Admiral using 10BP4... open cathode???

Has anyone ever ran into what I have pretty much isolated as an open cathode? The filament, G1, G2, and the anode all are getting proper voltages and if removed one by one, each impacts the image as you would expect. However, there is no change in the image no matter the setting of the brightness control, and it is delivering proper voltages to the pin on the CRT. In fact, I can disconnect the cathode and nothing changes at all. I tried crimping and resoldering the cathode pin, but it didn't help, but perhaps I should keep trying this. There are no shorts in the crt from cathode to anything. Running filament on an isolated transformer made no difference either. Ion trap seems to be a peak for what i have got. Basically image is created using only the contrast control as it is... and it looks like max brightness, although it sure ain't bright. Higher filament voltage improved things, but not the main issue. Where are the electrons I am seeing coming from, if not the cathode or the filament???

BTW, I assume it is normal for the contrast control to need to be advanced to some degree to even get a signal or sound in this set... it is located in the grid of the first IF tube.

Dan

[wiseguy]

the only real way to find out is to test it on a crt testor,i have never seen a 10bp4 that has cathode drive from the video amp,drive from video usually goes to G1..
sometimes these old sets the cathode is grounded..
i would just test it.. and recap the set if you have not done so..gain is almost always the contrast control in these old sets..
and it is possible to have an open cathode.. but testing would tell.
good luck!
terry

The video amp drives G1, the brightness control establishes bias voltage on the cathode in this set... although even though the voltage is there, it doesn't do anything in my case.

Dan