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Old 20th May 2018, 12:32 pm   #1
Lloyd 1985
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Default 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.

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Old 20th May 2018, 1:34 pm   #2
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Default Re: Testing a Mazda CRM-121 CRT with a Megger!

It is interesting the notion of CRT testing. The requirements of a CRT to produce a good TV image are actually tough. The beam energy is distributed over the entire raster and also must be capable of a range of contrast levels and at the same time maintaining good beam focus.

So a CRT could produce a single blob, or single line scan that looks ok, but by the time the focused beam is spread out to a full sized raster, the beam energy and light output can be low and the focus poor in an aged CRT, so its not suited to a TV image anymore. Which is why a lot got replaced and often the old ones stuck back into the new one's box for storage.

So the condition of a TV CRT is multi-factorial and is far more critical when it comes producing a good raster scan/TV image than say in any other application for a CRT like a scope or vector scan.

It really means the only way to be sure if a CRT is up to the task for a TV image is to try it in a set, preferably with a full contrast test pattern image. However other tests that prove the CRT is at least working can help exclude the total duds.

I think it is good that the circle of illumination on the screen looks fairly uniform though, that is probably a very good sign.
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Old 20th May 2018, 2:01 pm   #3
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Default Re: Testing a Mazda CRM-121 CRT with a Megger!

I got a good result many years ago with a power supply set to give a faint glow from the heater and then applying the output of an electric fence box to the final anode.
Both the heater and electric fence unit were running from the same power supply.
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Old 20th May 2018, 9:25 pm   #4
1100 man
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Default Re: Testing a Mazda CRM-121 CRT with a Megger!

Hi Lloyd,
I think that's a really interesting experiment! I wonder if it will actually produce a good picture when you fit it to a TV?
I am now tempted to do something similar myself!

All the best
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Old 18th Oct 2018, 12:00 pm   #5
Lloyd 1985
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Default Re: Testing a Mazda CRM-121 CRT with a Megger!

It's been a while, but just as an update, I got round to digging out an Ekco T161 and having a play with it, and whilst I was at it, I bunged this old CRM-121 in to see how it fared. Unfortunately at the time I had made a boo boo in the RF/IF stages, connected a cap wrong! so the picture was washed out and struggling for sync, but none the less, here's the results!

This CRT isn't staying in the Ekco, it's a bit under-rated for it, as the EHT when the sets working will be around 9-10KV, and the poor CRM-121 can only handle up to 7KV. The set was producing around 8KV when I had the tube fitted, and there was a bit of hissing and crackling going on around the neck. Needless to say, I think this tube's going to be a good one! No problems with the picture going negative at high brightness, or issues with focus.

The Ekco uses a CRM-121B, which is rated for 10KV, and I also have a Murphy V180C, which has a CRM-121B fitted, which is a good tube, and I have swapped it into the Ekco, as the one in the Ekco was rather flat. The CRM-121 that is the subject of this thread will end up in the Murphy, since the EHT for that is only 7.5KV, according to the manual. Then I will get back to the Murphy and see if I can get it going properly!

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