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White "gloop" used to mount capacitors in.

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treez

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Hello,
We have one of the subwoofer amps as in the attached video. If you look at 1:39 to 1:50 you can see the white “goo” at the bottom of the big electrolytic capacitors.
On our board, this white goo has , in many places, gone dark brown and is crispy and brittle. Is this due to overheating due to a fault?
When I scraped and picked the brittle brown stuff away off the board, there was a broken axial diode that had been buried in it. It’s broken up and the centre of its body has gone and we don’t know which part to replace it with.
Ours is a Mordaunt-Short MS519 digital subwoofer, but it looks just like the one in this video.

So is the "gloop" supposed to ever be dark bronw and brittle?

see 1:39 to 1:50
https://www.youtube.com/watch?v=at4tzEIe3UY&t=315s
 

It's a kind of cement used to improve the physical stability of large components. Without it there is a risk of vibration tearing the solder joints apart, especially on large components like those capacitors.

Usually it is harmless and as long as it isn't burnt black it doesn't have any side effects on the circuit. If in doubt, scrape it off, the chances of the the board being moved in the future are probably a lot less than when it left the production line and got shipped half way around the World!

Can you show a close up picture of the board around the damaged diode please.

Brian.
 
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Re: White "gloop" used to mount capacitors in.

I had an aging electrolytic break its seal in normal use (reel-to-reel tape deck). There was a 'crackling' sound as I watched goo start bubbling out from it. This could be similar to your case. I imagine the goo is exposed to high heat, turning it brown and brittle.

- - - Updated - - -

I post the above story because you mention "white “goo” at the bottom of the big electrolytic capacitors. However this doesn't necessarily explain your broken diode.
 
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Can you show a close up picture of the board around the damaged diode please.
Click to expand...
Thanks, ill get it soon, the amp is at work , as someone brought it in to be fixed.

The diode looks like a orangey/blacky 1N4148 type thing, but of course , it might be a zener. The middle bit of the body has gone...lost in the crusted gloop stuff
 

Re: White "gloop" used to mount capacitors in.

I think the diode might be the D502 of the attached schematic....i got this from another forum where they said that the JBL SCS150SI amplifier is similar to some of the mordaunt-short ones. I have no idea but its all ive got to go on at the moment. Schematic.jpg
 

Hi,

D502 is very unlikely to break. It's just a low current voltage regulating device.
The current is limited by R503.

If there was overvoltage ... causing overcurrent, then I think it's more likely that R503 burns.
Because with twice the current:
* you get about twice Ptot in the diode,
* but about 4 x Ptot at the resistor.

Klaus
 

Thanks, ill probe the pads and see what its connected to and take d503 into deep consideration. Do you think the crusty brown stuff (detioriorated glue) coudl have corroded the body of the diode.....the diode was certianly well buried in the "gone-off" glue. Apparently this glue the goes off and gets corrosive and conductive is well known for destroying amplifiers.
 

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