Inverter as a boat anchor? - Page 1
Forum section: electronics / beginners

author: jerryk
author_rank: Regular Contributor
timestamp: April 11, 2019, 09:09:38 pm
content: I have a friend that keeps having inverter mishaps, the spoils of which he passes to me. The photos show the latest disaster. It was new last year but got exposed to salt water while it was on. A major part of the damage is in the photos below. Boards are again covered in conformal coating to make repair prospects more distant. I think this thing is just toast but may practice on dealing first with corrosion then strip the conformal coating on some of the boards and see if how far I am willing to go.Beyond that I may salvage some of the parts. Is the transformer any good given the extent of the corrosion? Is there anyone that sees this as repairable given the proper level of skills?Jerry

author: james_s
author_rank: Super Contributor
timestamp: April 11, 2019, 10:10:29 pm
content: That doesn't look too bad, I've certainly repaired stuff that was in worse shape. Desolder the affected parts and scrape the coating off with a piece of stiff plastic or use a brass brush in a rotary tool. Do the initial desoldering outside or with some decent ventilation since the coating will probably stink.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2019, 08:05:13 am
content: If it wasn't for the conformal coating, it would be a lot worse. After you've done the repair, I'd recommend covering it with a layer of conformal coating, which is necessary as it's clearly used in an application were it might get wet.

author: jerryk
author_rank: Regular Contributor
timestamp: April 12, 2019, 09:39:21 pm
content: I am making some progress and feeling a little more optimistic about the prospect of this turning into a repair. The FET's were difficult to remove but I did manage to remove the damaged row of them and got things cleaned up. So far the rest of the inverter board tests out ok as far I can tell.Next question is what to do with the rusty transformers. This is salt exposure so it seems to have had a real impact to the cores of these guys. I also can see in the main transformer signs of salt/corrosion residue in the outer windings under the outer tape layer. Not much but enough that I don't want to leave it. I'm not sure if I might chase the salt further into the windings or if I should even worry about it. Should I try to wash the transformers in distilled water followed by IPA soak? Can I take the wire brush wheel on my bench grinder to the rust on the outside and then revarnish?I've attached some photos of the rust I'm up against and the progress made on the inverter board and the mcu after cleaning.Thanks for the help.Jerry

author: james_s
author_rank: Super Contributor
timestamp: April 12, 2019, 09:54:05 pm
content: Rinse out the salt with clean water, let it dry thoroughly, varnish if desired. The cores you can either leave alone or you can wire brush them and coat with paint or varnish but the rust on the iron is cosmetic and won't affect operation.

author: jerryk
author_rank: Regular Contributor
timestamp: April 12, 2019, 10:35:13 pm
content: Quote from: james_s on April 12, 2019, 09:54:05 pmThanks - I was over thinking things and wondered if a high speed wire wheel would induce some disruptive magnetism into the core. I have some rust stabilizing paint that we use on structural steel that works real good that I may try.The FET's were fairly difficult to remove and reinstalling them is a process I'm not too sure of. I have a hot air station and have used it to attach various ic's but this board is the heatsink that transfers heat directly to the case. It was difficult to heat the FET's enough to remove them. I did resort to cutting away the FET's casing till I exposed it's heat sink and things went much faster. I'm not real sure of what the best way is to reattach them given the boards ability to dissipate heat and my limited tool set.Any ideas are appreciated.Jerry

author: james_s
author_rank: Super Contributor
timestamp: April 13, 2019, 12:18:13 am
content: You can magnetize the core any way you like and it will be wiped out as soon as it's powered up. The degaussing coils used to demagnetize things and erase tapes and stuff are just an iron core with a coil on it, the magnetism induced by the coil is orders of magnitude stronger than anything you'd hit it with externally. Transformers are pretty much indestructible as long as the winding insulation remains intact.

author: jerryk
author_rank: Regular Contributor
timestamp: April 13, 2019, 10:24:15 pm
content: I have two items on the control board that I want to replace but am having difficulty identifying. One is the clock crystal labeled FS4 91 5P and the other is a small smd transistor package (I think?) labeled C175. Any ideas on a number to search on Mouser with?Jerry

author: jerryk
author_rank: Regular Contributor
timestamp: April 16, 2019, 09:01:20 pm
content: I am at the point of installing the new FET's on the inverter board.To recap - this board is giving me major difficulties with regards to heat dissipation. Attached are photos of one of the new FET's in position ready for soldering and the back side of the board where it will go. Removing the FET's took all the preheat and heat that my hot air station had. Eventually I broke off the top of the bad FET's exposing the tab and removal went much faster. As a further example of the boards heat dissipating properties, I have not yet been able remove the any of the ceramic caps next to the FET's even using my Hakko FR-300 desoldering gun on the back side at max temp with my Hakko FX-888D at 850f on the front side head the leads.So this is uncharted territory for me and I'm wonder if any of you have any suggestions on how to proceed with reattaching the new FET's. I do not have a reflow oven and am limited to a couple of soldering stations and hot air rework station.Any ideas are truly appreciated.Thanks - Jerry

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2019, 03:44:37 pm
content: The only thing I can think of is pre-heating the entire board. Try putting it in a cast iron frying pan and heating it on the hob. Obviously don't get it hot enough for the solder to melt, just enough to make it easer to solder.

author: james_s
author_rank: Super Contributor
timestamp: April 17, 2019, 03:50:55 pm
content: Do you have a heat gun like the type used for thawing pipes and stripping paint? With care that can give you all the heat you need.

author: jerryk
author_rank: Regular Contributor
timestamp: April 17, 2019, 06:18:12 pm
content: I don't have any cast iron pans around but have considered using ceramic tile in a pan to mimic the uniformity of cast. Of concern is those three diodes (type and temp tolerances unknown) on the back of the board. I thought of straddling the two tiles to keep the diodes from direct contact of the heating surface as an option.I do have a real nice heat gun that I can use to supplement the preheat process. I also have a fluke laser temp 62 max and the temp probe from my multimeter as temp monitoring options. I suppose heat just from the bottom of the board? The overheating the blue filter cap bank on top is a concern of mine. I looked at the datasheet and the only temps listed are operating temps with the max being 105C. Surely they can take more than that in a preheat but how hot can I get them without causing damage?I really want to replace the yellow ceramic caps but not sure on how to get an exact match or even if that is important. I think they are a 2.2uf MLCC but the C5K has me stumped. (Edit: added better shot of front and back of caps)The biggest concern of all is the close proximity of the drain tabs on the two rows of IRFS3004's. I'm interested in any type of soldering technique to keep the replacement row I have to install from bridging to the adjacent row. The only thing I can think of is to put a piece of kapton tape over the tabs on the existing row.As far as solder type and technique goes, should I use solder paste or should I pre tin the pads an try to melt them in that way. Any tips on type of solder or any experience you guys can share before I make the plunge would sure make my day.Thanks for all the help so far.Jerry

author: jerryk
author_rank: Regular Contributor
timestamp: May 01, 2019, 07:21:34 pm
content: I just wanted to follow up on this thread to say I might have fixed this thing. I'm waiting on the control panel to arrive so I can do some detailed fault checking. I have yet to put it under any real heavy load but the inverter produces a very nice sine wave and runs a small 3amp ac motor. The charger seems to go through all phases of charge without a hitch. So I'm on pins and needles an know that I'm aways from putting the "FIXED" stamp on it.One of the disappointments of the whole thing is because I'm in Alaska I can't get any conformal coating or thermal transfer epoxy without a very long shipping wait. I found a conformal coating pen which I will touch up critical areas but the thermal transfer epoxy I think is fairly important.I have attached a picture of the placement of the epoxy on the inverter board and am wondering if there is any off the shelf stuff that will work. The only stuff I have found at our electric supply place in Anchorage has aluminum in it and would mess up the 100k thermistor reading as far I know.Lastly is there any sort of conformal coating substitute hanging around the average hardware store that I might be able to sub for the real stuff?Otherwise, I really appreciate the help and am a little blown away that it's working. I also attached a photo of the end of the repair and hope this thing passes the test of time.It's been fun - Jerry

author: james_s
author_rank: Super Contributor
timestamp: May 01, 2019, 11:24:15 pm
content: Maybe an automotive clearcoat paint, transparent epoxy enamel or varnish? I'm not e chemist but I would think some paints would do a reasonable job of keeping moisture out.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 02, 2019, 04:31:21 am
content: You might also go really old school and dip or paint with molten beeswax which should be locally available. It is certainly not as good as modern coatings, but better than nothing. Also relatively easy to remove.

author: Per Hansson
author_rank: Unknown
timestamp: May 02, 2019, 03:50:38 pm
content: If the client has his inverters end up in salt water often a better way forward might be to design a water tight box with proper cooling for the inverters so they don't overheat.That might impress the customer more than repairing the inverter itself if he has many, just an idea...