Note
The HD-19 was Heath’s first phone patch. It uses a special hybrid transformer to achieve very high isolation between the receiving and transmitting lines.
The HD-19 allows both VOX and manual operation and is placed in operation by a single switch. The unit features separate controls for transmit gain, receive gain, and patch on/off, and a VU meter provides levels and permits a convenient check for null depth.
The patch is designed to operate with a standard telephone line impedance of 600Ω, and provides a null depth of 30 db minimum between receive and transmit lines. Receiver impedance: 3 to 16Ω. Transmitter impedance: 600Ω or high impedance output.
Eventually Heath redesigned the HD-19 to bring it in line with the SB series, rereleasing it as the HD-15. This was pretty much a matter of cosmetics, as the insides didn't change much at all. The HD-19 has a green front panel and a light green or gray cabinet. The panel meter is most often seen with a yellowish face, though white-faced meters have been observed.
Caution: Always turn the function switch to off after a phone patch has been terminated, and never dial the phone with the switch turned on. In both cases, ringing current could damage the VU meter.
References:
Review. 73 Amateur Radio. Feb 1961, p. 24.
General information. CQ. Jun 1966, p. 84.
Telephone line input impedance: about 600Ω
Null depth: at least 30 db isolation between transmit and receive circuits
Receiver impedance: effective match from 3 to 16Ω
Transmitter impedance: 600Ω or high impedance
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.
