[Anthony]

Sorry for the delay in replying. Decided (in frustration with the problem) to put it away for 24 hours to recover. My results should appear in italics below.

CDU110. The circuit in the manual is singularly unhelpful. It is drawn almost as if to make understanding the circuit difficult. I should not worry too much about the beam current setting, as long as you have a nice bright beam (which you haven't!). It (RV50) moves the base of the "cathode-focus-feedback to EHT oscillator-earth" chain up and down within 100V, thus limiting the maximum range of the brilliance control. Most other manufacturers use the "setEHT" control to achieve the same purpose.
I suggest your problem is either:

1. The top of that chain R66-focus-R64-RV51-R55 is low. It should be about 1kV. The sharp focus you now have obtained may be just luck for the particular cathode voltage you have.

I have double and triple checked the resistors on the focus and brilliance chains and indeed the original focus problem was due to R64 and 65. The rest are depressingly good.

Measured with my homebrew 100Meg probe the top of this chain is -943 volt so O.K.

2. The grid supply is low. The brilliance comes off the top end of the transformer winding, point e. The bottom end of the winding, point d floats on the bright up circuit, emitter of VT59. I suspect the extra resistor chain R85-R86 is a feedback mechanism to correct for main voltage variation, or to filter the grid supply, and shouldn't have much effect on your problem.
But have you checked the bright up voltage, at Vt59 e, or R77/R81. I would expect it to be around +60v for bright trace, going to +20v for minimal trace. Switch to XAmp to avoid confusing the issue by TB brightup pulses, when you should have a steady +56/60V.

This is interesting. The voltage at eVT59 is +89.3V with my DMM or +82.6V with the 100 Meg probe (On X-Amp). What also seems a bit odd is that it does not vary at all with the Brilliance control (?).

I have found that brightness problems are usually either the focus or brilliance resistor chains going high (under voltage stress), or the brightup high voltage transistors failing (as VT57,58,59 in this scope).

I have measured the P-N junctions in VT57,58 and 59 and they are all rectifying. Something at least.

You have measured the EHT voltages. Your curious results for the grid may be due to current taken by your voltmeter. It is normally bad practice to measure the grid voltage, as it is at very high impedance, and the current taken by your meter will alter the circuit conditions and so voltages drastically. An Avo on the grid will easily destroy the tube (that is why there are the two diodes from k to g -MR44-45). A 10M digital will confuse. A 100M probe will be satisfactory.

You caught me out! I did use a DMM to measure the grid voltage but even I would not have used an AVO. I remeasured with my 100 Meg probe but (sadly) pretty much the same results as with DMM, that is, -866V with Brilliance fully CW (brightest), -1117V fully CCW and 975V when the trace appears with little or no change as it goes fully CW.

3. Can you tell us what the brightup voltages are. That may be the key.
Do let us know what you find.

As above. You say 'voltages' - did I miss one or more?

You may be interested to know that the power supply has failed on my CDU110, eventually traced to failure of the dual electrolytic C10/C11. It needed reforming, as the positive plates of the two parts of the capacitor had shorted together, but one operates at +20V, and one at +0V, and so shorted out the -50V rail regulator.
When this is repaired, I will measure a few voltages for you.

That is a tough one.

I attach the circuit of the EHT supply so that people can follow the discussion, and contribute. Bill m0wpn

Given that the actual CR tube voltages seem resonable I still wonder whether the problem is not the CRT itself but very much hope it is not.

Can you interpret the bright-up results?

Sincerely, Anthony