Testing a Mazda CRM-121 CRT with a Megger!
Just thought I'd share this with you all, I picked up a Mazda CRM-121 from Stuart R at the NVCF, we had no idea if the tube would be any good at all, the box was marked up with 'NO GOOD', but this wasn't going to put me off!
I'd read before somewhere about cathode imaging, where you get a defocused blob on the screen just by supplying the tube with the correct heater voltage, and some EHT, not the full EHT, just a lowish voltage. I have already tried this on other tubes, with interesting results.
So here's how I set it up:
Connect heater to bench power supply, set to 2V,
Connect Megger, positive lead to anode on CRT, negative lead to cathode and grid.
Wait for the CRT to heat up, then select 1000V range on Megger, and press TEST!
The first result looked rather unpromising, with a very dim glow on the tube face, so I released the test button, and left the tube with just the heater glowing for 20 mins. After this I tried again, and surprisingly the cathode had woken up very nicely! I had a good bright blob on the screen, and it actually looks quite good, compared to some other CRT's I've tried. I think maybe the tube was condemned all that time ago because of the large ion burn in the centre of the screen, which the edge of can be seen in my photo's.
Next step, try it in a TV set! I've got a Murphy V180C which it will fit into for a test, but that won't happen for a while yet.
Regards,
Lloyd
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