Hi!
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Once more the magic eye iris changed colour to a gentle green (with central bluish pupil and a grey internal ring (arcus).
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That sounds suspiciously like your EM34 "magic–eye" balance indicator may be needing replacement due to the presence of gas in the envelope, causing a bluish glow from under the cathode/heater system light shield cap.
The price of these indicator valves is rocketing alarmingly every year because of their scarcity, but you can get a much cheaper Soviet–made 6E5S type that will do the job adequately enough without impairing the appearance of the instrument.
EM34 characteristics:–
Heater 6.3V 0.2A, Target Anode pin 5, 250V, Anode Resistors, pins 3 and 6, 1M, Grid volts for shadow–closure, –5V (1) & –16V (2).
Pin 1:– i.c., pin 2:– h, pin 3:– a', d', pin 4:– g, pin 5:– t, pin 6:– a", d", pin 7:– h, pin 8:– k, g'
6E5S characteristics:–
Heater 6.3V 0.3A, Target Anode Pin 6, 250V, Anode Resistor, 1M, pin 3, Grid Volts for shadow–closure, pin 4, –22V.
Pin 1:– Not used, pin 2:– h, pin 3:– a, d, Pin 4:– not used, pin 5:– g, pin 6:– t, pin 7:– h, pin 8:– k, g'
To rewire an EM34 holder to suit a Soviet 6E5S, disconnect the 1 megohm resistor between pins 5 and 6 and discard it.
Disconnect the h.t. lead from pin 5 and reconnect it to pin 6, and also disconnect the 1 megohm resistor connected to pin 3 from pin 5 at it's other end, and reconnect the free end to pin 6 along with the h.t. lead.
Also transfer the 4M7 resistor and 0u01 capacitor from pin 4 to pin 5, or simply connect pins 4 & 5 with a short piece of wire.
No other wiring alterations are needed.
Chris Williams