About a month ago I posted a question asking if others had
experienced local FM station interference at various frequencies on
the FRG-100. I finally found the cause and a solution. The FM gets into
the receiver thru the 12V power cable and any other connections to the
receiver (external spkr, tape recorder or computer connection, etc.) IT
DOES NOT ENTER THRU THE ANTENNA; in fact, you can ground the antenna
terminals and it's still there! The FM signal mixes with the 2nd
harmonic of the first local oscillator for some reason, and this allows
either the sum or difference signals of the oscillator's 2nd harmonic
and the FM freq. to fall at various frequencies from 1 to 30 MHz. My
worst were on about 5.06 and 13.1 MHz and this computed out exactly to
two of the local 100 kW FM station frequencies.

The FIX: I was tired of that oddball FRG-100 power connector and the
inability to use a standard 5mm coaxial power connector (common for
most receivers)with the radio on camping DXpeditions so I purchased one
of the standard types from Radio Shack (#274-1582), drilled an
appropriate hole and mounted it just to the left of the existing
connector in the blank spot on the back panel. I then soldered a
small powdered iron torodial filter of about 10 uH between the
connector's + pin and the top edge of the rear fuse holder (remove
fuse first!) I then a soldered a 0.1 uf ceramic capacitor to either
side of the toroid and the other side of the caps to chassis ground
forming a pi filter. I then changed the plug on the power supply 12V
cable to the standard 5mm male coaxial mating plug. The
FM "intermod" is now less than S-2 to S-3 with no external antenna as
long as I don't connect anything else to the accessory jacks on the
receiver's rear panel. Once the antenna is connected, atmospheric noise
or any signal that activates the AGC wipes it out.

I hope this helps some of you that may have this problem. I think the
direct ground connection to the chassis helps the most. The oddball
stock connector connects the power ground to the pc board ground plane
very close to the first mixer. This is probably the cause of the
problem due to common mode ground loop problems in the 100 MHz range.

Frank
K3YAZ