24V fan would seem to be variable to temperature sensed by thermistor close
to but not touching the heatsink at the o/p hybrid and no trace of white
goo.
Going by the number of problems actual and potential on this less than 2
year old PbF amp, has someone omitted some goo at assembly?
The green silipad under the TDA is puffy as though overheated , fan supply
works but does it work properly with thermistor positioned 4 mm from the
heatsink, just monitoring air/ board temperature ? Touching thermistor with
hot metal brings up the fan speed.

No abuse.
In for failed 1/4 inch input socket, failed spring action on shorting
switch/ signal contact, all other sockets would start to fail in a couple of
years, used or not because of inadequate solder. Solder on a number of W/W
going home, one failed on touching it, push on power handling connector pins
did not survive normal hand force of being pulled off the posts, more solder
failure, posts came away with the sockets. Force between pin and socket more
than the pcb solder strength. At least the clean/dirty switch has an oval
knob and close tolerance panel hole so that may survive longer than the
rectangular ones in other Marshalls.


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

Decided to bend it nearer the heatsink and add goo.
At switch on , at about 13V you cannot hear the fan at all. At 24V, after
heating the thermistor, it is
just about noticeable with absolutely no ambient noise around. So little
point to being variable let alone respond to air/board temp.


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