Cathode Projection: What is it?
Project the cathode onto the screen.....
Never heard of that one before....
I can't be certain but is this where you supply the CRT with filament and EHT voltage but no HT.
By doing this you then get a defocused blob in the centre of the screen, this blob looks a bit like the moon, even down to the craters. This is a projection of the cathode and from this you can somewhat determine the cathode condition. The craters you see are the defects on the cathode, the less you see the better the cathode will be and the subsequent quality of CRT and picture presented.
I might be totally wrong, I'm sure someone far more knowledgeable than I will be along to correct me and state I'm taking Bow-lax.
My (modified) Müter CRT tester can do something like that.
An MW22-16 gives a projection like this (@ c. 5kV final anode voltage):
And this what I got from a 6/6:
It is not as easy as it looks to get something at all and needed quite a lot of fiddling with the various electrode voltages.
I'll try some other CRTs in the future. At least it shows that these is a possiblility of a picture.
I couldn't find much in the literature about this at all. Articles are certainly welcome!
I would like to know what it actually means what we see in the projections. Like the ragged edges in the case of the 6/6.
It would be nice to see similar pictures from others.
Hope this helps a bit.
p.s. maybe this should all be in a different thread Chris?
I'm sure LLJ wrote about it once.
I've had a go at this before, first tried it on the CRT from my little Motorola TV, and more recently on the CRT in my Ekco TU211. With the TU211, I removed the focus assembly and used my Megger set on the 1000v setting, positive lead to the final anode, and negative lead to the cathode. The little 7" tube from the Motorola worked better!
I must try this on my TV22!
That's interesting Lloyd.
You just connected the cathode and anode to the megger @ 1000V ?
Heater connected I suppose? And the other electrodes?
Ah, forgot to say I also connected the grid to negative as well, but it also worked with it left unconnected, just a bit dimmer. The heater was connected to a variable bench psu, I had to wind it up to about 8v on the Ekco, as it's really low emission, the set actually had an extra dropper supplying just the CRT heater to give it more juice!! Not a nice arrangement at all. I can't remember what I connected up on the Motorola, but it can't have been much different.
I can remember doing it back in the 1970s on an old Phillips tube and cabinet I was going to use to make a big screen scope. I used a pulsed car ignition coil.
It flashed up a nice big circle of glow and quite bright each time it was pulsed. The wires just went between the dag and the cavity. with 6 volts connected to the heater.
This now famous 1938 set exhibits a very plain pitted cathode projection image. Watch right at the end!