R7000.23

                      Connection to R-7000

     As mentioned earlier, a DC voltage is  present  at  the
     R-7000 IF output connector and is used to power an ICOM
     TV accessory.  A direct connection between  the  R-7000
     IF  output  connector  and SM-220 would damage at least
     one of these units.

     To block the DC voltage, a 0.1 microfarad capacitor was
     soldered  inside the R-7000, between the IF output jack
     and the  adjacent  jack  labeled  "spare".   Connection
     between  the  R-7000  and  SM-220  is then made using a
     short length of RG-58/U coaxial  cable,  with  one  end
     plugged into the R-7000 "spare" jack, and the other end
     plugged into the rear of the SM-220.

     This leaves the original IF output jack undisturbed  so
     it can be used with the TV adaptor accessory.


                      Vertical Sensitivity

     When I connected my modified SM-220 to  my  R7000,  the
     SM-220 would display only the strongest of signals.  To
     improve the display sensitivity, I  inserted  a  20  dB
     gain  RF  amplifier  between the R7000 and SM-220.  The
     amplifier was a spare Ameco PLF2 FET  receiver  preamp,
     adjusted  for  10.7  MHz,  but  other amplifiers can be
     used, provided they have at least a 500 KHz  bandwidth,
     and sufficient gain, at 10.7 MHz.


                     False Readings - Images

     The SM-220 circuit is like a superheterodyne  receiver.
     The  panadaptor itself has a 455 KHz IF, and like other
     superheterodyne receivers with a low IF  frequency,  is
     prone to images.

     An image is manifested as  a  false  pip,  which  moves
     across  the screen as the receiver is tuned, and is 910
     KHz (twice the IF) away from the actual signal.

     The images discussed here are in the SM-220, not in the
     receiver.   Images are bothersome on the strongest sig-
     nals, like those  300  watt  paging  transmitters  that
     saturate  the  county with RF, belching out out strange
     digital noises or voices (now illegal to monitor).


                    Use With Other Receivers

     The modified SM-220 can be used  with  other  receivers
     having  a  10.7  MHz IF.  In other receivers, one would
     need to find the proper point in the IF  stage  (before
     filtering  takes place), install a connector, then wire
     a DC blocking capacitor between the IF stage  and  con-
     nector.