Electronics > Repair

conformal coating for tig welder PCB (milller)


I finally got that miller front panel working after a board swap and numerous repairs, and I need to repair and redo some conformal coating on the PCB because there is slots milled in it with epoxy and stuff like that, and tons of replaced parts.

I bought MG chemicals silicone conformal coat (with a strength of 1kV/mil) but I see that the go-to for repairs by qualified people might be something like the 4kV / mil corona dope.

The electronics have 800V rails.

I wanted to recoat the boards before I try welding with it.

So do you think the silicone is good enough, or should I buy corona dope?




My concern is that the board flexes a little (the main board is long and has alot of pressure to put in the 'cards' and its also bolted to wires and the big IGBT bridge, and it has some power resistors, so I thought maybe silicone would be better off even though it has a lower rating.

And the original stuff was kind of gummy, i thought it was silicone

bought a new fan too because the impedance test on the old fan is kinda dodgy its out of spec, damn them, I am scared to put some random shit in there because it looks like a contested electromagnetic environment lol

another question if anyone knows:

the manual says the fan should read 1.5 megaohm with a DMM. With a 34401A it reads 40kOhm.

I assume that this is bad, especially since there was arc and thermal damage to the PCB. I tried it on a 12v battery (D cells) and it works fine though.

However, reading a fan with a DMM seems bizzare. Its a 24VDC fan that uses 0.5 amps with 2 wires. Normally I would ignore it, but its an order of magnitude off, I assume that means there is something leaking or charring. If the replacement fan I get fits, I will take this one apart to see if I can find something, maybe. I don't want a bad fan leading to yet another rework of conformal coating.  :-DD

But here is the thing:
I got another old fan, that is 1/2 the size (from old spectrum analyzer), this fan reads 36kOhm. Is miller just making shit up with the 1.5 mega ohm? Do I have two bad fans that work fine? Am I using the wrong meter?

Because with a fluke 289, I read 500 kOhm. Which is still 70% off, which is better then an order of magnitude, but its still bad. I assume a damn fluke 289 should be similar to what they want.

and a 96 scopemeter reads 2.2 megaohm  ::)

I love well thought out test procedures!

TERRA Operative:
We just got the high voltage rated conformal coating just so we had a good overhead for the high voltages involved in TIG welders. Some have a pretty high striking voltage.
I have used it to repair arc-damaged PCB's (metal dust builds up and arcs across). Grind out the carbonized PCB with a Dremel (including adding some strategic slots to increase creepage distances), then coat/soak in the conformal coating to insulate it all again.
Never had a welder come back for that fault twice.

For the fans, if it spins at rated voltage and pulls about the expected current without too much weird noises, send it. :)

It was a common thing to pull control PCB's out and wash them in the sink with dishwashing liquid and a soft brush. Blow with compressed air and maybe leave in the sun for an hour or two.
This was especially the case with Cigweld welders that have the main control PCB oriented horizontally at the top of the machine. Metal dust would settle until it shorted out all the control electronics and the machine would just not work at all.
Clean the PCB, stick it back in and it ran happily until the next service interval.

oh yeah the boards were ultrasonic cleaned and I used THF to remove the conformal coating in rework areas.

I took out all the transformers n shit right now and ultrasonic cleaned, alcohol sprayed and 80C baked everything. Dust everywhere.

For the fan, I will just give it a drop of silicone oil near the top since i peeled the lable off. Hopefully I should have it back together soon and see if I managed to break anything else. They don't seem to have fine magnet wire transformers here so I am not too worried but never know... but I won't tolerate filth growing in everything lol  :o

For charred repairs in it right now, I used a ball end end mill and rubberized abrasives to remove char, then refill the areas with DP270 loctite epoxy (slots cut between a few traces) and teflon substrate behind it for curing. Maybe I sprinkle in some fiberglass powder in a few of the deeper cuts too, tried to follow PACE repair procedures. The arc starter board looks OK though, I did not work on it, and its clean, lots of goop on the spark gap solder joints too. Maybe I should remove that and put fresh electronics grade silicone sealant there hmm but I see no evidence of damage.

If the extra silicone goop over the spark gap is yellowing, replace it? I was going to use chipquik silicone if itis yellowed

well I was hoping to spray it on today but I don't want to coat those old caps on the hot power board touching the iGBT's so I need to wait... don't trust anything on this board...'

replacing all those power resistors that had the burned pcb around them,  I fixed the PCB before but I never bought new resistors because I was not at that point where I think it might work. Looks like I need two PCB rivets to fix some damaged crap.


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