[RDusel]

Gentlemen,
I'm stumped. I'm restoring a 1950 Zenith Porthole tabletop and almost done.
The set has a 12lp4 that tests like new, a new NOS flyback, new caps (paper and electros are changed).
The problem I am having is that the set only plays properly for a couple of minutes then it loses horizontal drive and HV.
Deeper inspection shows the following: (circuit diagram attached)
cathode current on the 6GB6 is a little higher than it should be, ~130-150ma.
When operating the set plays fine and the HV is 10-12KV, no heating of the flyback (but it doesn't run very long).
Secondary (P8) grid should be 255, comes in at 262.
Cathode (P3) is 15v, should be 14v.
The horizontal discharge produces a steady sawtooth waveform at 68V (checked at plate).
Now here is the issue. The control grid (P5) at the 6BG6 starts at -7 volts with a normal waveform (and normal voltage). Within 2 minutes it climbs (drops?) to -17 volts at which point the 6BG6 completely cuts off, stopping sweep and HV.
All components on the 6BG6 control grid circuit are new and tested (there are not a lot of them). Voltage on the horizontal discharge side of C79 does not change.
So I am assuming that electrons impinging on the control grid drive it negative when the current in the 6BG6 flows and this negative voltage fluctuates up and down based on the waveform passing through c79 from the Horizontal discharge plate creating sweep. And R77/R78 are connected to ground to bleed off some of the negative charge so the tube does not cut off like it is doing now.
Can anyone suggest why the control grid is going too far negative? Can it be linked in any way to the excess cathode current?
Grid emission on the 6BG6 tube (I have tried a couple of them)?
On the control grid I see one side tied to +85V through a cap and the other side tied to ground via resistors. So where does the negative voltage come from?
Any help very much appreciated as I am really close to having this one done and I'l love to watch it and move on to the next set.
Thanks!