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

[MindBender]

I wrote a MATLAB package for cone beam CT on the DX-50, it should run the MX-20 since it uses the same protocol.
http://www.rtftechnologies.org/physics/faxitron-DX50-CT-scan.html

Would you consider sharing the source?

[CwazyWabbit]

Looks like they are on his website, follow the link in his message.

[KaneTW]

I found a Faxitron unit on eBay with a DC44 for a reasonable enough price that I immediately bought it.

Has there been any progress in reverse engineering the interface for it, or is it possible to reasonably replace the imaging sensor?

[KE5FX]

I found a Faxitron unit on eBay with a DC44 for a reasonable enough price that I immediately bought it.

Has there been any progress in reverse engineering the interface for it, or is it possible to reasonably replace the imaging sensor?

This is still a good source of info to start with.  I archived it under docs/Haidekker in the zipfile in reply #52 above, if you haven't seen that yet.

There is probably no point in replacing the Bioptics sensor unless you can find a Hamamatsu panel.

[KaneTW]

I found a Faxitron unit on eBay with a DC44 for a reasonable enough price that I immediately bought it.

Has there been any progress in reverse engineering the interface for it, or is it possible to reasonably replace the imaging sensor?

Small update, the seller had a DC12 unit available and is sending that instead.

[KaneTW]

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.

[Fraser]

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

Fraser

[KaneTW]

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.

[KaneTW]

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...

[Fraser]

No objection to showing the schematics.

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

Fraser

[KaneTW]

Here's the set of links:
Gun board: https://i.kane.cx/dFTeBu https://i.kane.cx/8JmAtu https://i.kane.cx/xvgpbQ
Controller board: https://i.kane.cx/H7T6TI https://i.kane.cx/GmY9hV https://i.kane.cx/VGPG0v https://i.kane.cx/TGXgZR

Gun board annotated with measurements:


Thermal cam shot:
 

The component getting hot is the reed relay K1. The temperature did not noticeably change whether it was on or off.

Some scope shots of the mA current output:
10kV: https://i.kane.cx/Mh6Z1i https://i.kane.cx/qQl6TF
20kV: https://i.kane.cx/FQfNOj https://i.kane.cx/3cXARo
35kV: https://i.kane.cx/s8l59J https://i.kane.cx/gcR71o

The high frequency content is always present and I'm pretty sure it's just EMI/bad probing (the current output port only fits a multimeter lead). The ~25kHz content however is increasing with increased kV, so something's weird there. Note that the average remains roughly constant at -250mV.

[KaneTW]

After some debugging I noticed that L2Q was set to a way higher value than the manual specified, around -280V. It does claim that you need to follow the tube test sheet but I adjusted it to -200 or so and got closer to the 0.3mA of current that it should have. Still can't go above it (so really bad SNR at lower voltages) but that's good enough for me since I image at max voltage.


Some images of a 10-layer board attached.