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Faxitron MX20 x-ray problems...

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mikeselectricstuff:
So just as I need it for a teardown vid, my Faxitron has started playing up...
From power-on it will sometimes take one or two decent exposures, then the x-ray power falls off rapidly.

The beam current, measured from the test sockets, falls off substantially, from its nominal 300mv, down to about 20 or 30mV over a few mins - not very consistent but seems to be getting worse. 

Other voltages seem OK - EHT measured from test sockets is rock solid, as are filament & focus voltages.

One thing that does change significantly between standby and run modes is the grid voltage - this sits at its nominal -140v in standby , then rises to 4-600v when the beam is on. The manual doesn't say much about the conditions under which the voltages are measured though and there isn't any more info, though the way the HV modules are wired suggests this is normal. 
Disconnecting this & measuring the PCB side makes little difference, so it isn't something coming back from the tube.

Does anyone have a working MX20 they could measure some voltages on for me ?
I'd be really grateful if somone could measure  supply inputs to the HV modules, as these determine the HV outputs of the modules.  Points shown in orange on attached pic. Can be measured relative to chassis, in both standby and run mode at a known kv setting.

Fortunately these are accessible from the top without removing the PCB.


 

KE5FX:


I noted in Shahriar's recent Signal Path video that he identified the reed relays on this board as being the cause of his own trouble.  Not sure if it was the same problem you're seeing, though.

Edit: well, that's weird... I went back to double-check the measurements, and the second one was 7.90V during an 18 kV exposure, rather than 6.547V.  It read 8.5 at 35 kV, then showed 8.05 when I returned the voltage to 18 kV.

Beam current is solid at 324 uA at any voltage 18-35 (actually 19-37, I like to live dangerously).

mikeselectricstuff:
Thanks - those active voltages are rather higher than I was seeing, but it is tube ( and possibly HV module) dependent and I think I was measuring at 35KV

Annoyingly, I've not seen it work 'normally' at all since I started measuring things so can't compare voltages in "good" vs. "bad" mode.

Perhaps it might after it's had a night's sleep...!
 

mikeselectricstuff:
Just re-measured - at 18kv I get 4.7/8.7/2.2  and at 35kv 8.2/9.2/2.2
However as the beam current was decaying over a few exposures, these didn't change, so I don't think things are looking good for the tube...

I did test the reed switch - it wasn't really clear what actually fixed Shahriar's unit

Next step maybe to open up  the tube enclosure & check the EHT connection, though I think I'd hear some arcing of there was a bad connection there.

As far as I can tell, these drive voltages are closed-loop based on the measured HT voltages, so your difference in voltage could perhaps be down to warm-up, the tube drawing different current so needing a different drive level to the HVPSUs perhaps.

KE5FX:
Good point, tube warmup could account for it. 

Do you regularly season your tube? 

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