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Faxitron MX-20 Reverse engineering (with some good results)

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Unit arrived. Seems in good condition, except:

* Tube current output shows something weird. -0.20V in VDC, 0.06V in VAC, 0.21V in VDC+AC. Voltage is on point. There's an image produced and I have no idea what's causing the weird current readout.
* Rear USB-A to USB-A adapter didn't work, I had to remove it and connect the cable directly for my PC to detect the sensor.

Apart from the weird tube current issue everything looks ok.

SR Software and Technical manual link sent today. Hopefully that will assist you in testing your unit.


I queried Gamma High Voltage and the current monitoring is a 1kOhm resistor + capacitor + 2 Diodes placed across GND and HV return. So a negative voltage is expected. This is also confirmed by the schematic having an inverting amplifier (-10x) on mA feedback.

I believe the oscillation is actually occuring in that case and not just a readout bug. Once I have the chance I'll probe the current setpoint and see if it's oscillating as well.

I've probed the current monitor with my scope and the it looks like EMI to me. I've debugged further and noticed a peculiarity.

There's a reed relay (Fraser, can I post the schematic?) K1 that ultimately switches the L2-Current an L3-Focus outputs. It does so via switching the wiper output from the L2/L3 pot to an opamp.

The voltage before the relay is nominal. The voltage after the relay is much higher -- I've measured:
Wiper: 1.31V, node after relay (R5/R3) 3.3V
then 1.47V -> 5.26V, then 1.56V -> 6.08V

The inverting output is at most 2.3V, so the opamp goes to its max possible output of 10.5V pretty quick, which corresponds to about 9.2V on the BJT emitter that is upconverted to the tube control voltage.

I need to take out the gun board and probe some more, but it's a huge pain to reach the bottom srcrews...

No objection to showing the schematics.

I will look in the service manual to see if there is anything in there to assist you.



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