Note
Released in Fall 1976, the HR-1680 (and later the HX-1681) was designed as a state-of-the-art replacement for the aging technology of the venerable HR-10B and DX-60B pair.
The HR-1680 is a fully solid state rig with an analog readout and a built-in power supply. It is a superhet double-conversion type receiver covering 500 kHz segments on 80-15 meters and the 28-29 MHz portion of the 10-meter band. There are no provisions for WARC band coverage.
Front panel controls include main tuning, AF and RF gain, band switch, pre-selector, mode switch, and function switch. Note that the mode switch automatically selects fast or slow AGC. The front panel also includes a headphone jack for low impedance phones.
Rear panel connections include RCA jacks for speaker, sidetone, mute, and a 50Ω antenna. There also is a spare RCA jack and a two-pin connector for 12 volts DC input. The receiver is built on several plug-in PC boards and uses a wiring harness.
Features include a double-tuned RF stage on each band, diode band switching, a built-in 100 kHz crystal calibrator, AGC controlled IF and RF stages, a four-pole crystal filter, and a two-stage active audio filter.
The HR-1680 can be aligned without instruments. Sensitivity is better than 0.5 µV for all bands. The unit has only one filter (SSB), and there is no provision for a second. Selectivity is 2.1 kHz minimum at 6 dB down, 7 kHz maximum at 60 dB down. Audio response is 2100 Hz minimum at 6 dB down, 7 kHz maximum at 60 dB down (wide setting), and 250 Hz minimum at 6 dB down, 2.5 kHz maximum at 60 dB down (narrow setting). Center frequency is about 750 Hz. Image rejection is 50 dB or better. IF rejection is 60 dB or better. First IF is 8.395-8.895 MHz. Second IF is 3.395 MHz.
A problem you may encounter is related to the card edge connectors Heath used. The surfaces of these connectors may become oxidized over time. If you experience any erratic operation, try cleaning up these connectors.
The HR-1680 may be aligned without instruments. Light green front panel with red plastic dial window (illuminated) and dark green cabinet—same paint scheme as HW-100/101/104.
MODIFICATION FOR USE WITH HX-1681
Oddly, to use the HR-1680 with it’s matching transmitter, the HX-1681, two small modifications are required to avoid sidetone overloading and distortion.
Refer to Pictorial A and add a 10kΩ resistor at lug 2 of R4 as shown, then refer to Pictorial B and replace R204 (3300Ω, orange-orange-red) with a 100kΩ (brown-black-yellow) resistor.
References
Review. QST. Jan 1977, p. 35.
Review. CQ. Oct 1976, p. 43.
Add noise blanker, 400 Hz filter, more. 73 Amateur Radio. Jan 1980, p. 76.
Modifications. QST. Nov 1982, p. 22.
Use as a CW monitor. QST. Oct 1988, p. 39.
Frequency coverage: 3.5 to 4.0, 7.0 to 7.5, 14.0 to 14.5, 21.0 to 21.5, 28.0 to 28.5 and 28.5 to 29.0
Sensitivity: better than 0.5 µV for 10 db signal-plus-noise to noise, SSB
IF selectivity:
2.1 kHz minimum @ 6 db down
7 kHz maximum @ 60 db down
Overal audio response:
wide:
2100 Hz minimum @ 6 db down
7 kHz maximum @ 60 db down
narrow:
250 Hz minimum @ 6 db down
2.5 kHz 7 kHz maximum @ 60 db down
(center frequency approximately 750 Hz)
Overall gain: better than 1.5 µV input for 0.25 watts of audio power
Audio output power:
0.8 watts into 4Ω load continuous
1.2 watts peak power @ less than 10% THD
AGC characteristics:
blocking level: 3 volts
dynamic range: 120 db or better
time constant: attach time better than 1 mS; release time switch selectable
@ 100 mS (CW) or 1 second (SSB)
Intermodulation distortion: –60 db
Image rejection: 50 db or better
IF rejection: 60 db or better
Internally generated spurious signals: below 1 µV equivalent input except at 3.74m 21.2, 28.6 and 28.9 MHz
Modes: USB, LSB, CW
Stability:
less than 100 Hz per hour drift after 30 minutes warm up
less than 100 Hz drift for 10% change in line voltage
Tuning rate: about 15 kHz per revolution
Dial accuracy: within 2 kHz after calibration to nearest 100 kHz marker
Dial backlash: 50 Hz or less
Muting: short external ground at mute socket
Sidetone input level: 10 mV or greater (300 mV maximum)
IF frequencies:
first IF: 8.395 to 8.895 MHz
second IF: 3.395 MHz
Operating temperature range: 15F to +120F
Power requirements:
120/240 VAC, 50/60 Hz, 27 watts maximum
11.5 to 15 VDC @ 0.75 amps maximum
Solid State:
diodes: (1) 1N751 zener, (1) VR9.1 zener, (28) 1N458, (4) 1N191, (1) DO-7 zener, (2) GD510, (4) 1N4002
transistors: (2) 2N5770, 94) MPF105, (2) X29A829, (1) 2N3638A, (1) EL131, (5) MPSA20, (4) MFE131
integrated circuits: (1) MC1496G, (1) LM317T, (1) LM324N, (1) TBA820L
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.
