The Heathkit HR-1680 is now out of production, but during its time it was a low-cost and reasonably high-performing, ham-band-only (80 through 10 meters) radio receiver marketed as a build-it-yourself kit.
With the audio CW filter on the front-end dual-gate-MOSFET, it could pick 0.1 uV signals out of the mud. Not quite in the same class or selectivity performance as the Heathkit SB-series, but -- damn! -- it worked great for its price. Sometimes the front end would oscillate if the RF gain was turned up too high, easily cured with a resistor in series with the hot end of the RF gain control.
Its main problem that many Heath customers complained to me about was that all signals were S9. The S-meter tended to hang in the middle across a wide range of input signal levels, only very weak or very strong signals pushed the pointer below or above S9. I studied the circuit and came up with a modification to make the S-meter behave better, displaying a less-compressed response to the relative incoming signal level. I'm sure many HR1680s are still in use, if their owners happen to read this they might want to try this fix.
Heathkit Engineering approved my change, to be applied whenever a customer complained about this defect. All Heathkit retail stores received a weekly compilation of Technical Exchange Bulletins, an in-house publication to keep field technicians abreast of the latest product updates, modifications, repair techniques and troubleshooting tips. With the kind permission of Ken Kaplan, who has gone to great lengths to preserve Heathkit Technical Exchange Bulletins, the fix as published by the Heath Company is reproduced below. It was quite a surprise when I first stumbled across Ken's website and recognized stuff that I had written as a Heathkit retail store technician 35 years ago.
February 8, 1980
HR-1680 Bulletin No:
S/S Receiver HR-1680-15
S Meter Reads S9 or Pins for Most Signals
To improve the HR-1680‟s S meter response, make the following modification:
Parts Required: 2 1N4149 diode [PN 56-56]
1 560 ohm, 1/2 watt resistor [PN 6-561]
1 1 Kilohm, 1/2 watt resistor [PN 6-102]
1 PCB connector [PN 432-120]
-- Install a kilohm resistor [PN 6-102] on Q203, base to collector.
-- Disconnect the wht/blk lead from board D, pin 7 and reconnect it to board A, pin 9.
-- Connect the “+” meter lead to board B, pin C.
-- Install a 560 ohm resistor [PN 6-561] in series with the “-” meter lead at PCB connector.
-- Connect the PCB connector to board B, pin B.
-- Solder two series-connected diodes [PN 56-56] from the “-” lead of the meter to ground (cathode to ground).
-- Zero the meter with the zero control (no signal input), tune in
the calibrator signal on 80 meters and adjust meter sensitivity
for +60 dB (tuning dial at 300).
UV AT ANT. METER (BEFORE) METER (AFTER)
10 S9 S5
100 S9 40 dB
1000 Pinned 60 dB
1000 60 dB
Install this modification only if the customer specifically complains about the S meter response.
The astute reader will note that this is still not the proper response of 6dB per S-unit, the change from 10uV (S5 reading) to 100uV (40dB reading) is only 20dB, not 64dB, but the meter did respond much nicer and the customers went away happy. I think there is a typo in the last table entry, I seem to recall the final level into the antenna jack was 10,000uV without pinning the meter, not 1000uV.
The Frankenstein's Fix has just come to an end. Stay tuned to read the submissions and see what kind of difficult job of judging we have ahead of us! Submission details and full contest rules here.