was. The 10MP4 can handle 9.5KV easily, but wasn't intended to in this set.

If anyone decides to try this, make sure you start well above G1 cutoff. You don't want to burn a hole in the phosphor. That's why I practiced with a 5AXP4 first that was already suffering.

It helped a great deal having the ion-trap in the correct position to start with. When I wasn't able to get a beam spot at all, it would have been dangerous to have G1 set low and then move the ion-trap around to find the beam because it would have appeared quite suddenly and be much too bright. That meant move, adjust G1, move, adjust G1...

John

I'm glad you looked into this further John. I and probably many others had no idea about these type crts neverless how to test them. With what appears to be a healthy tube will give you some motivation to focus on the rest of the set now. I look forward to your progress. Good Luck!
-Tony

...

What about using a tester that only uses the cathode and G1 (like the Beltron)? It's obviously only an emission tester, but good enough to get an idea of what the CRT is like.

I actually did that. The emission was in the milliamp range. The problem with using G1 is that you measure the emission from nearly the entire surface of the cathode. It's the tip that gets ion damage that's important. That is reflected in the G1 cutoff and that's why I wanted to measure it. It gave me much more confidence in the CRT.

In collecting some used CRTs I found a couple that have decent emission, but the G1 cutoff is extremely low.

Here'a a link to an explanation by Sencore.

http://www.sencore.com/uploads/files/RejuveCRTs.pdf The important info is on page 5.

John