RE: Re: [OT] Baby monitor picking up "hum"

["Bricknell, Tony" <tony.bricknell@xxxxxxx>]

Sorry if I cover potential solutions that were discussed earlier (okay,
I
deleted them when I thought I had nothing to offer this thread)...

As the unit works from batteries it must be a 49MHz-type radio
thingy.  As
the hum disappears on batteries it is pointing to the mains Power
Supply
being at fault with one of the following (doing my best not to get too
technical in the explanations):

1. An internal smoothing capacitor has broken down / become de-soldered
and
is not smoothing the rectified stepped-down mains into a nice
"DC" level.
Solution: Try a different power supply, rated at least as high as the
existing one (if the one you're using is rated 12V @ 500mA, ensure the
one
you replace it with is rated at least 500mA @ 12V).

2. The mains power supply is not rated correctly for the baby
monitor.  For
example, if the baby monitor requires 12V @ 500mA and your power supply
can
only provide 12V @ 300mA, it's output will "sag" (drop below
12V), and the
DC level will become less smooth (increased ripple).  In the same way
as
above, this 'ripple', superimposed on your DC supply, will get through
to
the audio amplifier in the baby monitor, manifesting itself as a low
hum.
Solution: Use the correct power supply.  If not known, try a bigger
power
supply and see if the hum disappears / reduces.

As for filters, if it is high ripple on the DC supply that is causing
the
hum then you could try adding some suitable large capacitors across the
DC
supply into the monitor (but this would be an external, ugly solution).
Best to find a suitable mains adaptor that does the job.

HTH
Tony

-----Original Message-----
From: tefxp [mailto:roaming@xxxxxxx]
Sent: 22 July, 2002 13:11
To: ukha_d@xxxxxxx
Subject: [ukha_d] Re: [OT] Baby monitor picking up "hum"


--- In ukha_d@y..., Frost Neil <Neil.Frost@s...> wrote:
> Have you tried it just on batteries (if that's possible?)

I tried mine on batteries only over the weekend and the hum all but 
disappears if the xmitter is battery only.
this suggests some noise on the mains.

Is there some sort of filter I can buy that will stop this?


thanks,

Tony

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