4 December 2016
The SPR-4 is irritating in that it is not full-coverage.
Drake gave us 23 crystal sockets and offered us crystals to add 500 KHz bands to the SPR-4. The crystals were expensive and a nuisance to order and install. There are not enough crystal sockets to achieve "full-coverage" from 200 KHz to 30 MHz. Not having enough crystals makes it impossible to do a complete and accurate alignment of the SPR-4. Even with a lot of crystals, alignment is more cumbersome if you have to change crystals as part of the alignment process. Today it is relatively easy to replace the crystal-bank with a DDS generator that can be programmed to mimic any crystal on command. This lets the Drake SPR-4 fulfill its destiny of being a full-coverage HF receiver. The PCB with the crystal sockets can be removed and replaced by a new PCB containing the DDS generator, a microprocessor and an encoder. The encoder can be driven by the same knob that was used to select crystals but now tells the microprocessor which frequency to generate.
The SPR-4's original frequency display is two gear-driven dials driven by the shaft that drives the PTO. An optional crystal calibrator enables us to shift these dials a little whenever we change bands, to keep the dials accurate despite the small, unavoidable errors in the crystals. The SPR-4 was one of the first solid-state HF receivers sold for non-military applications. It was also one of the last HF receivers to use dials for frequency readout. A digital display is a big improvement, especially if it eliminates the need for the crystal-calibrator and re-calibration with every band-change. All of the dial's errors can be avoided by replacing it with an electronic readout that is driven by a frequency counter system that continuously measures all the frequencies used in the receiver and uses them to calculate and display the actual received frequency.
The Drake Technical Manual for the SPR-4 gives an abbreviated alignment procedure. I've never been able to find a really complete procedure write-up. I can't believe that the abbreviated procedure in the Technical Manual is what the factory used when aligning receivers coming off the production line. The most important things I've found that need to be remembered when aligning the SPR-4 is that the trimmers for the RF Amplifier, 1st Mixer and Pre-Mixer boards need to be adjusted at the high end of their "Range." I.e., for the "C" range the RF Amplifier and 1st Mixer trimmers should be adjusted while tuned to 2.999MHz. The Pre-mixer trimmers are a different story, though. Because the PTO tunes in the reverse direction, i.e. lower PTO freq. tunes a higher received freq., the Pre-mixer "C" trimmers should be adjusted while tuned to 2.500MHz.