CRT testing question.

[jeyurkon]

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