I've now taken this amp apart. There was one zinc plated nut with captive
star washer , loose inside, causing trouble. Now I've found another one,
lodged safely, in the base. Can I find where these 2 have come from ? -
anyone been here before?. They are loose fitting ,32 TPI , on the other
0.128 inch screws used inside , so presumably 6UNC. As this is in the UK,
unlikely 2 would have fallen in the fan access , in a metric-screw UK.
Exactly these style nuts are used elsewhere in the amp , but the next size
smaller. Most screws in there are tapped clinch nuts or chassis steel.

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