Hi Tony, A shorted diode in the bridge could cause this, and would rather quickly fry your new FET's. It is best if you can disconnect the bridge DC output, and see what it measures on a DC voltage scale on your multimeter. If it only shows 1/2 volt DC or so, especially with your Z-750 cranked up, the bridge is almost certainly bad.
See if GRJ chimes in with a better way to test. I have to get back into my train mode. Normally, I restore OLD electronics, and have a variety of electronic instruments to help me out. I am living in a less than 10" by 10" room, and am very challenged to work on anything, as all my belongings have to fit in the room. I just received a NOS Lionel C&O Hudson (1995 vintage) that I am converting to a DC motor and a ERR CC commander to drive it with. I have to send a fellow my Lionel Pulmore motor to have the gear? removed and put onto the new "Pittman" motor. I work on the fold down part of a small antique desk! My situation is comical!
I'm going to try to fit a loop of Fasttrack under the bed and sticking out on one side to run some new trains on.
