I have a circuit board from a car stereo amplifier. My vehicle uses an amp for each speaker, and this particular amp is well known for having electrolytic capacitors that go bad. I would like to replace the capacitors, but the caps in the middle of the board, and the adjacent inductors were partly covered with an amber colored transparent, slightly soft potting. I was able to remove enough of it without doing any damage (except to a couple of the capacitors I will be replacing) but I'd like to have an idea of what to replace the potting with.
The amp board itself gets enclosed in a shield that acts as a heat sink for the transistors, which are not near the potting, and the entire shielded amp goes in a sealed enclosure that also contains the speaker. So it's pretty isolated from outside environmental factors, and I don't know enough about this to know what effect the potting has on thermal issues or anything else, or even why it's needed. It seemed to have been applied somewhat haphazardly, covering the inductors about half way each, and burying some of the capacitors part way, and filling some of the space between them.
Is there some general explanation of why potting would typically be used here and what I should look for?
You'll find that amber adhesive on a great deal of car audio gear. It's used to prevent vibration. With sufficient amount of vibration (over time or even a one-time severe jarring), electrolytic caps can develop internal intermittent faults -- where the leads are welded, for example. That's why you'll see the stuff underneath caps. Smaller inductors are secured so vibration won't cause the body to bend over to one side -- this isn't as much of a concern with inductors that use very large gauge wire, as the wire itself is strong enough to keep them in place. With the really large components, however, cold/intermittent solder joints can result from vibration, simply because of their mass.
If the factory application looks haphazard, well, it is. They have to get it done in a jiffy and since it isn't conductive, they just plop it in there where it's needed and if it runs between components, so much the better, as that means even less chance of vibration moving things around.
As for replacing the factory compound, hot melt glue (hi-temp not low-temp) is an acceptable substitute. You can get various colors, including amber, and even different textures -- some are "waxier" than others, some cure harder than others, etc. You must make absolutely certain the pc board is CLEAN prior to applying hot melt glue. Scrub the area in question with alcohol first (91% is best). Or use one of the electronics cleaner sprays (the kind that leaves NO LUBE FILM, obviously). The surface of pc boards is generally smooth to begin with. It won't be if you bother scraping the factory glue off. Hot melt glue has a hard enough time lying down into tiny score marks, and it won't adhere well, if at all, where there's a film of dirt present. Remember, the idea is to stop vibration. If your glue is not secured to the board, it's probably still secured to (or hugging) the components, which means it is only adding mass to the components it's supposed to be holding down. Now the whole shebang is moving around (though probably not visible w/o help from a finger and maybe a magnifier) and that's just begging for trouble. Just try to get the factory stuff off a board and you'll see how well it has adhered to it -- that's what you want.
You could try to get your hands on the real factory compound, but if it's like other assembly-line chemicals I've looked into, you likely won't want to pay their asking price for it. Aside from hot melt glue, there may be other consumer adhesives on the market which may work. You don't want something that cures rock hard like epoxy, as you may have to remove it for future repairs. Rubber silicone caulk might work, but it's... rubbery... and therefore does allow some movement. Also, you want something that won't crack with age or heat.
Thanks. The potting came off the board itself cleanly (maybe too cleanly) so I don't think I'll have a problem. I do need to clean some residue off near the leads of some former caps, but it's not from the potting, and was probably the original problem. I have a can of circuit board cleaner somewhere that should fix it.
As for future repairs, if I need to do any because I mess up with the glue, at least they will be easy. :-) But I don't think that will be an issue. There are only 9 electrolytics, so I'll just change them all and be done with that section of the board.
For the benefit of anybody who comes across this thread by doing a google search, I'll mention that these are from an Infiniti Q45 with the Bose amplifiers known for bad capacitors.
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