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  Improving the
Power Supply

Includes Load Tests
Written by Eric Lemmon WB6FLY
Posted by Robert Meister WA1MIK
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The MFJ-4225MV power supply is a compact and very capable switch-mode unit. As is true for most switchers, it is much lighter than the typical linear power supply of similar rating, and has about one-quarter the volume. It is rated for 22 Amperes continuous, and 25 Amperes intermittent. A cooling fan runs continuously, and its speed varies with the voltage setting, which can be adjusted with a front-panel knob between 9 and 15 VDC. The front panel is shown below.


When I ran a load test, I discovered that the AC current exceeded the 5.0 Ampere rating of the AC Mains Input fuse at 21 Amperes DC load. The provided 5.0 Ampere 5 x 20 mm fast-blow fuse should be replaced with a 6.3 Ampere fuse.

The significant deficiency I found in this power supply is that the negative output terminal is bonded to the grounded chassis. When used in a base or repeater station, where the radio itself is grounded, having the power supply negative terminal also grounded will often create a ground loop that may inject AC hum into audio circuits and/or may upset the operation of a controller or similar outboard equipment.

Unfortunately, the connection of the negative terminal to the chassis is not an easily-removed jumper wire, as found in many Astron power supplies. Instead, the connection is made by contact between a trace on the PC board and one of the PCB mounting tabs. As shown in the photo below, this contact can be broken by lifting the PCB slightly and sliding in a thin #4 nylon or fiber flat washer. I always apply a "FLOATING NEGATIVE" label to any power supply that I have modified to disable such a connection.


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This page originally posted on Monday 13-Jul-2015.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.