I am new to forums and train repair. I am seeking information on the Marx 666, specifically the correct Ohm readings on the field coil, armature and E-Unit, so I can diagnose and attempt repair of the motor.
Last year I replaced a broken reduction gear, but when the holidays rolled around we found that the engine was drawing too much current from the transformer and won’t run. A 12v battery jumpered directly to the armature and field coils makes the motor spin, but the field windings get very hot very fast. I suspect I may need to rewind some of the motor’s coils.
The fact that you got it to run on DC suggests shorted turns. When you “jumpered directly to the armature and field coils”, did you have the windings in series or parallel? If in parallel, that alone could explain the heating.
The best test for the armature is to connect an AC (not DC) source, through some ballast element like an incandescent lamp (like a number 53), to two of the three commutator segments, in all three possible combinations while observing the armature voltage. If it is okay, each combination will show the same voltage. If the voltages differ significantly, that suggests a shorted turn on one of the armature windings.
Thanks! The field coil and armature are indeed wired in series, with the E-unit reversing the direction of the field coil for each on/off cycle. I had assumed they were in parallel and I jumpered it that way when testing with a car battery.
Testing of the E-unit shows that sometimes its switch mechanism does not complete the circuit and energize the motor, but when it does (90% of the time) the motor runs fine. The armature coils measure about 1.2 Ohms in each of the three positions, using a digital multimeter, so I think it is OK and survived the 12VDC across it, as did the field coil (both appear visibly overheated though).
I think that the E-unit was the problem all along, and what appeared to be humming and a high current draw due to a short, as described to me last Christmas, was really just the sound of the E-unit buzzing from the transformer’s AC current. (The throw rod rattles quite a bit when I energize the solenoid with an AC charger, which I presume is normal.)
At this point (so close to Christmas) I can live with an E-unit that fails 10% of the times it is engaged, so I reassembled the whole engine and will take it home this Thanksgiving to see if it will run for real (i.e. with an actual train transformer and pulling some cars). I’m keeping my fingers crossed.
After Newyears I may decide to address the faulty E-Unit if that turns out to be the only problem. I’ll have 12 months then to get things right, but for now the priority is to get this family heirloom running again for the holidays for all to enjoy.
Thank you again for pointing me in the right direction, there is no telling how much more damage I could have done to it if it hadn’t been for your help.
For parts and all questions Marx, Robert Grossman in Ohio is a great resource. Bob Nelson knows his electronics, but for any parts, Grossman is a great resource. I have a handcar that I had some questions on…he just sent me a PDF file with all the diagrams and parts lists. Very good to deal with.His phone/fax (440)209-0535. His web site: www.trainpartsformarx.com I have no affiliation with him, he has just treated me right. He also advertises in the back of CTT magazine.