What is this compound on Orbitsound circuit board.

I've just repaired my Orbitsound T12 sound bar (on about the third attempt).

The bass amplifier is a bridged TDA8932B (https://www.nxp.com/docs/en/data-sheet/TDA8932B.pdf) which is a class-D amplifier.

I found that one of the bootstrap capacitors for the TDA8932B had corroded and most likely gone open circuit. It broke as I removed it.

There is some sort of compound or sealant around each of the inductors, which has spread over some of the surface mount components. I have found water under the compound in places.



The surface mount components that were partially covered by the compound have corroded, as can be seen on the right of the photo. The bootstrap capacitors are the two components with the out-of focus numbers nearest to the IC.

My question is what is the brown compound, and why has it caused corrosion?

It's hard to be sure from such a close blurred picture, but it's most likely glue, used to hold large capacitors and inductors in place - if there's water underneath it, it sounds like something has been spilt in it?. Although liquid could be leakage from a duff electrolytic?.

The material is very hard, not like glue. I may have seen something similar on common-mode chokes. It has been put on all the inductors.

I am fairly sure that nothing has been spilt onto it. It's not where there would be any liquids. I didn't see any evidence of leaking electrolytics. I changed all of the electrolytics and there were a lot of high ESRs but no leakage that I noticed.

In normal use the circuit board is horizontal, and the housing doubles as the speaker enclosure, so there isn't much air circulation inside, and I am wondering changes in temperature could lead to condensation, like cargo sweat. If there is condensation, it might creep under the material by capillary action.

Diver300 said:

The material is very hard, not like glue

Click to expand...

Adhesives can be of almost any hardness and even "soft" adhesives can become glassy after 12- to 15-years of thermal cycling on a PCB. Also, a high quality manufacturer with a reputation to uphold like the well-known brand, "Orbitsbound*" has likely done all kinds of durability testing with each material and component used in their devices (thermal cycling and humidity chamber followed by various physical testing afterwards).

It is most likely adhesive for the heavier components.

* Oops, read the top line...

Diver300 said:

The material is very hard, not like glue. I may have seen something similar on common-mode chokes. It has been put on all the inductors.

Click to expand...

It's glue, and is extremely common - glue is often hard, it depends on type.

I hadn't thought that the glue could harden a lot over time. The bass amplifier is in the middle of the board, and the sound bar takes quite around 7 W when turned on and no sound is coming from the source. It takes about 3 W on standby, so it's probably been quite warm for about a decade.

Recently I've been turning off the soundbar when we're not watching the TV, and I'm wondering if condensation got in when it's cold.

Diver300 said:

I hadn't thought that the glue could harden a lot over time. The bass amplifier is in the middle of the board, and the sound bar takes quite around 7 W when turned on and no sound is coming from the source. It takes about 3 W on standby, so it's probably been quite warm for about a decade.

Recently I've been turning off the soundbar when we're not watching the TV, and I'm wondering if condensation got in when it's cold.

Click to expand...

If the warm air cools and it's humid in the area, the warm air will contract and draw in humid air. That mid air will only condense to liquid if the hosing is cooled below the dew point. Not unusual in general - it happens to certain headlights and taillights in cars. It is unusual for an indoor product - especially if you have air conditioning. If it's on your patio, it is very possible that you get condensation by morning - just like the dew in the grass.

I'm in the UK, so like just about all houses here, there's no aircon.

I've had condensation in the unheated garage occasionally, and a bit around the edges of the windows when it's cold.

Condensation inside the soundbar doesn't seem likely, but there's been water and corrosion in there, and I'm curious about what caused it.

There has been speculation that some of the adhesives used can cause corrosion after extended periods. But finding liquid around it sounds like some kind of external source.