[DaveWM]

I would start with a HV prob on the CRT anode, just to see where you are.

Make sure its the HV, I was reading up on some zenith issues, on HV there is a .22uf and 200 ohm (IIRC) resistor in the cathode circuit of the 6HS5, check those out. The VDR is always suspect.

IF the HV is holding steady then suspect the video out tube (12bh7 i think), if there is a problem there it could be biasing the CRT to cut off. My book mentioned some diodes in the video driver transistor circuit that could open or short causing excess or no brighness.

but it sure sounds like HV reg and the VDR to me, I had the exact same prob (CRT would bloom and could be recovered with constant adj to the brightness), it was the VDR, you could see the anode voltage jumping around 5k up and down, and the HV adj would not bring it down below 27kv with the screen brightness turned down.

The problem I found with the VDR's (they can be got new from talon and moyers) is they are not all the same, BRAND NEW, I ended up paralleing two and it finally worked.

My guess is back in the day when the VDR's were new and not NOS, you would put one in and it would fix the prob, 5 min fix, but now with VDR's not being what they should be its more like pin the tail on the donkey as far as it fixing the problem, hit or miss.

Hopefully in your case its a tube (the HV recifier or HV regulator), if not the .22 or 200 ohm resistor, if those check AND the HV is still to high (and jumps all over the place) with the HV control having little effect on bringing it down, then I would just try subbing in some high value resistors (like 2 meg to start with) for the VDR. There is a term strip behind the 6HE5 that has the vdr connected to it, you should be able to get some jumpers on that and bring it out to the back of the TV, this would let you try different resistors (or vdr's if you get some).


One last thing, check the resisor that goes from one side of the pot to the ground on the HV, make sure it has not driffted high. that would cause a Low HV, I just got thru a zenith that had that happen (it was a roundie with a shunt tube 6bk4). Main thing is to get that 6he5 the correct bias so it can track the fly and load it down to the correct voltage, that keeps some reserve so when the beam current goes up, the HV will not drop.

welcome to the HV woes club...