UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Testing a Mazda CRM-121 CRT with a Megger!

Argus25 · 20th May 2018, 1:34 pm

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.

20th May 2018, 1:34 pm	#2
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	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.