Note
The SB-401 represents a general refinement of the SB-400. Refer to SB-400 for a discussion of the basic design and specifications.
Heath made a few significant improvements in the 401. Most notable among these improvements is the addition of a front panel switch to change from separate operation to transceive operation. In the SB-400 it was necessary to open the hood and swap a couple of cables. On the SB-401, a front panel switch effects the change—a major improvement to overall operating convenience.
In addition to numerous small circuit tweaks, other changes include the swapping of a 6BZ6 for the 6AU6 in the LMO, and the addition of a driver coil for 29 MHz to improve output on the high end of ten meters. Refer to illustration. As there was no room for this coil under the shield (without significant re-engineering), it was added immediately adjacent to the driver variable capacitor.
Heath also added a sidetone level control, which is mounted on the chassis, and the 120 VAC two-blade external antenna relay power receptacle of the SB-400 has been replaced by a nine-pin molded nylon connector on the 401.
The most significant change may be the heterodyne oscillator crystals. Assuming that most purchasers would be using the SB-401 with its companion SB-300 or 301 receiver, Heath made the heterodyne oscillator crystals optional. What this means is that if the SB-401 you’re thinking about buying doesn’t have the crystals, the transmitter will not function except with the SB-300 or 301. Refer to Figures 1 and 2. To determine if the crystals are present, open the top of the cabinet and look in the left front corner of the chassis directly below the final tuning control shaft. If the crystals are installed, all is well. If not, you have two choices—find some crystals, or find an SB-300 or 301 to use with it.
Note: because of the different size and mounting hole configurations, SB-400 and 401 crystal filters are mutually incompatible.
Refer to SB-300 for interconnection requirements.
References:
Review. QST. Mar 1967, p. 42.
Review. 73 Amateur Radio. Jul 1967, p. 98.
Improved switching from transmit to transceive. QST. Dec 1966, p. 21.
Spurious emission, relay problem, low drive. QST. Jan 1969, p. 16.
Operation outside the ham bands. CQ. Jan 1969, p. 96.
Improved sidetone. Ham Radio. Oct 1969, p. 73.
Improved ALC response. Ham Radio. Jan 1970, p. 71.
Instant frequency change. QST. Jan 1970, p. 28.
Dual frequency operation. QST. Sep 1970, p. 50.
Foot switch operation. QST. Sep 1970, p. 53.
VOX adjustment. QST. Dec 1971, p. 40.
Increase friction in worn zero set. QST. Jan 1973, p. 52.
Faster relay response. QST. Feb 1977, p. 43.
Sidetone and AGC mods. QST. May 1979, p. 45.
Sidetone for. QST. Jul 1979, p. 49.
Zero-set dial modification. QST. Feb 1980, p. 44.
High plate current fix. QST. Jun 1981, p. 35.
Power supply upgrade. Electric Radio. Apr 2007.
Mod for stand-alone operation convenience. Electric Radio. Feb 2007.
General information about SB series. Electric Radio. Sep 2016.
Frequency range (in 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
Stability: < 100 Hz per hour after 20 minutes warmup; < 100 Hz for ±10% line voltage variation.
Emission types: LSB, USB, CW
DC input power:
SSB: 180 watts PEP
CW: 170 watts
RF output power:
80–15 meters: 100 watts
10 meters: 80 watts
Output impedance: 50–75Ω
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
Carrier suppression: 55 db down from rated output
Oscillator feedthrough or mixer products: 55 db below rated output (except at 3910 kHz crossover which is 45 db)
Harmonic radiation: 35 db below rated output
Unwanted sideband suppression: 55 db down from rated output at 1000 Hz and higher
Third order distortion: 30 db down from rated PEP output
Noise level: at least 40 db below rated carrier
Audio frequency response: 350–2450 Hz ±3 db
Audio compression: 10 db nominal at 0.20 mA final grid current
Audio input: high impedance or phone patch
CW sidetone: internally switched to speaker with transmitter in CW mode and key depressed; about 1000 Hz
Keying: break-in CW, provided by operating VOX from keyed tone, using grid block keying
Power requirements: 120/240 VAC, 50/60 Hz; 80 watts standby, 260 watts key down
Tubes: (1) 0A2, (1) 6AU6, (1) 6AV11, (1) 6AW8, (1) 6CL6, (1) 6D10,
(1) 6EA8, (2) 6EW6, (1) 6J11, (1) 6BZ6, (2) 6146
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.
