4 December 2016
The SPR-4 is irritating in that it is not full-coverage.
Drake gave us 23 crystal sockets and sold 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. 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 becoming a full-coverage HF receiver. The PCB with the crystal sockets can be removed and replaced by a new PCB containing the DDS chip, a microprocessor and an encoder. The encoder driven by a knob and is used to tell 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.