Potting question
 

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?

wrote in message
oups.com...
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.

Good luck,
Ray

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.