Linear Power Supply Upgrade

Astron RM-50 Rebuild

I was using an Astron RM-50 13.8vdc power supply for my Kenwood TS-480HX transceiver.  The power supply was sagging badly with only 150 watts output from the transceiver.  A  refurbishment of the regulator board helped a little, but still wasn’t satisfactory.  A ham friend put me on to a regulator circuit designed by W5BWC (  I recommend you read his posted articles.

Circuit boards are available from Far Circuits,,  for a modest price.  The parts or suitable replacements are available from Mouser or DigiKey.   This regulator is useable on most any 13.8vdc analog supply.

This is not drop in modifications.  One should take their time and do it right.  Remember, you are dealing with a lot of current, albeit not high voltage.  When finished, my unit held at 13.79vdc from 0 to 30A and it didn’t seem to strain with the TS-480HX drawing approximately 425 watts.

Larry, W0QE, built the regulator circuit in LTSpice and I spruced it up a bit mostly with text designations.  LTSpice will provide you with the voltages at any point and is quite useful if you need to trouble shoot any issues.  I can provide the necessary files.  LTSpice is available free of charge to download.

Completed RM-50 rebuildRM-50 Wired-2   Brackets for regulator boardCNTL-3 Top

Wiring of pass transistorsRM-50 Pass X's R

Meter boardMeter Brd -1

RM-50 Upgrade Notes


Observed no continuity in chassis ground.   This is because the panels were painted before assembly.  Drilled out two pop rivets, one on each side, clamped back panel to side panel, tapped with #8 threads and placed brass bolts, NoLox and jam nuts.  Drilled through front panel and chassis bottom lip, tapped with #6 threads, countersunk head and placed bolt.  (Threading holes provides more surface area.)

Observed no continuity from power safety ground to chassis.  Cleaned paint off chassis under the tie strip with a Dremel tool and abrasive buffer wheel.

The original ground post was bad as well.  Tapped the hole, cleaned off the paint  and placed a brass screw with a retaining nut and lock washer.

Now any point on the chassis is less than .1 ohm (limit of my Fluke 87) to the ground post.


Mounting Pass Transistors

Chose to ground the TO-3 cases.

Chose not to rely on the chassis ground connections despite my chassis improvements..

Noted the old TO-3 sockets were built in a sandwich with a plastic base.  Clipped of the plastic heads on the retaining posts with a razor knife and disassembled the sockets.  Inserted the plastic pieces from the inside of the chassis to center the bolts in the existing holes and center the pins.  Put ¼” Teflon sleeves on the pins in the heat sink holes to assure they would not make contact.  (probably not necessary)

Bridged all of the collector connections (through bolts from the TO-3 housing) with #12 tinned bus wire.  Connect a #6 ring terminal on one end and solder, place a ring terminal on the other bolt, cut the wire to length, slide the wire into the loose terminal, tighten nuts and solder the remaining terminal.

The collector bus wires are close to the chassis, but it’s all supposed to be at ground potential anyhow.  I used braid stripped off of RG8 coax because it laid flat, is flexible and it slid it under the bus wires.  Generously soldered each bus wire to the braid.  Placed heat shrink tubing over the braid at the ends near the regulator board to avoid shorting out anything on the back of the board.

Used the existing tie strips to place a #12 bus wire that connects each emitter via the .01 ohm resistors.


Mounting the Regulator Board

Used 5/8” brass strap to create two U shaped (flat sides) brackets.  Drilled ¼” holes in the top and one end of each bracket.  Placed appropriate length ¼” brass bolts through the holes, heated them up with a 250 watt soldering iron and flowed solder under the heads.  This provides a large current capacity and assures that when tightening the nuts the bolts won’t turn.

Temporarily mount the brackets in the chassis and place the regulator board against them.  Use nylon washers to correct any length differences preferably on the positive terminal.  Mark the mounting holes on the brackets, remove the brackets and drill holes.  I used #8 bolts to loosely mount the board to the brackets. Install the unit back into the chassis and tighten the nuts through the chassis.  If all looks good, gently tighten the bolts through the board.  Place washers and nuts on the vertical bolts to retain them in place and solder the board to the brackets making sure to primarily heat the brackets.  The #8 bolts supply the mounting strength and the solder assures a continuing connection.  Once all together the board and brackets are easily removed as a unit if necessary.


IRF520 and Shunt Resistor

Mounted the IRF540 driver on a 1” piece of 1 x 1/8” aluminum angle.  Threaded the holes to make assembly easier.  Used a small piece of perf board to create a connector.


Mounted the Shunt resistor on a 1 x 2” piece of 3/16” aluminum.  Again threaded holes for easier assembly.  Cleaned the paint off the rear panel with the Dremel tool and applied heat sink compound between the resistor, pad and chassis.  The pad was added to assure clearance between the chassis and resistor pins.  The extra mass could not hurt for heat dissipation either.

Rather large wires need to be connected to the resistor pins as well as the amp meter leads.  Created two connectors by taking short pieces of #12 bus wire, rolling appropriately sized eyes on each end with a needle nose pliers with round jaws and then folding it in half so the eyes match.  The folded end solders on to the resistor pins and the eyes accommodate the large wires.


Fuse in Positive DC Line

Used a chassis mount blade (Maxi) fuse holder, Littlefuse 01520001U. (32 volt, up to 60 amp)  Used 40A fuse.


Meter Connections

Astron hung the trimmer pots off a pin soldered to a ring terminal on the meters.  Made a 2.25 x .75” circuit board that mounts on the meter terminals and mounts the trimmer pot and wire connections.




Place a jumper between S201A and S201B. It’s easy to miss this one.  A jumper to the low side of the shunt resistor is then not needed.


A bleeder resistor should be placed across the output.  It takes a long time for it to discharge.


LTSpice was very useful in helping me determine I had the wrong value zener in D202.


Unit now stays within .1v with powering a TS-480HX running 200 watts output (est. 425 watts load, 29.7a).  It sagged badly before.


The loud clunk on startup with these units is primarily the magnetic field of the transformer acting on the steel top panel like hitting a bass drum.  Adding a surge resistor on the AC input doesn’t help much.  I don’t worry about it.


In my case (rack mounted, metered RM-50) I saw no need for the onboard circuitry for the meters or the LED indicator.  Had it been a more simple task to add the LEDs as meter lights I would have done that.





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