Re: 11801 questions - funny fan noise and light

Ragnar S

Thank you gentlemen for your replies!

So it is unclear if it is supposed to do this, but it is at least not
unheard of.

I checked the voltages on the feed wires to the fan relative chassis ground.
From start, the positive wire has about +6.3 V and the negative about +0.7 V.
When the fan revving occurs, the negative wire goes to about -6 V for about
35 ms, making a total voltage over the fan of about 12 V. For some reason
the pulses come irregularly, around 1 or a few times per second.

My current hypotheses is that the intention is to regulate the speed of
the fan, but I would be surprised if this actually is how it was designed
to work. It is somewhat annoying. :-)

Are there any schematics on these boxes to be found?

Also - the neon (?) light in the power supply, right behind the fan, that
starts to flicker as soon as it has power (also in standby mode) - do you
guys have that too?

Thanks for you help and best regards,


On 25 Jan 2020, at 16:01, Chuck Harris <> wrote:

Mine is normal until after the scope has been turned on for
about 30 minutes, and then it starts going "Brummm..... Brummm..."

I suspect a capacitor failure, as it is annoying, and would have
caused notice if it were a "normal" condition.

Annoying, though it may be, I can't see how it would cause any
harm... unless it presages a total failure.

What it isn't is a bearing failure, or lack of lubrication. My
fan spins for what seems like half a minute after power is turned

-Chuck Harris

Reginald Beardsley via Groups.Io wrote:
I don't recall mine ever doing that. I'm very sensitive to noise as I investigate anything I can't identify, so I think I would have noticed it.

A fan on one of my HP Z400 workstations was doing that. I shut it down to reduce the heat in the room and when I tried to boot it the fan was dead. I've not had time to correct it yet.

Good luck,

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