Thank you so much for this. Lots of very good information.
Interesting that you mention airflow direction. That's one thing that has puzzled me about this scope. There has been a lot of discussion lately on other lists regarding whether fans should should push air into the instrument or pull it out and the consensus is that pulling it out is best but now I find that this scope pushes air in. I presume part of that is to maximize the air flow directly across the pass transistors but that's just a guess. I had considered reversing the airflow but now I don't think I will.
Thanks again,
Barry - N4BUQ
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Hi.
There are now three bearing types available:-
Ball bearing.
Sleeve bearing (often sintered bronze* sometimes a plastic material.)
And Magnetic.
(* With care, these can last a very long time.? Some are possible to
extract and "refill" with lubrication, in a similar way to how we used
to re-lubricate motorcycle chain's!)
The last type (magnetic) are (mechanically) virtually silent, but stray
DC magnetic fields may be an issue in some cases.? Often found on
smaller higher speed fans, at a price.? They can still fail, if dirt
gets into the bearing area though, or they overheat.
The other source of "whine" or "turbine like" noise, is caused by
something being too close to the rotating blades, often on the "air
outlet" side.? That can be the motor strut's themselves (later types are
profiled to reduce such noise) or a nearby cable, coax, or even other
metalwork.
(Large items too close to the inlet side can also cause noise level to
increase, as well as restricting airflow, depending on how the
obstruction allows air to pass into the fan itself...)
Sometimes, changing the fan for one with very similar airflow
characteristics, but with the inlet over the struts rather than the
outlet (if that is the case) can help with the noise level.
Take care not to reverse the airflow through the instrument.? Many "wont
like it" over the long term!? Most fans are marked showing the rotation
and airflow direction, but it is not difficult to tell just by looking
at the blade shape.? (The concave side is "usually" facing the outlet.)
Instruments in a rack, can have conflicting airflow requirements too.
It's not that uncommon to find issues caused by two or more exchanging
each others warm air within a rack, rather than working together to pull
ambient air "through" the rack.? In such cases, the rack is likely to
need it's own fans too!
It is sadly too, not uncommon for a rack to be "over populated", so that
instruments that pull air in from the underside, have little to no space
for that as the unit below it is too close.? There is a reason for those
1 or 2'U' blank panels often seen, not just to make the front look
neat.? And again, the type of support for the instrument.? Shelves and
"L" shaped supports front to back, can sometimes obstruct airflow
to/from the sides as well.
Over the years since many older instruments and equipments were
developed, there have been vast improvements in fan technology, in
particular blade and other airflow profile design aspects.? Some to
benefit the ability to push or pull air through a restrictive passage,
others to greatly reduce fan noise, without compromising (too much) on
the airflow characteristics.? Most will use less electrical power for
the same "duty" these days than the older designs, as motor tech' too
has advanced.
The last thing you want, is for a fan to run, with the blades
aerodynamically "stalled", often a hint is a "fluttering" sound to the
airflow, and/or air exiting a fan being spat out radially, more than
axially.? The resulting overall airflow is usually very low in those
cases, regardless to if the fan is pushing air in, or pulling it out of
a casing.
Old catalogues to identify the original fan type and performance
(assuming you have the "originally fitted at manufacture" fan, and also
new catalogues (web pages) so as to choose a modern unit with the needed
airflow, and possibly reduced noise is what you need.? It does take more
time than you might expect though, as often the choices are either vast,
or limited.? :-(
Keeping air filters clean too, can help with noise levels, as well as
keeping the instrument internals cooled correctly.
Hope something helps.? I hate to see otherwise great instruments
compromised (or even damaged as a result of overheating) just because
someone didn't like the noise, so put a "quieter" fan in.
HP (and other manufacturers) designers will have done the math, and
specified the part used originally for a good reason.? Not just to
prevent overheating, but also to produce a sensible warm up time, that
results in relatively stable operating temperatures, for a given range
of ambient temperatures.?? Compare the air outlet(s) temperatures once
they have stabilised, for the same ambient and operating conditions,
before and after replacing the fan, to be sure.? Much higher with the
new fan, and it's probably not doing the job needed of it.? "How Much"
is a moot point though.
Cheers All.
Dave 'KBV
(Who's populated and retrofitted customer racks with various "high
dissipation things" in the past than I care to think about.? Not unusual
to end up either fitting large fans to the top to pull air out, install
ducting if a unit has the outlet hardware for it, or just remove back
and sides, to allow free airflow, when in use.)
Noise?? You haven't lived until you work near a host of things with
400Hz fans in!? Some units we worked with had no less than three of
those noisy things!? They do move heck of a lot of air for the size
though...
<
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