[OT] hum in baby monitor when running on ac

[MCH]

It could be the filtering in the PS (both) or the unit.

What are the specs on the OEM PS and the one you tried?

Joe M.

Hi All,
I have a stubborn problem with a FisherPrice baby monitor. It's an older
49 MHz wireless unit.
The receiver has constant hum whenever it is run with a wall wart. On
batteries, it sounds as quiet as a church mouse.
I've tried plugging it into different outlets, I've swapped wall warts
with one from my junk box. I've tried wrapping it in aluminum foil (for
shielding), no difference. If I listen to the signal in my scanner, it
sounds like a million bucks.
I'd really like to use this thing for home security as the thing can
hear a mouse fart at 100 yards, just the thing to put in my garage.
Any ideas?
TY, Art

[James Cameron]

Perhaps RF is arriving via the wall wart lead. Try running the wall
wart lead through a ferrite toroid several times as close as possible to
the receiver?

Is the frequency of the hum the same as power line frequency at your
location? If so, perhaps AC line frequency is affecting the audio
ampifier; use a regulated DC supply with capacitors closer to the
receiver, or replace the capacitors in the audio amplifier.

[Sean Breheny]

Do you know whether it ever worked quietly on a wall wart? It may simply be
that the ac supply option was an afterthought and they never provided
adequate filtering and regulation. I assume that the wall wart is supposed
to be a DC-output type and not an AC output one? Which type did you use? Is
it the OEM unit?

As James said, it can also be some funky RF behavior. One receiver I
designed (it was a direct-conversion type) would develop audio oscillations
if the headphone lead was too long or placed too close to the antenna. It
turned out that the local oscillator was being radiated via the headphone
lead and this produced a feedback loop since variations in the
self-reception of the local oscillator caused audible signals by the nature
of how a direct-conversion receiver works. The fix was to add a ferrite
internally to the cable running up to the headphone jack. Something similar
could be happening with the external power lead (although I highly doubt
that this is a direct-conversion receiver because it is almost certainly
FM).

If you can, try a regulated DC supply. If that helps, then you might
consider a regulated wall-wart (like a switching type - as long as it
doesn't radiate at 49MHz!). You can also try adding a large electrolytic
cap to the output of the wall wart (provided it outputs DC) but you have to
be somewhat careful because having a large capacitance on the output of
rectifiers reduces the conduction angle (duty cycle) of the diode
conduction, raising the RMS to average current ratio in both the
transformer and the diodes. They can overheat if the cap is way over-sized.

Sean

Post by James Cameron
Perhaps RF is arriving via the wall wart lead. Try running the wall
wart lead through a ferrite toroid several times as close as possible to
the receiver?
Is the frequency of the hum the same as power line frequency at your
location? If so, perhaps AC line frequency is affecting the audio
ampifier; use a regulated DC supply with capacitors closer to the
receiver, or replace the capacitors in the audio amplifier.
--
James Cameron
http://quozl.netrek.org/
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[Van Horn, David]

I've seen bridge rectifiers cause this sort of problem. Bypassing each diode with a 0.1uF gets rid of it. Switching transients.

Could be hum in the power supply, mic pickup, or receiver.