I cleaned out all old grease, regreased. Polished commutator. Replaced fingers on reverse unit, cleaned drum. Brushes looked ok. Can move wheels by rotating commutator with field and brushes detached. It ran for a short while then quit. No hum. At one point actually smoked a little from engine while under power from transformer. Does this seem like field unit needs to be replaced? My first attempt at servicing and repair. Had it running for short time but now, nothing. Appreciate any thoughts.
The 300 is a fairly simple engine with no smoke or choo choo. They usually are among the best running Gilbert steam engines. Here are a few items you could check.
The field resistance should measure about 1.3 ohms. The armature resistance between any two poles (commutator segments) should be about 1.5 ohms. the resistance between any commutator segment and the shaft should be “infinite.”
Make sure the ends of the brass brush tubes are fully seated in the brush bracket and not contacting the commutator. Any spacers that were on the armature shaft should have been reinstalled after cleaning.
Make sure all the fingers are making contact with the copper segments on the reverse unit drum. Many of the S gauge suppliers had replacement fingers that are too long. These can be used if the “spoon” end is carefully bent to put it in contact. The best source of correct length fingers is Jeff at the Train Tender, ttender.com.
Gilbert field failures are rare and when one fails it is usually a broken wire. The far more common failure is the armature. They can be rewound but it is not easy. Replacement tested armatures are available. The 300, depending on its year of manufacture used one of two different armatures. Early production armatures had no oil slinger. Later production armatures are slightly longer and have an oil slinger. They can be interchanged as long as the brush brackets are also interterchanged to use one made for the longer armature shaft.
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I’ll check the resistances. Thank you for all the info.
Bill
I would check for short circuits and open circuits with a multimeter if you have one. Tom’s advice is also quite solid.
-El