| T O P I C R E V I E W |
| bialex |
Posted - 04/03/2026 : 12:42:41
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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| 1 L A T E S T R E P L I E S (Newest First) |
| 3z_phoenix |
Posted - 04/07/2026 : 06:03:08
Hi bialex,
looks like you slide into the worldwide easter-bunny rabbithole :)
for my thinking, you have all possible check done.
I see it same way, there is highly possible a defective M4011 tube1.
Best option for this is, let do GQ-electronics support-team do the repair, whrite them an email: support@gqelectronicsllc.com
maybe they can also send you a fresh new tube, and you can do the replacement.
But keep in mind, its some tricky to measure the HighVoltage above the tube, it not only need a voltmeter with inner high resistance
typical Ri~ 10 Megaohm is too low, you need a voltage-divider also! and with do some math.
The problem is not the high voltage, the prob is the high resistance of the HV-source.
What else you can do, check the area around the tube-contacts, it have to be clean.
Any dust and some wet air can shortcut the voltage above the tube.
(Like an old TV-set, the high voltage atract statical the dust in the air...)
Some years i still read the forum here,
and found user EmfDev looks like in very close contact to GM support.
...keep my thumbs under pressure...
kind regards,
3z_phoenix
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