DG7-32 CRT questions

Hi,
I'm trying to wire up a DG7-32 CRT. I've managed to get a visible spot,
which is more-or-less focussable down to a 0.5mm dot. Maybe not as good as my
oscilloscope, but pretty good for a 1950s CRT.

The tube itself is a Tungsram of unknown age, and was bought new-old-stock.
I'm using the Philips DG7-32 data*** because I can't find a copy of the
Tungsram data*** anywhere. Given that it's a standard tube, I doubt the
specs vary that much between manufacturers.

Now, the problem is, I can't get the spot to extinguish completely. Here's
the circuit I'm using to derive the G1, cathode and G3 voltages:

======

100k 120k 68k 220k 680k
-500V <<<---/\/\/---/\/\/---*---/\/\/----/\/\/---/\/\/----------------> 0V
^ | ^
(actually | | |
-530V) | | |
| | |
Vg1 (inten) Vk (cathode) Vg3 (focus)
-522V to -465V -396V -366V to -276V

Anode (Vg2,g4,a) voltage: +243V

Voltage across tube (Va-Vk): 639V.

======

First point: The deflection plate pairs (D1/D1' and D2/D2') are wired
together to disable them, but the pairs are left floating. The deflection
amps aren't wired up yet; I wanted to get the core of the CRT circuitry
working, then move on to the deflection control circuitry.

Now, if I set Vg1 to -522V, I get a large round spot on the CRT face which
can't be focussed down at all. Stepping Vg1 down to around -500 gives a
mostly round spot that can be focussed down fine, but it's far too bright.
This is the problem I want to solve. This CRT is going to be used as an X/Y
display for both a scope clock and an I/V curve tracer, so I NEED to be able
to blank the spot (for the scope clock at least).

Vg3 seems to work OK, but doesn't have much in the way of range - the CRT
spot focusses almost all the way to one end of the 220k potentiometer's
travel. Fiddling with the balance of the 68k and 680k might be enough to
shift the center point closer to the middle of the pot's travel though; it's
ranging from K+30V to K+120V and the data*** specs the range limits as K+0V
and K+200V. This is more of an ergonomic issue than anything else, though.

At one point I also had a 47k potentiometer wired between +240 and GND,
with the CRT anode connected to the wiper. This has since been removed to
simplify part of the circuitry, and the anode is now wired direct to the
+240V rail.

The voltages are derived from a 6VA 9-0-9/230V mains transformer run in
reverse, with a half-wave rectifier, 10uF capacitor and Zener circuit
generating the +240, and an unregulated voltage doubler generating the -500V
(which is actually supposed to be -600, but is actually closer -530).

Thanks.
--
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.