FINAL REPORT:
To all who took time to respond - my sincerest thanks.
With the calm help of Harry Leeming we resolved all the silly voltages sag.
The drawings earths are correct (though there is a very confusing difference between PCB lay out and the drawings). There are also a lot of worryingly drawing and component errors in the service manual.
On this set:
The earth to the secondary (HT) winding on RECT 'A' board had been left disconnected by a previous incumbent - my original hunch about no current return path was correct. It was a combination effect: The missing earth was for the HT and Screen supplies, but due to another wiring fault (the mains 0v and Fan connections reversed) plus an important bleed resistor across the 300v supply (very high) caused the current to seek out alternative paths, In this case - screen cathode and heaters. The flash seems to have been: screen to grid (in side the valve) - then back through the bias supply chain to transformer. The sag simply was the losses through the Bias chain dragging the volts down. Sounds easy, But it produced a very strange slow sag on Tx and even slower return to normal voltage profile.
Interestingly one of the the flash-over valves is OK. So some good news.
Lesson 1 - check every connection, especially earths.
Lesson 2 - always use a variable transformer (Variac) to isolate and start voltage low. it helps - but beware these old sets don't like the voltage being too low(60%~70% of line is OK). They Hum and make some very worrying noises giving you more grey hairs than you need, if too low.
Lesson 3 - Ask for help.
This FT101ZD is now back in service - though a tune up on 160m and a few minor alignments will tidy it up. But later!
My special thanks must go to Harry, and to all those who helped.
Moderator: For now this case is happily closed. Thank you
