1033 transformer question

I have a question for the 1033 experts. I soldered on a new cord on an old 1033 that was given to me. The voltage drops out as I increase the black throttle voltage. What would cause this.

THanks

You might want to send this question to one of the model trains forums or to the classic toy train forum. You can find them all at http://cs.trains.com/forums/