Old Marshall Valvestate Amplifier's circuit board is covered in a hard, porous (foam-like)substance?

Question

Hi,

I own an old 100W Marshall valvestate amplifier. It hasn't worked in years but recently I opened it up to take a look, in the hope that I could fix it.
However upon taking it apart I noticed that the circuit board was covered in a hard foam-like substance!?

The foam-like substance was very much like 'expanding foam' (i.e., the stuff builders use to fill gaps in walls etc). It was a brownish colour, porous and hard to the touch, but it was easily breakable (I could chip away at it using my fingernails)

There looks to be a fairly large capacitor on the circuit board, from which it appears that the foam has emanated from, but I cant be sure as the foam has literally covered all the components on the board!
Furthermore a blown capacitor wouldnt spew a foam that hardens??!!

1) What's going on with this circuit board?

2) Any suggestions as to what this foam is or how it got into the amp? (Is it put there intentionally as a heat sink or something crazy?)

Also the amp used to work but over time it started to cut out until one day it stopped working completely!

Steve4Physics · Accepted Answer

That is very strange!  You could try asking in the Engineering section, as electronics people often read that. Also try the music/guitar section.

If it were me, I'd pick-off as much of the gunge as possible. Maybe gloves and a mask are appropriate - depends what it is.  Then try cleaning up remnants with something like switch-cleaner and an old toothbrush.

Then replace any obviously damaged parts such as the capacitor.

And take if from there,  At least you can try and diagnose any remaining problems if you have a multimeter. Maybe you can find a circuit diagram online.

Good luck.

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Old Marshall Valvestate Amplifier's circuit board is covered in a hard, porous (foam-like)substance?

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