DC power backlight hum

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xpawel15x

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My Pro-96 hums when I turn on the backlight while it is powered with a 9V DC power cigarette adapter. Is this normal?
 

SCPD

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Sounds like line noise from the alternator or engine in th car perhaps. make sure its grounded right
 

rvictor

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If the "hum" increases in frequency as the engine rpms are increased, that's a pretty good indication that it's alternator or ignition related. You can buy filters, but it's easy enough to make your own following the directions posted here:

Curing Alternator Whine by KB1MVX

Dick
 

kb2vxa

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No, provided you're using the proper accessory this condition is not normal. Using a substitute it's anyone's guess what is "normal". Since the scanner was made by GRE one may assume the 100-014 adapter is the proper one.
100-014, GRE CIGARETTE CORD FOR PORTABLES - Scanner World - The Largest Dealer of Scanning Radios in the World

The adapter is a DC-DC power inverter which completely isolates the load from the vehicle's electrical system rather like a transformer does with AC. You may discount any likelihood of AC ripple (alternator whine) on the vehicle system, it appears the output current is insufficient to power the receiver properly when the lamps are on and drawing additional current. It's all in the active regulator/filter on the 9V side being overloaded so AC ripple from the inverter circuit is not being entirely smoothed out.

There are two solutions to the problem, use an adapter with sufficient output current capability to power the unit at full load or just put up with the hum when the back light is on. You really don't need it lit all the time now do you?
 
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