[Test-Equipment] Hickok 539B with shorts light problem

[Pat]

All:

I have a Hickok 539B with a non-functioning shorts light.  I have a 
known "bad" tube that a BK600 and a TC162 flag as leaky that will not 
fire the bulb. Even using a simulated short using a 100K resistor at a 
socket will not fire the bulb. Fooling with the switches will not 
aggravate the lamp into firing. The switches and interior are still 
relatively clean.

Here's what I've done so far:

1. tested the lamp by using a fixed series resistor and a variac, no 
problem, bulb is okay firing at about 60 volts
2. checked the three fixed resistors shown near the lamp in the 
schematic and tested the 250k sensitivity pot and toggle switch, all okay
3. checked plate supply, okay
4. checked screen supply, okay
5. general visual inspection for any cold solder joints, none found
6. replaced paper caps with mylar, none of the originals found leaky
7. none of the postage stamp caps were replaced
8. There were two 47 ohm 1/2 watt resistors embedded in the shorts 
switch assembly that don't show on my schematic, one was discolored, 
both were replaced

The tester works but reads high due to the bias meter reading too low. 
Using a DVM to set bias gives a plausible reading.  I can measure a 
resistor across a tube socket on the gm meter but it is off about 100k.

Is there anything that commonly goes bad after 50 years in this shorts 
test circuit that I have over looked? I've probably got about 20 hours 
of spare time into this thing already. I've not really been able to 
follow the schematic back to the voltage source. Perhaps someone already 
knows how to check that source.

Pat