Note
Although the SB-10 was the first Heath product to use the “SB" label, the SB-300 was really the first product in the classic Heath SB series. The SB-300’s companion transmitter, the SB-400, would not appear for another 6 months.
The SB-300 is designed around 11 tubes (including the LMO), and while two PC boards are used there is still a great deal of point-to-point wiring, much of it with a wiring harness. The SB-300 is a dual conversion superheterodyne unit with a 3395 kHz IF and covers 500 kHz portions the 80-10 meter ham bands. The 10 meter band is divided into four separate 500 kHz pieces. The receiver uses a crystal controlled front end with tunable inputs. The BFO is crystal controlled as well.
At the heart of the SB-300 is a pre-assembled and aligned VFO Heath called an LMO or Linear Master Oscillator. The LMO was a remarkable piece of engineering. It had an output from 5 to 5.5 MHz on all bands, was linear from one end of the band to the other, provided 1 kHz dial markings with reset-ablility to about 200 Hz, and was accurate to within 400 Hz of the nearest 100 kHz marker. Frequency stability is better than 100 Hz per hour after warm-up. Heath used the LMO in all but the last of the SB series rigs.
The receiver's sensitivity is rated at better than 1 µV for 15 db signal plus noise-to-noise ratio. Selectivity is 2.1 kHz at 6 db down for SSB operation. The SSB crystal filter was supplied as standard equipment with the SB-300, but Heath also offered an optional 3.5 kHz AM filter, and an optional 400 Hz CW filter. The presence of these filters may be determined easily by opening the top cover. The filters are small, (usually) black, rectangular modules about 2.5 inches wide mounted in a row between the LMO and the S-meter and are (from front to back) CW, SSB, and AM. A total of three filters may be installed. Without the proper filters installed, the SB-300 will not operate with the mode switch in the AM or CW position, however, reception of these signals is still possible in the SSB mode.
Note: because of the different physical size and mounting hole configuration, SB-300 and 301 filters are mutually incompatible. Refer to SB-100 for a discussion of filters.
Features of the SB-300 include a built-in solid state power supply, a built-in 100 kHz crystal calibrator, a lighted S-meter and tuning dial, and smooth backlash free vernier tuning that provides nearly 5 feet of bandspread.
There is no built-in speaker and there are no provisions for crystal controlled operation. The SB-300 can be configured to work separately from the SB-400 or to transceive with it. In the transceive mode, the SB-300 determines the operating frequency. Switching from separate operation to transceive requires that you swap a couple of cables inside the SB-400. Refer to SB-400 for details. This clumsy chore was streamlined in the SB-401. Note: Do not attempt to transceive using the SB-400 with the SB-300’s mode switch in either the AM or CAL positions as the units will not function properly.
The SB-300 has provisions for two plug-in converters—the SBA-300-3 for 6 meters, and the SBA-300-4 for 2 meters. These converters are selected from a switch on the top side of the chassis at the right rear corner of the rig. Refer to Figure 2. On the SB-301 this switch was replaced with a front panel control.
Front panel controls include main tuning, function, mode, AGC (fast/slow/off), band, AF and RF gain, and preselector. There is also a front panel headphone jack.
Note: Heath specified that the LMO, BFO, and HFO interconnects must be 24 inch lengths of RG-62. It takes no fewer than seven coaxial cables to connect the SB-300/301 to the SB-400/401.
Note that the FUNCTION and A.F. GAIN controls are mounted slightly lower on the front panel compared to the similarly positioned FINAL tune and mic/cw LEVEL controls on the SB-400. Refer to Figure 4. The SB-400 was still in development when the SB-300 was released, and engineers had failed to account for control shaft mechanical clearance requirements of the SB-400 when they designed the front panel of the SB-300. The resulting mismatch was corrected in the SB-301.
Alignment requires only a VTVM. The SB-300 is finished in the classic SB two-tone green wrinkle paint. In 1966 the SB-300 was replaced by the improved SB-301.
References
Review. QST. Jul 1964, p. 82.
Review. 73 Amateur Radio. Aug 1964, p. 66.
Review. Popular Electronics, Aug 1964, p .85.
Review. CQ. Sep 1964, p. 40.
AGC trouble. CQ. Dec 1964, p. 101.
60 Hz hum. CQ. Sep 1965, p. 84.
Improvements (very brief). CQ. Ap 1966, p. 79.
Improved switching from transmit to transceive. QST. Dec 1966, p. 21.
Increase BFO output (brief). CQ. Dec 1966, p. 79.
Front panel converter switch. CQ. Apr 1968, p. 61.
MARS reception. CQ. Jul 1968, p. 112.
Operation outside the ham bands. CQ. Jan 1969, p. 96.
Spurious emission, relay problem, low drive. QST. Jan 1969, p. 16.
Improved AM. CQ. Jun 1969, p. 79.
RTTY reception. Ham Radio. Jul 1968, p. 76.
Narrow shift RTTY reception. Ham Radio. Oct 1971, p 64.
Narrow shift RTTY reception. Ham Radio. Jun 1973, p 54.
Increase friction in worn zero set. QST. Jan 1973, p. 52.
Improved stability. QST. Nov 1975, p. 37.
Zero-set dial modification. QST. Feb 1980, p. 44.
AM noise limiter. Electric Radio. Mar 2007.
General information about SB series. Electric Radio. Sep 2016.
Frequency range (MHz): 3.5–4.0, 7.0–7.5, 14.0–14.4, 21.0–21.5, 28.0–28.5, 28.5–29.0, 29.0–29.5, 29.5–30.0
IF: 3395 kHz
Stability: < 100 Hz per hour after 20 minutes warmup; < 100 Hz for ±10% line voltage variation.
Visual dial accuracy: within 200 Hz on all bands
Electrical dial accuracy: within 400 Hz on all bands after calibration to nearest 100 kHz point
Backlash: < 50 Hz
Sensitivity: < 1 µV for 15 db signal plus noise-to-noise ratio for SSB
Modes: LSB, USB, CW, AM
Selectivity:
SSB: 2.1 kHz at 6 db down, 5.0 kHz maximum at 60 db down
AM: 3.75 kHz at 6 db down, 10 kHz maximum at 60 db down
CW: 400 Hz at 6 db down, 2.5 kHz maximum at 60 db down
Spurious response: image and IF rejection better than 50 db down. Internal spurious signals below equivalent antenna input of 1 µV
Audio response:
SSB: 350–2450 Hz nominal at 6 db
AM: 200–3500 Hz nominal at 6 db (with optional filter)
CW: 800–1200 Hz nominal at 6 db (with optional filter)
Audio output: unbalanced nominal 8Ω and high impedance headphone
Audio output power: 1 watt with less than 8% distortion
Antenna input impedance: 50Ω nominal
Muting: open external ground at mute socket
Crystal calibrator: 100 kHz
Power requirements: 120 VAC, 50/60 Hz, 50 watts
Tubes: (1) 6BZ6, (4) 6AU6, (1) 6AB4, (1) 6AS11, (2) 6BA6, (1) 6HF8
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.
