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Since the 51J-4 did not include a product detector, the Military contracted for product detectors to extend the life of the receivers in service. The picture below shows a ISB-1 by Delta
Electronics (discrete solid state) that has been aligned to work with the 500KHZ IF in the 54J-4 The ISB-1 is a product detector and audio amplifier. The ISB-1 is shown mounted in a cabinet with a small speaker
panel. In reality I think the 54J-4 does fine receiving SSB by using BFO injection. In addition some applications wanted a squelch,
the Audio Threshold Detection Relay - 761G manufactured by Scientific Prototype fit the requirement.
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