Curious buzzing sound on AM car radio

[Spooky]

Hi All,

I have a Motorola push-button AM car radio which is plagued with a
buzzing sound on its audio. It is present at the low end of the medium
wave band and it is right across the long wave band too. At first, I
thought it was noise coming through the DC power adaptor I was using,
but it was still there when I ran it in my car from a DC outlet. The
buzz is only present with an antenna is connected and it almost
disappears when you place a hand on the top of the radio chassis. I
can't pick up this interference on any other radio and my ground
connections to the chassis seem fine. I'm at a loss as to where to
start fixing this! Any suggestions will be gratefully received.

Regards,
Gary

 

[Porky]

Your antenna lead may have a break in it or something loose at the
antenna end. Check the routing of the cable under the dash.

 

[Dave Plowman (News)]

I have a Motorola push-button AM car radio which is plagued with a
buzzing sound on its audio. It is present at the low end of the medium
wave band and it is right across the long wave band too. At first, I
thought it was noise coming through the DC power adaptor I was using,
but it was still there when I ran it in my car from a DC outlet. The
buzz is only present with an antenna is connected and it almost
disappears when you place a hand on the top of the radio chassis. I
can't pick up this interference on any other radio and my ground
connections to the chassis seem fine. I'm at a loss as to where to
start fixing this! Any suggestions will be gratefully received.

In the AM only days, it used to be the practice to use a thick braided
earth strap from the radio chassis to a nearby body ground, even if it had
a ground power wire. There was usually a captive 1/4" bolt on the back for
this purpose. Think the theory was that RF needs a lower impedance ground
than the power lead which may well be grounded someway off at a central
point, and pick up interference.

 

[Mark]

is it really old i.e. does it have a vibrator type power supply? does
it have tubes? or is it all solid state? how about if you drive your
car to a field away from power lines and houses?


Mark