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Is there supposed to be a dust filter in addition to the metal screen "filter"?? The screen is good for keeping fingers and larger animals out but for dust, not so much.? The manual states to wash and allow the filter to dry which makes me think there might have been something else behind the mesh.

Thanks,
Barry - N4BUQ

Pulling air out of an instrument means sucking air and dirt into
the instrument, all around the edges. Not a good thing.

Jeremy


On Wed, Apr 3, 2024 at 7:38?AM n4buq <n4buq@...> wrote:

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

> 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...
>
> ><
>
>
> --
> Sent from a Micro$oft free zone:
>
>
>
>