[David Dunlop]

Work is still plodding along with the Supply Unit in a number of areas. The Covers and Knobs, Metal are curing quietly away until the end of the month, when they can be reinstalled and I have been working on two other objectives in the interim.

The first item I have been struggling with is the output capability of the two Rotary Transformers (Dynamotors), MG1A HP and MG2A HP in the Supply. The MG1A HP unit I am comfortable working with as its output is well within the testing limit of all my available test equipment, which has an effective DC Voltage limit of 1,000 volts. The MG2A HP unit, however, is a different animal entirely, for which I had to dust off my math skills to really understand it. So if you are ready for some really boring trivia, here goes.

The information Robbins & Myers provided on their data plates for the two rotary transformers is as follows:

MG1A HP:

Input: 11 Volts @ 8.5 Amps
Output: 285 Volts @ .15 Amps

MG2A HP:

Input: 11 Volts @ 25 Amps
Output: 1,300 Volts @ .12 Amps

These values would have been obtained under a full load test and I am assuming that since these two rotary transformers were the upgraded ones produced in 1945, the load would have been the Sender Unit of a complete 52-Set very likely provided to Robbins & Myers by Canadian Marconi for the purposes of developing the new dynamotors. What really caught my eye was the input voltage used by Robbins & Myers was only 11 Volts DC. Then I remembered a cautionary note in the 52-Set Operator’s Manual advising that once the sets wireless batteries reached 10.5 Volts DC, they must be replaced with fresh batteries and recharged immediately, because the performance of the 52-Set will be compromised otherwise. So Robbins & Myers probably set 11 Volts DC as the lowest effective voltage to test the dynamotor performance against.

While trying to glean as much useful information as possible out of the Overhaul Manual for the 52-Set, I found a reference to the output voltages for the two rotary transformers from Canadian Marconi’s testing.

“The output voltages of the supply unit shall be within the following limits with an input voltage of 12 Volts measured at the terminals of the supply unit end with full load on the output circuit:

MG1A supply: 300 Volts +/- 10% at 175 mA.

MG2A supply: 1,430 Volts +/- 10% at 120 mA.

End Quote.

Again, from the Operator’s Manual, the optimum voltage from the wireless batteries is noted at 12.5 Volts DC, and that value pops up from time to time in the manual, but 12 Volts DC is the normally expected operating voltage for the 52-Set. So Canadian Marconi used 12 Volts DC for their testing.

When you look at the DC Voltage increase between the two tests, you get an upgrade of 7.09%. The increase in output for the MG1A dynamotor comes out at 7.5% and the similar calculation of the MG2A output gives us a 10% increase. So a 1 Volt DC input increase produces a close similar percentage increase on output.

Where things got really interesting was a later comment on the Overhaul Manual from Canadian Marconi:

“The voltage regulation of the supply unit from full load to no load on the output circuits shall not be more than the following:

MG1A Dynamotor 35% at 175 mA.

MG2A Dynamotor 40% at 120 mA.

End Quote.

So if the Supply Unit of the 52-Set is in a stand-alone mode on the bench being tested, as mine will be at the moment, the “no load” output of the two dynamotors will be in the area of:

MG1A 405 Volts DC

MG2A 2,002 Volts DC



So the MG1A HP unit is well within the safe test range of my available multimeters, but MG2A is alarmingly over this limit. On the bright side, back when I was restoring the ZE-11 Remote Supply for the Remote Receiver, I had to but some 10-Watt Resistors to fabricate the correct test load. I think I have 4 x 8.2K Ohm and a pair of 3.3K Ohm ones tucked away and will have to sort out if that will be enough to get a testable output voltage under the 1,000 Volts DC mark.

Never a dull moment with this project.


David