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Repairing X-ray machine

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echase
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Username: echase

Post Number: 675
Registered: 07-2007

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Posted on Wednesday, 05 December, 2012 - 12:21 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Anyone got any experience of mending X-ray machines? I brought back the main power PCB from an X-ray machine in South Sudan and am trying to repair it. Geckos have used the PCBs as toilets and there is severe corrosion of tracks and component leads. I have repaired 2 tracks and replaced 4 components and it now looks as if it might work but tehr is still all the rest of the machine, which visually looks less damaged but without the main power PCB working there is little power to the rest to test it. Fortunately General Electric publish the 285 page service manual on their website so I have partial circuit diagrams of it. Ideally Iíd buy a new PCB but no one seems to sell them and they would be hundreds of pounds even though not very complicated as anything for medical use is much more expensive that any other product. More that the South Sudanese medical service can afford.

Thanks to previous generosity from some here I had previously replaced the mouse chew ribbon cables but this did not clear these other faults. We are blocking all the ventilation holes with grills to prevent further vermin attach. Fortunately the 135kV generator for the X-ray tube is sealed against such attach so we donít have to play with these high voltages. But there is still 680V DC on a huge capacitor bank that feeds that generator.
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joe
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Post Number: 1323
Registered: 05-2005

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Posted on Wednesday, 05 December, 2012 - 01:07 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

When you say the PCB isn't very complicated, could you make a new one ?

Could you post a picture of it ?

Joe
My projects, technical info and ramblings at www.hobbyelectronics.net/
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ant
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Post Number: 929
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Posted on Wednesday, 05 December, 2012 - 03:35 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello Ted, have you tried GE's generosity factor to the full?

Regards Ant
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echase
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Post Number: 677
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Posted on Monday, 10 December, 2012 - 01:10 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Sorry for delayed response. I could try GE.

The board is 250 x 100mm, double sided, plated through so would be quite expensive to replicate. I am also worried that if I do a copy it would loose the analogue calibration of some circuits that do current measurement using current transformers. I think I can fully repair it as long as I carefully check all its functions so I know it is giving out all the right outputs. It wonít be as reliable as a new board but then the rest of the machine isnít great either.

I am more worried about what happens when the local electrician plugs this board in again next month without my there to supervise him. 680VDC could create a spectacular arc across the contaminated PCBs attached to the capacitor bank, which is the size of a large desktop PC. We did wash those PCBs and brush off all the dirt but there will still be contamination between components and PCB where washing does not fully penetrate.

People say water and electrics donít mix but I find washing PCBs very effective at cleaning them up, especially when I dropped my mobile down a dirty toilet there.

We repaired an ultrasound machine that had so much dust in it that one DC power rail was shorted out. Never met that sort of fault before. Its cooling fans are 50mm off the floor so just suck in loads. OK in clean Western hospitals but not there. Fortunately they had a tool like a leaf blower that we used to raise a huge dust cloud.
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zeitghost
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Post Number: 1777
Registered: 01-2006

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Posted on Tuesday, 11 December, 2012 - 08:40 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I read somewhere that the best way to clean pcbs is to use a dishwasher.

Probably from Bob Pease, a chap who knew what he was adoing of.

Dunno which is the best detergent to use though.

After the wash, place in a warm fan oven at about 100 deg C for that ultimate drying effect.
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echase
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Post Number: 678
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Posted on Tuesday, 11 December, 2012 - 10:41 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Dishwasher sound OK for some PCBs but not good if you have components on there that can take in water, e.g. relays, switches, loudspeakers. Suggest though that only a very low concentration of detergent be used as might attack paint on resistors bands and corrode some metal plating, etc. This power PCB would not take dishwashing as has contactor and relay on it. But I could remove them and then dishwasher it.

Is one small problem though, in South Sudan, there arenít any dishwashers there! During a huge thunderstorm I did wash a big inverter in the large flood outside my hotel door. The people with me thought I was mad but I can now point out to them itís not a mad as using the Chatzone approved dishwasher method!

(Message edited by echase on 11 December, 2012)
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zeitghost
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Post Number: 1780
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Posted on Wednesday, 12 December, 2012 - 11:53 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Cleaning stuff in 3 youtube vids:

http://www.youtube.com/watch?v=kOPPFgHOBKc
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atferrari
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Posted on Wednesday, 12 December, 2012 - 02:31 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Ask Dean Huster

He posted, some time ago a long text with details for washing and drying PCBs.

Long time in the oven at not so high temperature IIRC.
AgustŪn TomŠs - Buenos Aires - Argentina
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bowden_p
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Post Number: 167
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Posted on Thursday, 13 December, 2012 - 12:22 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Echase,
I certainly wouldn't recommend the dishwasher treatment - ours strips paint quite nicely off wood/plastic handles, so would also attack the coating on components too. Long immersion in water could damage the board material or other components. I have used IPA to clean boards effectively, and this dries quickly too. Use a stiff brush to agitate the dirt.
With regards, Paul.
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echase
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Posted on Friday, 14 December, 2012 - 03:06 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

IPA is good for some things. In the link above http://www.youtube.com/watch?v=kOPPFgHOBKc he is using soap (dishwasher liquid?) and water to clean a power amplifier. I would not recommend any soap on components like the aforementioned relays as any soap residue inside them will affect contacts and cause later corrosion. So just use clean water or IPA (although not sure IPA will shift dried mouse/gecko urine!). But I have used dishwasher liquid on sealed PCB components. IPA would affect any switches or variable resistors that have grease/oil in them whereas clean water should not.

In manufacture PCBs are often washed after soldering to get rid of flux etc. What liquid is used then?
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zeitghost
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Posted on Friday, 14 December, 2012 - 05:13 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Used some sort of green gunk from RS diluted with water in an ultrasonic bath back in the days when I worked for a living.

As long as the piezos weren't washed & the boards were baked for a time at 100 deg C it was ok.

Washed piezos went an interesting corroded green colour. Still worked though.
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bowden_p
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Post Number: 170
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Posted on Friday, 14 December, 2012 - 07:08 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Echase,
For total immersion cleaning after assembly, all components have to be sealed. Many components which used to be "open" - like relays - are now sealed for this reason. Often a solvent cleaner would be used with some ultrasonic agitation. More recent fluxes are water soluble, so a more ozone/environmentally friendly water wash is possible. In your case, where the contaminants are not assembly related, it sounds like a case of making it up as you go. I'm sure your experience will guide you, but I guess you would be advised to remove components not intended to be dunked in water - unsealed pots, variable caps etc. Device sockets present a problem - perhaps a clean round them with a cotton bud, then sealing up with some tape before the "main" overall wash perhaps? And put some disinfectant in the water too! What a horrible problem to deal with!
It sounds like some mods are needed to keep the contaminating wildlife out in future - perhaps some fine perforated sheet metal pop-riveted over air vents?
All the best, Paul.
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echase
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Username: echase

Post Number: 680
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Posted on Monday, 17 December, 2012 - 02:31 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Getting to the end of the repairs and cleaning of this board. The total damage was 2 PCB tracks corroded through and 4 LEDs with no leads left. I removed 3 component sockets that were looking dodgy and soldered 2 new ICs and one old relay from them straight to PCB.

Of course it is not as good as new as I have not replaced every corroded component, so there is the risk that the residual corrosion/dirt will cause later failures. But as there are several other PCBs that I did not bring back to UK, as too big, that have corroded bits there is not a huge value in making this PCB pristine whilst the rest of machine is not.

Would a spray can of conformal coating help to slow down any further corrosion? Could be useful around the 680VDC area too as added insulation, but am wary of sending a can to the electrician and asking him to spray it around as he wonít be able to distinguish between components that are OK to spray and those that are not, like semi open relays, switches and connectors.

Can the spray penetrate between mating faces of connections and cause high resistance, e.g. in IC sockets and multipole connectors? That would prevent me using it on large areas of this PCB.

(Message edited by echase on 17 December, 2012)
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bowden_p
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Post Number: 171
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Posted on Tuesday, 18 December, 2012 - 02:12 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi echase,
Is the coating available as a paint-on formula?
It might be a bit more controllable than a spray in inexperienced hands. A conformal coat is often used to provide extra insulation and environmental protection. (e.g. against humidity in particular.)

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