I have a Hickok 550x that I've been tinkering with. I've...
- replaced the 0.1 MFD cap, left all the mica caps alone
- tested the two tubes (83 and 5Y3)
- Freed up stuck rotary switches
- Removed two 0.5 MFD caps down in the multimeter section
but these have not been replaced yet.

Just for fun I fired up the set in the tube tester mode - I
was hoping the two .5 MFD caps only functioned when the
multimeter mode was used. The "fuse lamp" had lost its slight
glow and I took this to mean the caps had been leaky.

The tester acted funny though. I tested some 12BA6 and
12BE6 tubes and got the same very low reading for a number of
tubes, regardless of new, used, previously tested good, or what.

I also tested a diode tube (6W4GT I think)and it pegged the
meter off the scale.

Was I an idiot to expect correct readings without replacing
those two .5 caps in the multimeter section, or is something
else going on like resistors out of spec? I have the caps
on order.

If I'm reading the schematic correctly, the two 0.5µF caps are only used for the AC voltage ranges of the VOM.

I would start with testing some diodes since those are emission only, and use less of the tester circuitry. Then progress to triodes to keep things simple.