I have a little 4.5" monochrome CRT Display in a deposition controller I have. It runs on 12v and uses NTSC. Pretty generic.
Its Dead.
I have high voltage, getting about 260v to the screen grid, filament is glowing. Scope shows activity on horizontal and vertical deflection. I can see the amplified video signal at the cathode on the tube.
What am I missing? Not enough HV? Poisoned cathode?
Checked the video output with another display and the video is good. If all else fails I will just run an external display.
Short the CRT grid & cathode together, it should illuminate the screen very brightly.
One thing to check is the video O/P Vcc - it would flood the screen if low, so suspect high ESR reservoir cap.
Even with a mono screen (say about 7 - 12kV EHT) you should be able to het a corona discharge (fuzzy blue glow) by holding a large screwdriver to the casing of the EHT transformer overwind or the rectifier stick if its external.
Its worth examining the line scan tuning & S-correction capacitors, although the usual failure mode results in excessive EHT - and often breakdown in the EHT transformer.
A tricky one can be the linescan energy recovery system, the forward drive only drives the electron beam from the center to the RHS of the screen, the flyback (the bit that generates the EHT) takes the beam very rapidly to the LHS, then the energy recovery (ringing in the EHT/linescan inductance) drives the beam from the LHS back to its starting point.
Naturaly a defect in the energy recovery system can deplete the EHT so you can't see any observable symptoms.
Thanks for the info guys. I eventually figured out the CRT itself was bad. Tried hooking external HV to the anode cap and nothing. Found another unit on ebay and with a good CRT and tried it with the other board and verified the CRT was bad.
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