bilou wrote:

"Irv Finkleman VE6BP" wrote in message
...
I have been wondering which way the fans on my TS-930 should be blowing.
Should they take the ambient air from outside and blow
it on the heatsink, or should they be evacuating the inside air
in the vicinity of the heatsink and blowing it out? I have been
restoring the unit and both fans required replacement.

I've surfed a number of sites on the subject but can never get
a definitive answer.

Thanks in advance for any assistance...

de Irv, VE6BP
Hi
My opinion is quite simple:
Pulling air out of an enclosure will ,at the limit, create a vacum inside
In such case radiators are totaly useless only ,by inertia,delaying
complete failure.
Pushing air in allows filtering it from dust and is clearly the way to go.


As a retired broadcast and CATV engineer, I find all of this amusing.
The worst install that I've run into was at a TV station. Six or seven
full sized racks for the control room for the video gear, and three 1"
Sony VTRs. They were framed into a closet, with sliding doors on the
outside, in the hallway. They had a separate 15 ton A/C for that
closet, yet equipment had a high failure rate. The HVAC contractor had
installed the supply and return vents in the ceiling. They had been
screwing around for several years, and losing about $500 a month in
failed capacitors. I took one look, felt around in the racks and told
them that the supply line should come in at the floor. Since they racks
were sitting on a poured concrete floor, there was no way to feed the
air into the bottom of the racks. They called me a fool, and told me
that engineers from the equipment OEM had been to the site. I got them
mad enough to prove me wrong by removing the ceiling vents and using a
piece of flex duct on each, that dropped to the floor. In under five
minutes, there were no hot spots in any of the racks, and after a month,
the capacitor failure rate dropped to an acceptable level. BTW, one of
the racks had a 5 kW linear 5V power supply in the bottom. Even that ran
cool.

Another site used open racks for microwave equipment at a CATV
headend. They had a large A/C mounted through the wall. MOving one rack
just four inches to the side eliminated the problems. it was deflecting
the air flow away from the other equipment racks, and the return air was
being pulled behind it, back to the A/C.

You are never going to create a vacuum with the fans used to cool
relay racks, they just aren't designed to be that tight. Instead, they
are designed to run quietly, and for a long operating life. Decent
vacuum motors are two or more stages, and the individual fans are in
enclosed areas. AMETEK/Lamb made most of the vacuum cleaner motors in
the US at one time. They were reliable, and easy to repair. Their
website has some good information about blower/vacuum motors. I
repaired some of the motors for a local steel mill, where they were used
for air quality sampling. I also rebuilt a few truckloads of them for a
guy who rebuilt and sold used vacuum cleaners. He would give me a
truckload of motors that he considered scrap. I would repair them, and
sell them back to him. He always wanted to see my armature lathe. He
freaked out when I showed him how to clean and true an armature with a
variable voltage DC power supply, and a hard gray in eraser. ;-)