The black plastic cover just pops off, but it's easy to crack it. It's held in place by a few small raised points that clip into recesses in the alloy housing.

With the cover off, you will see the tops of 4 transistors peering through the epoxy. You need to clean away the epoxy and replace the failed ones.

You should be able to get the transistors from somewhere like Maplin/Tandy/Radio Shack. They are NEC transistors, but I can't remember the number.
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Andy.
NSR-WORLD.COM

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