On Wednesday, December 25, 2019 at 2:08:45 AM UTC-8, William Beaty wrote:
Monitor 4, "Radiation Alert"
S. E. International Inc, Summertown TN

These little GM counters have a typical failu the 500VDC voltage falls low,...
three zeners in series are used as feedback reference: one of 1N5272 at 110V, and two of 1N5281 at 200V (to create a 510V zener.)

I found that all three were damaged and leaky

For HV use, these parts should have long lives (they're glass/metal packages, no epoxy).
How old IS that unit? Has it been in any odd environments (neutron flux?) ?

Replacement with 1N5388 might be a good idea; the axial parts are stil metal/glass,
just have an epoxy overcoat. Surface-mount packages, though, I'd be less sure of.

The glass-package parts DID have some light sensitivity, sometimes, so testing
for leakage can be an interesting challenge.