[MRCA] Technical question! and the PRC-70

[Ray Fantini]

The AN/PRC-70 has three internal DC to DC converters. One provides 10 volts for analog circuits, the second provides 5 volts with two separate outputs for the analog and digital circuits and a third that provides -5 volts.
All three power supplies live in two sealed cans 1A4 and 1A5 when they were designed they were cutting edge technology, maybe so cutting edge that things may have been over looked.  Regulation is controlled by varying the width of a switching pulse to 2N3720 by the power supply controller a SG1524 regulator chip. The problem is that as voltage decreases the duration or width of the pulse increases to try to get more umph from the LC circuit the switch feeds and at some point the wide pulse appears to be almost DC and kills the switching transistor. This happens whenever the input voltage drops to low for the power supply to keep up.
To try to solve this problem the military adapted the battery box to accommodate the Frezzi board, that board went between the battery and the radio and when its input dropped below twenty volts or so it opens a relay on the board that disconnects the radio to prevent damage, may have been a higher voltage but I always set them to cut out at twenty. Somehow there are more radios and battery boxes out there then Frezzi boards, know that I have repaired a couple but beyond that not a lot of them around.
Jim Karlow KA8TUR did a lot of work on the PRC-70 and realizing this design flaw developed a fix for the regulators that disables them before they can damage themselves. He also published a lot about the care and feeding of the Frezzi board. His modification to the DC to DC converters is great but wedging it into the power supply cans is a choir so myself would prefer to use the Frezzi board if available.
Think your idea of the external DC to DC converter may be the first "New" idea and work done on the PRC-70 in years and would definitely be an improvement over the old Frezzi board.

Ray F/KA3EKH

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