"N_Cook" wrote in message
...

20 degrees is about the most or you block off access to the TO3s. Maybe
only
tinkering at the heat build up problem. Most air must go through the 25mm
or
so wide gap in the ps direction (over the tips of the vanes), rather than
the 2 off 5mm gaps to the duct let alone through all those vanes. I would
have thought there should be some shuttering to block off most of that
large
gap along the ps side , to force most air round and through the heatsinks.

They were assembled with enough white goo to keep a goth happy for a week.
So much that she filled the B & E holes so pins pushed through it, not
cleaned off, and some white goo actually under the solder joints (not
actually failures there). Those failed caps 47uF, 63V - in similar HK amp
they were rated 40V 47uF , what is the problem in that area of the
circuit?




On reflection, I don't think that these are actually designed with 'forced
air' cooling in mind. Rather, I think that that type of heatsink is intended
to suck the heat off the transistors, and then radiate it to free air, and
that the purpose of the fan is to shift new cool air through the chassis /
cabinet, to help the radiation take place. When the air intakes get clogged,
the internal ambient temperature goes up, stopping the radiation from taking
place efficiently, with a consequent large rise in the case temperature of
the transistor, made worse by the dried out thermal paste.

I don't quite agree with Gareth that the heatsinking is inadequate. I think
it is just about adequate, as long as the designed level of airflow around
the chassis can be maintained. If it can't by virtue of the intakes being
clogged, then it becomes very marginal.

However, I suppose it could be argued that it is 'inadequate' in that there
is little or no margin built in for compromised airflow.

Arfa