At 4/14/2006 16:16, you wrote:
>Chuck Kelsey wrote:
> > Maybe I'm missing something here, but I've never subscribed to the
> theory of
> > having fans run beyond PTT. What purpose does it serve? The transistors
> are
> > generating heat only when they are "on." As soon as they are "off" they
> > begin to cool down on their own. If the fans are doing their job, they are
> > keeping temperatures to a prescribed level the entire duration of
> transmit.
> > While running the fans beyond the end of transmit will cool the finals
> after
> > the end of the QSO quicker than without, why should it matter?
For a long time I've always run my PA cooling fans 24/7. Occasionally I
lose a fan but usually they give warning before they go.
Lately I've been experimenting with using integrated CPU fans/heatsinks, as
they fit nicely on the back of G.E. MVPs (where the heatsink for the 35
watt UHF MVP is found). This allows the cooling to be integrated with the
radio instead of relying on a separate rack panel to hold the cooling fan,
which is important for my portable repeater application. Since the power
for the fan comes from the radio, it's very easy to tie the negative fan
power lead to PTT instead of ground to switch the fan with PTT. Does save
wear & tear on the little CPU fans, & they don't collect that nasty carbon
grease which would probably bog it down faster than a full-size muffin fan.
As far as cooling efficiency goes, the CPU fans easily remove enough heat
from the 25 watt VHF HB MVP PA for continuous duty operation. The UHF PAs
aren't quite as efficient: the 35 watt PA generates too much heat even with
the original larger heatsink in place & needs an additional "backing"
muffin fan to keep it cool enough. The 20 watt UHF PA seems work with the
CPU fan but more testing is needed.
Bob NO6B
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