"Arfa Daily" wrote in message
news

"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



That was my definition of inadequate. The fact that 3 of us here have seen
large quantities of the same units with the exact same problems bear this
out.
I'm sure we've all seen plenty of amplifiers with a whole ton of dust
inside that have been working hard for years.

I had a theory that because once repaired they tend not to come back, this
may point to either a manufacturing issue or dodgy batch of Darlingtons (?)



Gareth.