Quote:
Originally Posted by jr_tech
Perhaps it is indeed a straight gun...Do you get any beam to the screen with NO magnet?
"I have one unrestored set that has a good original 12LP4 in it. I hate to take it out but I may have to eventually try that."
It seems that many times the most difficult experiment yields the most useful data!
Since you mentioned charging effects, I had another terrible (perhaps far fetched) thought about the key-stoning/focus... since the tube is not aluminized, is the landing energy of the beam constant as it travels across the screen ? Is it possible that the phosphor surface discharges somewhat during retrace? 
jr |
O.K. I do get a raster with no magnet. I tried the other rebuilt CRT and it behaves the same way. The raster is best without the magnet. However it is still badly focused on the right side.
The keystone is much reduced without the ion trap. I did have a keystone with the 10MP4 also. I suspect it is the -125V supply not being rigid enough, but I haven't tested it yet.
In defense of your odd theory
when I had the keystone effect pretty bad with the 10MP4 I thought about charging since the raster would move if I got my hand near the front of the CRT. I blew on the CRT expecting moisture to drain any external charging. The raster blew up and disappeared for a second then returned with only a tiny keystone. I think the small keystone is a supply issue.
I did some research and the only way I can see that I could get a raster without an ion trap is if G2 and G3 are at the same potential. I think this would be extremely unlikely. Especially for both CRTs.
I will have to try the old 12LP4 that is in a Sonora that is further down the restoration queue.
BTW, I did look up the specs for a 12LPC. Although it is aluminized, from the factory it still requires a dual field ion trap.
I also found this page where they converted a set from a
Triode to Tetrode CRT. They had to reduce the focusing field, which I also observed.
Something else that is happening now is that I get a bright spot for a couple of seconds after turning the TV off. With a gun that requires an ion trap, the electrons have to have the right velocity to make it out of the gun. As soon as the HV drops they get lost. But the straight gun without a trap doesn't filter by velocity
John