Note
HM-9 1983-198?
HM-9-A 198?-1991 (see text)
The HM-9 is another product designed for the QRP and VHF markets. It is a 50-watt power meter and SWR bridge that can be wired for one of three frequency ranges: 1.8-30 MHz, 50-54 MHz, or 144-148 MHz. The range is chosen during construction. It is built on a single PC board and features two ranges: 0-5 watts and 0-50 watts. Accuracy is plus or minus 10 percent of full-scale reading. The SWR sensitivity is less than 1.5 watts and the unit is fitted with SO-239 connectors.
FREQUENCY OPTION
The frequency range is determined by a variety of resistors and capacitors, and by a toroidal inductor, on the circuit board. Probably the easiest way to check the frequency range is to determine the value of resistor R1 on the circuit board. R1 has a unique value for each band. Refer to photo for the location of R1.
At some point, Heath revised the values of R1, presumably for better or more stable performance. As a result, units from the original production run will have one set of values, and units from the revised production run will have a different set of values. Note that the value for the 144 to 148 MHz band did not change. The exact date of the revision is unknown.
Original Production Run
Frequency Band Value of R1
1.8 to 30 MHz 6340Ω 1% (blu-org-yel-brn)
50 to 54 MHz 4990Ω 1%(yel-wht-wht-brn)
144 to 148 MHz 5490Ω 1% (grn-yel-wht-brn)
Revised Production Run
Frequency Band Value of R1
1.8 to 30 MHz 6650Ω 1% (blu-blu-grn-brn)
50 to 54 MHz 5230Ω 1% (grn-red-org-brn)
144 to 148 MHz 5490Ω 1% (grn-yel-wht-brn)
The unit can be rewired for other bands by changing the value of several common resistors and capacitors, and a not-so-common toroidal inductor (L1). Consult the manual for details.
An “A” version of the HM-9 is known to exist. As of this writing, only one example has been found. The date of introduction is unknown because the unit was never shown in the catalog. It is likely that the only difference between the HM-9 and the HM-9-A is the color of the cabinet—the HM-9 being brown and the “A” version being gray. This would be consistent with other contemporaneous “A” units including the HD-1420A, 1422A and 1424A, for example.
Frequency range: 1.8 to 30 MHz, 50 to 54 MHz, or 144 to 148 MHz, builder option
Wattmeter accuracy: ±10% of full scale reading (with 50Ω resistive load)
Power capability: 50 watts
Power ranges: 0 to 5 watts, and 0 to 50 watts
SWR sensitivity: less than 1.5 watts
Impedance: 50Ω nominal
SWR bridge: continuous to 50 watts
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.
