W4THU's Selective SSB/CW Demodulator
|The idea was obvious. The Drake 2-B
receiver has a four-pin jack on its back to accept an external Q-multiplier.
This plug could be easily rerouted to the input of the 455 KHz-to-50 KHz mixer.
Thus, it was easy to get an external 455 KHz signal into the 2-B. Only a small
value capacitor was needed to provide very light coupling to the mixer.
The next trick was to cut the wire from the variable capacitor controlling the VFO frequency and running a jumper to the 505 KHz oscillator circuit feeding the third mixer. This would provide very slow tuning across the passband of the receiver proceeding this demodulator. A small value silver-mica capacitor limited the tuning range to about 6 KHz, thus, the tuning range is 452 KHz to 458 KHz. Only minor other modifications were necessary. The 2-B can be returned to its original condition in minutes.
The dial is simply a piece of card stock printed with the appropriate calibrations. The card stock simply fits in front of the 2-B's dial and blocks the ham band calibrations. A narrow slit is cut in the card stock to permit viewing the dial pointer. Calibration marks where determined experimentally using blank dial cards. The calibration marks are based on the needs of an R-390A and its available filters.
You can see from the photograph below that calibration is quite precise. 100 Hz calibration marks appear above the dial pointer and bar-graphs represent the -6 dB bandwidth of the 2 KHz and 4 KHz filters. Below the dial are the actual set points for putting the demodulator on the proper frequency for demodulating USB, LSB, or CW with either the 2 KHz or 4 KHz filter selected. Repeatability is excellent. A further feature of this scheme is the fine tuning of SSB signals available. Moving a couple of hundred cycles to find the SSB signals "sweet spot" is not a problem with this demodulator.
Audio quality is very good. The 2-B's passband tuning and filter selectivity are still active and permit exceptional CW reception with the R-390A. The passband tuning feature is useful on both CW and SSB. All-in-all, this was a very worthwhile project with exceptional results. Of course, it can be used with any receiver utilizing a 455 KHz i.f. frequency.