[MRCA] Technical question! and the PRC-70

[jkarlow at comcast.net]

Ray -

With your excellent analysis if the PRC-70 power system and the low voltage shutdown, I have one small note that might be interesting.
The internal low voltage shut-down circuit I incorporate into the A4  module of the PRC-70 to shut down the power convertors when the input voltage drops below 20 volts DC was done to make the radio more robust when a variety of DC power Systems  are used.

In my latest battery incarnation for the PRC-70,  the BB-542/U has been reconfigured, with the use of two strings of 8 ANR26650M1B LiFePO4 cells in parallel with proper fusing and elimination of the original electronic board. I use a SONEIL CANADA Model 2412SRF-B 24volt 6amp battery charger, which plugs into the side of the battery box when re-charging is needed. This solution was presented to me by Peter Gottlieb in a radio I got from him. The big advantage of this configuration is that it reduces the weight of the portable configuration of the PRC-70 by 4 pounds when compared with the NIMH battery version. It also is lighter and lasts much longer than the original BB-542/U battery with the 1/2D NICD Cells and the Frezzolini board.
Hopefully this offers some additional ideas.

Jim
KA8TUR

Jim Karlow
4907 Oakwood Court
Commerce Township, MI 48382
E-Mail: jkarlow at comcast.net
Tel: 248-961-2034


________________________________
From: mrca-bounces at mailman.qth.net <mrca-bounces at mailman.qth.net> on behalf of Ray Fantini via MRCA <mrca at mailman.qth.net>
Sent: Tuesday, March 10, 2026 9:15 AM
To: W2HX <w2hx at w2hx.com>; KD2GIY <kd2giy at gmail.com>; mrca at mailman.qth.net <mrca at mailman.qth.net>
Subject: [MRCA] Technical question! and the PRC-70




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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