overview:  the pro2021 is a 200 channel scanner, mode is implied by
    frequency, missing the 220 ham band, 800+ cell fone/ISM band, the
    170ish thru 380ish military? air band...
    sold for about $200 from radio shack, first on special in 1986 time
    frame.
    bands which the cpu will honor may be excluded by the presence of
    diodes near C151 (on silkscreen) top view, left end, rear.
    in order from left starting with D44:
    D44 - 'U2' - 810-960 MHz Mode: FM Step: 12.5 kHz (installed - cell fone)
    D?? - 'U1' - 380-512 MHz Mode: FM Step: 12.5 kHz
    D?? - 'Hi' - 138-174 MHz Mode: FM Step:  5.0 kHz
    D?? - 'Ar' - 108-136 MHz Mode: AM Step: 25.0 kHz
    D45 - 'Mi' -  68- 88 MHz Mode: FM Step:  5.0 kHz (installed - TV ch2-4)
    D?? - 'Lo' -  30- 54 MHz Mode: FM Step:  5.0 kHz
  further: when the cpu selects a frequency, IC8 (a shift register) is
    used to enable the rf front end for that band.  IC8 is near center,
    about 1.5 inches from front, thru-hole mounted on top.  the outputs
    of ic8 associated with D44 and D45 are not connected to anything.
    R112 leads from pin 4 of ic8 to base of q10, and when the output
    at pin 4 is active (low) enables VHF LO band reception.
    Pin 5 of ic8 is active for "Mi" or "VHF-Mid" band recption.
    another (10K) resistor may be connected from pin 5 to the rf-enable
    end of r112.  this will enable the lo/rf for 68-88.
    this mod is incomplete, and any "scanner men" are invited to
    enter this discussion.

    pin 13 is associated with the 810-960 MHz band, and similar modification
    of adding a 10K resistor from pin 13 to the rf end of r113 should
    enable that rf front end.

  note: search mode does not function with either of these mods, so
    i'm probably missing something, but direct (manual) frequency entry
    of the 2nd harmonic of the 440 MHz ham band coupled with the use of
    a 440 HT who's 2nd harmonic is about 50 - 100 dB below carrier result
    in a full quieting signal at 5 watts at a distance of about 2 feet.

  so: have we stimulated anyone who knows stuff about modifying scanners
    to join in  and help us 2021 owners out?  if so, that would be great.

  btw: cycle the power after clipping diodes, the cpu seems to sample the
    diode array at power up only.  reset is not necessary.
-- 
-tw (Tom Walsh, kb7efz, 503 696 5384, Intel Corp; Hillsboro OR)