Swan Mark 2 Power Supply upgrade
by Mark Coker KC7JOG


The Swan Mark II linear amplifiers were manufactured by Swan in Oceanside California over 40 years ago (1967-1972). The Swan Mark II amplifier is an excellent linear amplifier and many hams are still running these amplifiers. The fact this amplifier is still in use is a testament to the sturdy design and construction.


The problem with the power supply in old linear amplifiers, is that the Electrolytic capacitors dry out and become leaky with age. I recommend replacing the original capacitors with a modern equivalent. I have two Swan Mark 2 amps, both with the original electrolytic capacitors in the high voltage power supply. Since both amps are now over 40 years old, it is a good idea to rebuild them both. Both amps are very well built, and have been my faithful amateur companions.

Rebuild Considerations

There are two approaches to rebuilding older linear amplifier power supplies with aging electrolytic capacitors. You can replace the original capacitors with new old stock originals or replace them with the new printed circuit board snap-in electrolytic capacitors. I decided to use the snap-in electrolytic capacitors since they are less expensive and more readily available. Steve designed a new voltage-doubler board to facilitate mounting of the snap-in electrolytic capacitors. This new upgrade circuit board replaces the original voltage doubler (capacitor and resistor) board that was used to mount the screw mount style Mallory capacitors. Steve: N9HE, decided to give me a push on the project, by going to the trouble of purchasing all the necessary components, as well as getting the supplies to manufacture the upgrade voltage-doubler circuit board required to upgrade both of my Swan amplifers.

Upgrade Description

This upgrade board uses quantity 8, 330 Mfd @ 450 volt United Chemi-con SMQ series snap-in electrolytic capacitors (available from Digikey Part No 565-2795-ND). The bleeder resistors were changed to Xicon 68K 5W power resistors which are mounted on the top of the upgrade board in parallel the caps. They were increased in ohmage value, which slowed the H.V. bleed down, but reduced the heat. One could easily reuse the originals, as 6 watt resistors are getting tough to find. I also changed the H.V. relay that's mounted on the filter board, but one could reuse the original, if need be.

I decided to reuse the original diode board which was upgraded by replacing the 600Vdc 1Amp diodes with 1N5408 diodes rated at 1,000 volt @ 3 amps.

Below are pictures of the original power supply configuration. A clear plastic template board was made to duplicate the original filter board size, as well as locate all 6 mounting holes. Also, necessary component locations were figured.

Steve was then able to manufacture the replacement boards with the necessary copper trace locations from the plexiglas template.

Click on image for larger view


Original, before modification

New Capacitor Board - Top View

New Capacter Board - Bottom View

Original Diode Board with upgraded diodes

New Resister and Capacitor Board with upgraded CW/SSB Relay installed
(Modified control circuit and relay upgrade to be described in follow up article)

Final assembly with new Resistor and Capacitor Board and Upgraded Diode Board

Final Assembly showing both boards

Steve choose some mil spec circuit board (blue) that was a little thicker than normal. However, due to the terminal lengths on the electrolytic capacitors, the second board was made with conventional thinner glass board. Above are pictures of both boards, as well as the finished products mounted in one of my Swans. Anyone that desires to do this project, may contact Steve at N9HE (at) QUARKED.COM. Steve has the prices, part numbers, vendors, as well as costs. On request, he can provide you with the required upgrade voltage doubler circuit board layout. He may even be willing to put one together for you.

Additional Note:

I decided to sit down with a pretty good capacitor checker, and test each of the 8 mallory electrolytic filter capacitors that came out of the first swan supply that was completed. Before starting, I also tested a brand new 330 MFD capacitor at 450 volt, just to make sure the test instrument was OK. The results may seem startling, but they are what they are.

Follow up Testing of the original 40 year old Computer Grade Mallory eelectrolytic capacitors removed from the Swan Linear Amplifier Power Supply

All 8 capacitors tested within an acceptable range for their original value of 325 Mfd:

Below are each of the capacitors, with the amount of milliampere leakage discovered for each one:

* The reason for the asterisk, is that 6 caps were so leaky, they pulled down the test instruments power supply. Even after I set the testing instrument to the max of 600 volts for the bad ones, they still managed to pull down the test voltage to the levels listed above. I repeated the test twice for all 8 caps, changing the order that each electrolytic capacitor was tested. The results were identical. Keep in mind, the Swan Amp was working well, when I decided to rebuild the power supply. The above results will surely call into question, the phrase "if it ain't broke, don't fix it"