During WWII Army aircraft electrical systems ran by distributing a 24-28V supply globally and then boosting it up locally to the hundreds of Volts required by receivers and transmitters. This boost was accomplished using “dynamotors.” These devices consisted of a DC motor that ran off the 26V supply, turning a generator that output a high-voltage AC supply. Filters in the dynamotor or receiver smoothed out the AC into a high-voltage DC supply that was used by the avionics.

The MP-28 was the dynamotor for the Bendix TA-12 transmitter. The TA-12 was used by many commercial transport and passenger aircraft and by Canadian Lancaster and Mosquito bombers during the war. Many became available on the surplus market after the war and were bought and used by HAMs. They are still available today on surplus sites and eBay, but the MP-28 power supplies are few and far between. We looked for over a year for the one in this exhibit and had to bid aggressively on eBay to obtain it.

One of the very interesting things about the MP-28 is that it is one of few Argus WWII radio products that appeared in Argus’s wartime ads. It is shown below three B-17’s in an ad titled “Communications for Victory.”