Glue.

Feel free to clean it off, though. Certain types of glue are known to go conductive as they age.

Thank you, that is reassuring 😀

I'll have a go at cleaning it eventually, just for peace of mind.
But for now I can enjoy some nice MT-32 music worry-free!

Now that I look again with a bigger monitor, that 47uf cap does look a bit deformed at the bottom. Not from age/leakage but probably caused when it was mounted.

arnoo wrote on 2019-12-29, 00:54:

I'll have a go at cleaning it eventually, just for peace of mind.

Normally, that glue is there for a reason and should be left this way. However, in your case it's different. In the first and third picture you can see that it caused quite a bit of rust on many connections, including some vias. This must be fixed, as it's only a matter of time before the unit fails otherwise. Remove the glue, then the caps (replace them with new ones), then apply some rust converter and reflow all connections that had rust on them.

That makes sense. Thanks for explaining!

Good catch on the rusty spots, need to check my units as well!

The corrosion is a problem when the manufacturer 'costs-down' to using general purpose glues instead of using electronics grade which have a low acid content.
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The glue is there to hold the caps (and other parts) in position long enough to get them soldered in at that factory.
That's the ONLY reason it is there.
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In very old electronics they would sometimes goop up coils with it to reduce hum but this isn't applicable here.
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gdjacobs was correct.
When it gets old it can cause problems, and not just corrosion.
https://www.badcaps.net/forum/showthread.php?t=5466
https://www.badcaps.net/forum/showthread.php? … light=bad+glues
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I just checked one of my units, I expect the 2nd one will be the same:

anyone has any advice on how this glue can be safely/easily removed? I want to clean it and replace the caps it holds since I doubt the removal process will go well with them

edit:
any idea what kind of capacitors are the 2 green Nippon Chemi-con NX-IV ones?

allright some good news - I was discussing this with Tiido and he told me this is degraded glue and not corrosion
sure enough I removed some parts at the green caps area and there is no rust or pcb corrosion under the glue, it's the glue that looks like that
the very minor scratches near the via are made while removing the glue - on the pic I have not cleaned anything yet

so it seems one can leave it like that or do some glue removal and forget about it

I've actually had some success removing glue like this just using lots of IPA to soak up around it. It seems to help loosen it from the PCB. I do this when I recap Intellivisions as the two large power filter caps on the power board have some really nasty old glue that just won't let go holding the caps down to the PCB.

Some times what looks like corrosion turns out to be old soldering resin residue but that doesn't appear to be the case on that one.