HeathKit SW-7800 Mods Web Page

The Heathkit SW 7800 is a classic. In it's original condition, some would
say it's worth more money just for it's originality. It is now considered
by some to be a collector item. The author would actually like to have
another (E-mail me if you don't want yours). There is room for improvement,
though. Here is a list of modifications that will provide very appreciable
improvements in performance....

                                 RCVR Board

Audio frequency response: Change C478 to 1uF, and C488 to 220uf. This
increases low frequency audio response.

AGC speed: Change C497 to 15 or 22uF. Originally, the AGC responds so fast
that low frequency audio is attenuated by negative feedback, and distorted.
You may prefer speed though, for those fast fading DX signals.. The larger
the cap here, the slower the AGC response.

Audio distortion: Place a 220 ohm resistor between Q402 emitter and ground,
remove R468, and add a 220K resistor from the B+ side of R467 to the base
of Q402. This improves the severe non linearity of this amplifier stage.

Front end filtering improvement: Change R402,403,404,405,409,414 to
100ohms, AND place a 3.3Kohm resistor across C409,414,418,424,428,433. This
improves forward bias through the ON-state diodes, and reverse bias on the
OFF-state diode switches, to prevent leakage of strong out-of-band local RF
sources (such as MW broadcasts) into the front end. This problem is
manifested by reduced sensitivity and muffled band splatter noise on bands
outside of the localized transmitter frequency. People who live next to
powerful transmitters will really appreciate this improvement.

Signal meter accuracy: Depending on the radio, the signal meter does not
even get up to the top of the scale on the signal meter with the strongest
signal you can receive. You might consider it desirable for the meter to be
at least at the "60" when a maximum signal is received. Place an
appropriate size resistor across R493.

NEW- Sensitivity: Add a 100K resistor from the cathode of D422 to ground.
This lowers the quiescent AGC voltage going into U403 during low RF input
signal conditions. You will see a large gain in sensitivity on weak
signals.

                              Controller Board

8mhz VFO drift: This problem causes the radio to drift by 3 or more Khz
when it warms up; On the controller boar: try changing C331 and/or 332 to
the same or near value capacitor that has a NEGATIVE temperature
coefficient. Or, E-mail me for details of a method that uses a negative
temperature coef. thermistor and varactor diodes to compensate for
temperature.

                                Synth Board

10mhz clock and cpu radiation:
    1: place a .01 disc cap directly on the anode lead of C221 to ground.
This helps to bypass some of the RF generated by this board to ground so
that it is not radiated by the electrolytic cap.

    2: wrap aluminum foil around the ribbon cables that go to the LED
display, and wrap wire around the foil and solder it to grounds in at both
ends of the foil wrap. This shields the display driver circuit harmonics
that sound like a bee's wings on certain frequencies.

                                   Others

Change the signal meter illumination to LEDs: Place 2 or more LEDs of the
color of your choice in series with a current limiting resistor. Solder
this to the light bulb socket. These won't burn out.