Hi all,
I’m having an issue with my GMC-500+.
Tube 1 (M4011, marked 2022) constantly shows 0 CPM, while Tube 2 still detects background radiation (~1–2 CPM), so the device is not completely dead.
What I’ve tested so far:
Tube 1 voltage set to ~42% (~417V), later reset to default (~52%): still 0 CPM
Performed factory reset: no change
When I manually short/touch the Tube 1 input terminal, the green LED flashes and CPM1 increases, so the counting electronics seem to work
So it looks like:
counting circuit works
Tube 2 works (low sensitivity as expected)
but Tube 1 produces no pulses at all
My question:
Is this more likely a dead M4011 tube, or could it still be a HV supply issue on Tube 1 channel?
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GMC-500+ Tube 1 (M4011) shows 0 CPM – need help
Posted - 04/03/2026 : 12:42:41
