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My reliable 1942 completely original USN 10EE MG lathe stopped working.


May 6, 2024
Ann Arbor
Good morning all you wonderful lathe experts from all around the world!

So, I have a 1942 MG round dial that is a war finish machine. Pretty nice for a war finish machine, but the tag is there so it is.

I have owned this lathe for about 10 years. Picked up in eastern US from a university sale. Condition- excellent for age.

Power: 220VAC 3 phase generated by a 20HP American Rotary phase converter. All input power checks out. I have been running it on the ARPC for 10 years with no issues.

Fuses at main lathe disconnect- fine, checked for continuity with meter. Main disconnect appears to be fine, and I can trace continuity through the disconnect, when connected.

MG startup push button momentary switch- repaired a few years back. just need cleanup and reassembley. works fine. PB exciter "runs", and also the Generator belted to it "runs". Need to check Generator variable resistor.

PB exciter- worn shaft repaired, bearings, re-brushed and dressed commutator and brushes. Brushes checked out last night. no visible damage. replaced 2 years ago. NO SPARKS AT BRUSHES.

Generator- brushes seem good. physical connections seem good, brushes not stuck. brush tensioners seem fine. NO SPARKS AT BRUSHES.

All wire screw terminals tightened and checked. no change.

Main contactor at back of headstock. works perfect. does what it should.

Rotary switch- good, checked it last night, no obvious physical issues. Clean action.

main electrical box- very old, but seemingly ok, all parts/relays/etc seem to be functional. note: I had to rewind the AP Relay (pain in the ass .003" wire) and its been functional for 5 years. Resistors- from left to right- 750 Ohms, 500 Ohms and (Need to check the farthest right variable resistor)

The cessation of the lathes operation was instant. Machine refused to produce DC on a Cold start after several weeks. no odd noises or pops, just lack of DC.

Main DC motor. not re-brushed (yet) but looks fine.

DC Contactors crusty, but they have been functional for many years. They are not moving at all and certainly are not arcing upon contact connection or break. When I toggle contactor manually, I have no movement or arcing.

I strongly suspect that my MG unit has stopped outputting DC power.

Anyone have any thoughts? Thanks so much for your time. I appreciate all of you.
I should add the following-

I probably have not actually used the machine in 5-6 months.

So, I am thinking according to my reading that the best things to check now are for exciter voltage and check the resistance of the variable resistor located in the end bell (pulley side) of the Generator.

I am also thinking that the Exciter needs to be flashed.
Capacitors are one thing that can go wrong, just sitting without power. I don't know enough about the MG to know if that might be your problem.
The belt-driven unit on top of the motor/generator (MG) is the DC exciter, not the generator. The generator is in the long tube on the bottom, AC motor on one end, DC generator on the other.

Check the exciter DC voltage, terminals E1 and E2 on the terminal panel on the side of the MG. Should see around 115 VDC. If there's no exciter voltage, nothing works. If you have no exciter voltage, see this link:
The belt-driven unit on top of the motor/generator (MG) is the DC exciter, not the generator. The generator is in the long tube on the bottom, AC motor on one end, DC generator on the other.
Thank you. most helpful. I am a bit confused though- are you saying that under the Exciter, that the "long tube" assembly contains not only a generator but also an AC motor sharing the same shaft? ok, i might be getting it now. the Exciter does not provide roptary motion for the generator below. an internal AC motor actually spins the generator. Am I getting it? Mucho thanks for your input.
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Yes, the 3-phase AC motor is in one end of the long tube and the generator is in the other, sharing a common shaft. The belt on the generator end transmits power to the exciter on top.

Older 10EEs had everything on a common shaft*. They were called "inline exciter" machines.

* The inline exciter wasn't actually on the same shaft, instead it plugged onto a taper on the end of the generator shaft.