[BC] exciter problem fix of note

[Tom Taggart]

Indeed, Mike.  Usually it's not difficult to get the same
fan from the usual electronics part suppliers, the on-line
catalog will usually show the specs, too. The RVR boxes I've
worked on (branded Bext, Energy-Onix, Armstrong) use either
an AC or DC fan. In two I repaired I substituted the Pabst
AC fan (with similar spec's), for the DC fan because the
Pabst fan is more rugged.

These RVR boxes seem to be a study in evolving design. The
basic chassis is the same--power in on the back right, power
supply mounted on a heat sink in the middle right, power amp
block immediately to the left of that, and a horizontal
board to the left of that with the audio and frequency
synthesis.  

Just about everyone I've worked on had a different design
for the power supply.  Not so much the circuit, but the way
the parts were arranged. And two I repaired had power
supplies that burned up--probably because of a dead or
inadequate fan. 

The earlier units used this Pabst AC fan (the 8500N model,
which, as I remember, is also used in the FX-30). The later
models used a Pabst DC fan, while some used a $10 fan made
by someone else. Most used the fan to pull air through the
top of the case, through the PA and power supply, then out
the back.  One exciter that was overheating was much happier
when I turned the fan around and pushed air into the box
directly over the supply.

The DC fans are quieter (somewhat academic issue when you
are using this as an exciter rather than as a stand-alone
transmitter). The 10 year old Armstrong STL transmitter I
just rebuilt had one of those "quiet" fans.  A noisy fan
might of been the issue back at the CC cluster it came from
(with the station I bought), but not in our new studios. I
stuck in the somewhat noisier but more efficient Pabst AC
fan because the only one bothered by the noise would be me
(my shop has the processing/STL rack).  From the amount of
dirt inside, and where it was distributed, it was apparent
there was not that much air circulation in that box anyway.

In brief (which I rarely am), while you have an exciter on
the bench for repair, and it has some age on it, can't hurt
to replace the fan, exact copy or something similar). 
Murphy's law will otherwise cause a fan failure some hot
weekend next summer during your vacation.