Bridgeport table power feed

The power feed on my 70's vintage J head Bridgeport is running erratically. This is the original power feed setup that came with the machine and is a Bridgeport OEM part, with the SECO Electronic controller. The motor turns well and adjusts for various speeds in one direction, but cogs in the opposite direction. There is no visual evidence of any burnt components on the printed circuit controller and the gang connector on the main board, appears to be free of corrosion and bent pins.

I cleaned and polished the commutator, replaced the brushes and checked for defective wiring. The motor spins freely in either direction. I replaced the two micro switches located inside the directional control housing and adjusted the cam action of the directional control. In either direction, when I press the jog button, the motor takes off at full RPM, as it should, for rapid traverse. This is driving me nuts and I can't figure out why it works properly in one direction and not the other!

As I view the wiring diagram for this machine, it appears that the function of the the micro switches is to reverse the polarity of the field windings. The polarity of the armature appears to remain fixed. This is, I believe, the way it should be.

Any thoughts on this are welcome.


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This is a total WAG, worth (at most) what you paid for it, but maybe there's something wrong with rectifier for the field winding and it's getting half-wave rather than full-wave. On a small DC motor this would typically be some 1N400x diodes, easily tested (with power OFF!) using the diode test function (usually low-ohms setting) on your DVM.

Best regards, Spehro Pefhany

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Spehro Pefhany

Check to see if there is a different set of diodes used for the different directions (may be different halves of a bridge rectifier). Sounds like one of the pair for one direction is open.


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Gary Coffman

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