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x-ray head disassembly and next course suggestions

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JonIverson

Sun Aug 16 2009, 09:12PM

Registered Member #2300 Joined: Sun Aug 16 2009, 01:49PM
Location: Hollywood, CA
Posts: 17

Hi,

I'm brand new to the forum here. I recently acquired a dental x-ray head which was leaking oil. I tried to locate the leak with the hopes of sealing, but I was not successful, so today I disassembled the head. It was soldered shut, so the task was difficult but I managed to open it up without internal damage. The unit is a 70 KV, 10 mA dental unit. Everything inside appears to be intact and there is the main transformer, filament transformer, and of course tube. along with some lead shielding here and there.

My goal is to reuse the system for some radiography. I already have a 1/4" lead box that the unit would fit into perfectly, but I am not sure what direction to take next. I could continue to use the current transformer with tube (assuming I can figure out the correct wiring for the transformer) or I was thinking of perhaps building a flyback driven multiplier (with the multiplier and tube in oil), but I don't think I could obtain the same current like with the original transformer.

I welcome any suggestions. I may post a couple of pics of the interior of the head later.

Thanks,
Jon

3l3ctrici7y

Sun Aug 16 2009, 09:47PM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

Post pictures, and perhaps we can help with the wiring and other info.

My xray head has a number of connections, Two are for the mains, two are normally shorted but can have a mA meter attached for measuring the anode current, and four are used for selecting what input voltage is being used.

Unless you have a reason to power it differently, and your goal is to take pictures with it, don't fight battles unrelated to your goal; just power it the way it was intended, and that will give you the fastest results.

Unless you have proper radiation measurement equipment, don't put it in anything (ex lead box) and then expect that you're safe. Simply setup the shot as necessary, then evacuate the area and take the picture.
It is easier and faster if you just leave the area. You can then have a smaller package and are able to image more things because you have a more versatile unit, not encumbered by the lead box. Though the safety of those around you is your responsibility; if you're in an apartment with people on 5 sides of you, this is more difficult.

At 70kV and 10mA, that is 700 watts of power dissipation in the tube alone. Do not run it for more than a few seconds at a time, and then allow many seconds for it to cool. As an example, if you run it for 1 second, and allow it to cool for 9 seconds, you will have 70 watts of average power dissipation.
Personally, I would not run it for more than 3 seconds, and then allow at least a minute or two for it to cool, though more cooling time is better.

Myke

Sun Aug 16 2009, 09:52PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Welcome to the forum.

First, I would check if the oil had any PCBs. I think that PCB testing kits don't cost all that much.
There may be air bubbles in the windings if the transformer was in air. You don't want to power up the transformer out of oil otherwise it will most likely arc over internally. To remove the air, you need to put it back in oil and put it all under a vacuum.

JonIverson

Mon Aug 17 2009, 10:46AM

Registered Member #2300 Joined: Sun Aug 16 2009, 01:49PM
Location: Hollywood, CA
Posts: 17

Thanks, guys, for your fast response. It's nice to see a forum that is quite active.

Myke: Thanks for the tip on PCB's. There used to be a simple flame test to check for those. It's on the web somewhere. IMO, this head didn't have any being made in 1991 and is actually still currently produced. The transformer/ air issue is something I've wondered about. I know some folks re-oil theirs without vacuum successfully and that's the way I had hoped to do it. If only I knew where those instructions where.....

3l3ctrici7y: Thanks, I intend on trying to grab some pics today and posting them. There are four connections on the housing that lead to the filament and main transformers. I suspect two is all I will need with the other two either shorted for use without meter. Not sure, which is why I'll post a pic. Myke had pointed out the air in oil issue, which was the only reason I was thinking about using a flyback driven multiplier instead, but if I can re-oil successfully I may not go that route. Plus, I doubt I could ever get 10 mA at 70 Kv with such a circuit but could be wrong. As for the radiation concern, I have done similar experiments in the past with old style stuff (Crookes tubes, etc) and did, at the time, obtain detectors capable of measuring low energy x-rays which are often missed by regular geiger counters. The lead box is fairly safe, but it does leak in several spots which is why I'm not near it when anything inside is operating.

Jon

LutzH

Tue Aug 18 2009, 06:24AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Dear Sir:

X-ray is my bag, when you get the pics I will be happy to walk you through the whole process if need be. I have been playing with these for over 20 years. I agree completely with the other postings especially the response urging cation due to heat buildup in the tube, they are spot on.

Many old dental units are self-rectified, and can benefit from rectification and filtering. You can get the same x-ray output with less than half the heating this way. X-ray production is only at a reasonable level of efficiency at the crest of the sine wave peaks, the rest of the energy just heats the tube, so when you rectify and filter with a HV cap. then your efficiency goes way up. This is one reason X-ray equipment makers tout "High Frequency" in there adds.

Send me a private email and I will give you my regular email address which I check more often. I have gotten a lot of help in this forum from others, so I am happy to pay it forward.

Aloha......Lutz :)

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