GE dental x-ray heads?

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Rich Feldman
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GE dental x-ray heads?

Post by Rich Feldman » Tue May 05, 2020 4:45 am

Sort of a long shot here, and an ephemeral topic. From that same dental office, in mid March, I picked up a second x-ray generator. This one is bigger & fancier; supposed to go up to 100 kVp and 15 mA. Five wires between control box and the head. Registered with the state bureau of radiation safety and taxation.
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This venerable GE product line has a set of slip rings to allow 360 degree rotation of joint between head and manipulator.
The wires run in ordinary flexible line cords, one 2-wire and one 3-wire. So old that the inner insulation is hard and cracked in many places, at both ends of the cords.
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After pulling out the 5-brushes board, I was unable to disengage the slip ring shaft from its bushings. Anybody know how that works? Hidden screws? Press fits? Ended up taking down the awkward combination of head and manipulator still joined (minding the strong springs inside manipulator).

Anybody know the wire connections? Gnd and mA are obvious. The other three slip rings, P1 and A1 and X, are connected with low resistance inside the head - I guess primary windings of separate HV and filament transformers.

Yesterday I put the brushes board back in place and restored all the wires, but am having trouble with continuity at the slip ring interface. Anybody got any hints? Thanks!

[edit] Might as well add that the wall bracket was put together with a wire feed welder. :-)
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John Futter
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Re: GE dental x-ray heads?

Post by John Futter » Tue May 05, 2020 7:24 am

Rich
glazing of the copper is common for something not used for years. I've seen the same on brush excited gen sets.
i use the 3M green scouring pad with a little water with 30% detergent. wash afterwards with alchohol and dry with hot air gun

bet it works after this

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Richard Hull
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Re: GE dental x-ray heads?

Post by Richard Hull » Tue May 05, 2020 5:44 pm

John's advice is sage. I have stacks of the 3M green abrasive cloth and use it in all machining, finishing and cleaning work where sand paper would just scar and marr the piece. If polish is needed I use a buffing wheel with appropriate "stick" alumina or rouge. Also for glass windows and plastic I use the large bottles of buffing and polishing compounds found at Habor Freight. (3 grades, but $35.00 per large botle.)

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Rich Feldman
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Re: GE dental x-ray heads?

Post by Rich Feldman » Thu May 07, 2020 7:54 pm

Made some progress with mechanical disassembly. Not the joint in question but the next one up. It will make it much easier to move things around now.
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I bet this unit will never again serve in a clinical setting. :-(

Online pictures of similar heads show a U-shaped groove in slip ring shaft, near its root.
That would be engaged by two parallel round steel pins, like ones I removed from the fixed part.
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Shaft still refuses to move axially. Hard to believe assembly in the field, as seen in manuals for similar products, would include a press fit for this product line. OTOH, the parallel retaining pins never served as the thrust bearing which carries the weight of gimbal arm and x-ray head.
All models are wrong; some models are useful. -- George Box

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Rich Feldman
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Re: GE dental x-ray heads?

Post by Rich Feldman » Sat May 09, 2020 11:36 pm

After scrubbing the slip rings and brushes as suggested, I put new (less old) wires on & checked everything with an ohmeter. Then reconnected control box & found the x-ray generator unit to be functional.
The very first 1/2 second zap, with kVp turned down to 50, registered 100 mR on a quartz fiber dosimeter.

This was an opportunity to demonstrate analog metering of current in a HV circuit.

Put the old Soltec V-O-M in series with the "mA" wire, and found that the direction of current is from control box into the x-ray head. That tells us which secondary coil (cathode or anode) has its start-of-winding brought out, a detail I will leave for some reader to figure out.
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Dennis, I was close enough to push the x-ray button and camera shutter button with my own fingers. But had the x-ray nozzle well covered with lead bibs.

To noobs interested in radiography: these things are not toys. Energizing one is equivalent to uncovering many whole curies of a radioisotope. For an emitter of "low energy" gamma rays (60 keV) it would be more than 1000 curies. The dental x-ray beam would peg a civil defense radiation meter on any scale lower than "nuclear war fallout survival" range.
All models are wrong; some models are useful. -- George Box

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Rich Feldman
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Re: GE dental x-ray heads?

Post by Rich Feldman » Mon May 11, 2020 5:54 pm

The unit has been stowed in a shed to avoid distraction.
Not before producing familiar blue glow on the alpha-viewing film.
The nose cone assembly starts with a 17 mm aperture stop made of lead.
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That comes after a nonremovable 1.5 or 2 mm solid aluminum sheet as a beam-hardening filter.
All models are wrong; some models are useful. -- George Box

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