Author Topic: Fine White Dusting / coating on components (capacitor) in power supply  (Read 684 times)

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Offline Purduephotog

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I've seen this before on some parts that failed- but I couldn't find a specific cause. On my last subwoofer I had a pack of resistors that looked like they were the source- but they all tested 'good'.

This is the power supply for a vacuum sealer. One of the chamber vacs you put a whole bag into, it evacuates the air, and then seals the top of the bag. It wasn't pulling a hard vacuum so I took it apart (again) to see if maybe the pressure sensing valve was bad. (I've subsequently remembered that there is no sensing valve in this unit, just a timer. Silly me).

Anyway, the power supply has a fine white coating on 1 of the capacitors, but not the other. It's also present on some smaller caps on one side and heat sink, but again not on others. Present on the bottom of the board in one spot that didn't have the insulating plastic too close to the surface- but again, not on others.

At first I thought a capacitor venting but doesn't appear to be any issues with the tops or sides. There is a metal film resistor near where the issue is but again, seems intact.

Any idea what I'm looking for here?

Offline Purduephotog

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Photos of the components since I apparently still can't figure out how to post content.

Offline Relayer

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Hello Purduephotog,
Does the unit use a cooling fan?
If so, it looks like the airflow of it is only passing by that particular cap.
Strange as it seems, air can flow in funny ways.

Offline Purduephotog

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It's inside an enclosed unit that pulls vacuum. The air flow isn't direct to that, and the 'inlets' that are inside are supposed to pull air in only...

It really is very well coated. I'd be tempted to say it looks and feels 'powder coated' if that made any sense- like a component got very hot and vaporized the surface. Even went to the trouble to look at everything under UV light and see if anything looked funky. No Dice.

I know I took photos of this before when I bought the unit, just don't have that system up and running to take a look for. I think it's new... but I'm guessing this isn't a common phenomenon by the lack of google-fu results.

Offline Doctorandus_P

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My best guess is that this is simply very fine dust, which is attacted to some traces / components because of electric fields.

It is very likely this has to do with some kind of leakage or failed component because of the way it is spread uniformly on some components, and not others, and is also on the bottom of the PCB.
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Offline slbender

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Hi, I just noticed a similar non-uniform white compound only on the outside of a single control in my older HP 3580a spectrum analyzer, its powdered up only on the "focus" control and that might have a high voltage internally, since there seems to be a problem with the display on the CRT.  Maybe the voltage is abnormally high on this control due to some other as yet undiagnosed failure, which could attract microscopic particles for some unknown reason. Set has no fan, and the CRT is well shrouded in metal shields, so it must be the presence of high voltages that causes this effect. So Peculiar!


Online james_s

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The focus voltage in a CRT display is typically quite high, second only to the final anode voltage. It's common for dust to collect on both of those due to the static charge.

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