PITTMAN/BOWSER DC-71 TO NEW MAGNET


There are many cases where older motors do not perform very well, but replacement with newer types is not too practical without major surgery and drive train alterations. To test the possibilities of improving these older motors, newer neodymium magnets were installed with improved materials that have drastically increased the field strength. Used in larger locos by Penn-Line, Varney, Mantua, Hobbytown, English, Japanese imports and others; the Pittman DC-71 was probably the most common, HO and other scales, motor during the latter 40's through to the 60's. It is stiill offered by Bowser and used in their loco kits. Having many DC-71's in stock and finally locating some possible magnet replacements, a trial was made.

First tests were run with the original. Then the magnets were exchanged , followed by tests for comparison. The motor space is 5/8" wide, a little over 5/8" long at the pole pieces and is about .652" high. Since only 5/8 x 5/8 x 1/8 magnets were found, 5 of these were stacked for 5/8" and the remaining height was shimmed with steel, cut from 22 gauge sheet.


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Original Pittman DC-71.



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DC-71 With new stacked magnets and shim.


CAUTION: Do all gear pressing before exchanging magnets, while the motor is in the original, more rigid state. Since the rear screw, which holds the frame in alignment, will not be used; pressing gears on shaft can distort pole pieces and front bell alignment. I learned the hard way and had to line things up by ear. Applying some ACC to the pole piece to bell joint beforehand can prevent bell rotation and movement.

Check the polarity of both magnets to assure the same orientation in the motor. Simply remove rear screw and magnet, which will now lose most of its field strength. This is not the case with the new magnets. Place the new ones in the opening. They will probably snap in, pulled by the magnetic force. Line them up with the sides and rear. In most cases the magnetic force will hold it in place, but for security a few drops of ACC around the edges and down the screw holes will help. Tests were run without cement with no problems. Just mount and test. To use the "Z" bracket, screw on with a 4-40 x 1/8" into the tapped hole at the bottom.

The speed of these motors varied over the years and with the Bowser versions. The one used ran light at about 13.6 k RPM, while it dropped to close to 10.5 k with the new magnet. This may be attributed to the expected increased torque. Current dropped from about .29 amp to about .17 amp. The current reduction does not appear to be as great as expected. But running at a lower speed the counter voltage is reduced and thus the actual armature voltage is higher with the same coil resistance. This counteracts against reduction. Stall torque and other measurements are compared in STALL TORQUE DATA ,

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