Incidently my test for whether such bypass switches need attention , even
before dismantling the amp.
Take one cocktail stick/toothpick . Hold at one end between thumb and index
finger and poke the other end in the socket - if the contact breaks before
the cocktail stick breaks ,about 150 gms over length of 65mm.

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Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/