Note
In 1969 Heath decided it was time to update the venerable GC-1A “Mohican” receiver. The result was the GR-78, a receiver that was smaller and lighter than its predecessor but worked every bit as well. The GR-78 is a portable solid-state superheterodyne receiver with double conversion above 18 MHz and a lot of features packed into a small cabinet. Its frequency coverage is 190 kHz to 30 MHz in 6 bands. The GR-78 uses 11 transistors, 5 FETs and 7 diodes, and has four ceramic IF filters. The IF below 18 MHz is 455 kHz; above 18 MHz Heath employs double conversion using a first IF at 4.034 MHz and a second IF at 455 kHz.
Features include a built-in 500 kHz crystal calibrator, an electrical bandspread, a lighted dial and S-meter, an ANL, and an AVC. The GR-78 provides switch selection of AM, CW, and SSB, and has a receive/standby switch, receiver muting connection, and a headphone jack. The receiver comes with a built-in speaker, telescoping antenna, external antenna connector, and carrying handle. The unit can be assembled with either of two optional bandspread dial markings, depending on the intended use. One is calibrated for general coverage use, and the other is calibrated for the ham bands. No wiring changes are required.
Sensitivity varies depending on mode and frequency but generally is good and typical of the genre. Stability is very good. The GR-78 is built on six PC boards, four of which plug into the main board. The unit will operate from 120/240 VAC or 12-15 VDC. It also will run on internal Nicad batteries kept charged from a built-in charger. The batteries are trickle charged at 20-25 mA whenever the unit is plugged into an external power source. The Nicads may be dead, but if you’re handy you can figure out a modern-day replacement. Before you buy a GR-78 it would be worth your time to remove the bottom cover to see if the batteries have been leaking.
Note: The receiver never actually runs on AC power. When plugged in, AC power is used only to charge the batteries. But the batteries are charged at a rate that is lower than what the radio uses when it is operating (see specifications below). So if run long enough, even when plugged into AC power, the batteries will eventually discharge to the point that the receiver will no longer function correctly.
Note: Unscrewing the whip antenna will cause internal hardware to fall off.
The locking DC power plug that fits the GR-78 rear DC power jack is the Cinch Jones P-302H-CCT-L. As of this writing, available from Mouser, et al.
The GR-78 is housed in a charcoal gray cabinet, and the front panel varies from light gray to light green, depending on the vintage.
References:
Review. QST. Oct 70, p. 48
Brief description. Popular Electronics, Dec 1972, p. 49.
Review. 73 Amateur Radio. Dec 1974, p. 109
Frequency coverage: 200 kHz to 30 MHz in six bands
IF:
Bands A to E: 455 kHz
Band F: 455 kHz and 4034 kHz
Sensitivity (µV, for 10 db S+N/N):
AM CW/SSB
Low end High end Low end High end
Band A 10 3 6 2
Band B 3 1 2 0.7
Band C 3 0.6 1.5 0.4
Band D 2 0.3 2 0.2
Band E 3 0.2 3 0.2
Band F 2 0.3 2 0.2
Image Rejection:
Band A: 65 db or more
Band B: 60 db or more
Band C: 48 db or more
Band D: 35 db or more
Band E: 25 db or more
Band F: 35 db or more
Selectivity: 7 ±1 kHz wide, 6 db down
Audio output: 300 mW
Crystal calibrator: 500 kHz
Charge rate:
120/240 VAC: 22 mA
12 VDC: 15 mA
13.5 VDC: 25 mA
15 VDC: 35 mA
Power requirements: 120 VAC or nominally 12 VDC
Battery drain: 65 mA @ 50 mW audio output; 40 mA with no audio. Note: Battery life is about 8 hours at normal listening levels.
Solid-State: (2) 40673, (5) 2N3694, (2) 2N3393, (1) X29A829, (1) 2N2430, (1) 2N243, (4) 1N191
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.
