Note
Just as the SB-640 permitted split frequency operation with the SB-101 and 102, so too the SB-644(A) permits split frequency operation with the SB-104(A). And like the 640, the 644(A) provides the same kinds of operating modes. It is interesting to note that Heath no longer referred to this device as an LMO (linear laster oscillator), a term the company used for its proprietary tuning device since it was introduced in 1963 with the SB-300. Now Heath called it a VFO. Moreover, the VFO is not preassembled, as with the LMO.
The SB-644(A) permits transceiving from either the SB-104 or the SB-644(A) as well as split operation. Unlike the SB-640, the 644(A) also permits control of either the transmit or receive frequency from the remote VFO with the SB-104 controlling the other. The 644(A) also permits operation on one of two crystal frequencies. Crystals used are the smaller HC-25U type, rather than the HC-6U, which Heath had used previously.
The 644(A) uses an illuminated slide rule dial as a general frequency indicator, but whenever it has been selected the 644(A)’s exact frequency is displayed on the SB-104’s readout. When operating split frequency, the 104’s display will change between transmit and receive frequencies.
Other than the main tuning knob, the only other controls are two rows of front panel pushbuttons which allow you to select the mode of operation. Status lights behind the red window indicate in what mode the 644(A) is operating.
There is no internal power supply. All voltages are derived from the SB-104(A). Rear panel connections include two RCA jacks labeled “in” and “out.” These match up with jacks on the 104(A).
In 1977 the SB-104 was given a major overhaul to fix a variety of problems—among these were some minor problems with the VFO. The 104A’s new VFO was incorporated into the 644 and the unit was renamed the 644A. Problems with the original VFO included FMing and bleed-through between the remote VFO and the VFO onboard the 104. This caused a variety of tweets and heterodynes.
References:
Improving the SB-104/SB-644 combo. QST. Aug 1979, p. 28.
Improving the SB-104/SB-644 combo (more on). QST. Sep 1979, p. 23.
Improving the SB-104/SB-644 combo (more on). QST. Mar 1980, p. 49.
Improving the SB-104/SB-644 combo (more on). QST. May 1980, p. 41.
Crystal Characteristics
Frequency range: 5 to 5.5 MHz
Mode: fundamental
Tolerance: 0.005%
Type: HC-25U
Load capacity: 42 pF
Series resistance: 40Ω maximum
Reference series resistance: 13Ω
Drive level at reference series resistance: 5.2 milliwatts at 20 mA
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.
