Transformer smoke / hot. under voltage light on

I would really like to run my problem past someone before I possibly burn up the new transformer? Or maybe the transformer is the problem?
The MEP-805A that I just purchased is a NOS unit with 3 hours on it. It fires right up and runs great. When it first fires up the gauge goes the 60 Hz then falls back to 50 when you release the start switch and the under voltage light
comes on, if you hold the start switch on it will stay at 60 Hz (maybe that’s what hurt the transformer?) the light still stays on. Any help would be greatly appreciated.