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The problem is discovered and apparently fixed. I already had bought the
necessary parts.
The key, and it was absolutely necessary, was to disconnect C134 and C107,
the caps that send the vertical and horizontal blanking pulses to the grid-cathode circuit.
This done, all the voltages measured with my 200MOhm
probe made sense.
C109 and especially C108, the oil HV filter caps, were leaking badly: C108
was at best (cold) 25 MOhm, worse when warm. I replaced them with
0.47 uF 1600V film caps. The voltage on the iconoscope went too high
so I added a 1 MOhm 1 watt resistor in series with R125 (330K). The new
caps are larger than the originals, so I mounted them on a terminal strip
under the chassis. (They are polypropylene high reliability ones, and are very large.)
Its now making better images, the problem being mainly horizontal streaking
after bright or dark areas. Its quite stable.
I will now check C116, C117, and C118, the other oil caps. Only C116
is critical: if leaky it would kill V104. Well, no, I looked at the schematic
and it would just raise the cathode current. But I checked and its OK.
And I'm now the owner of two good iconoscopes.
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