HP67 motor in HP65 card reader - too slow

[Harald]

I am trying to fix a very worn out HP65 that I accuried a while ago. Etienne verry kindly sent me a replacement for my broken card reader motor (thanks a lot for that Etienne!). I have now installed the new motor (I had to drill new holes in the plastic, as the mounting holes on the 67 motor are closer togather than on the 65 motor).
Everything works fine now, appart from the card beeing pulled through too slowly. Are the two motors different electrically? I believe to have seen a pot to adjust the motor speed when I repaired my HP67 card reader, but the 65 doesn't seem to have one? Is there a way to adjust the motor speed on the HP65 card reader?

Cheers,
Harald

[Randy]

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Are the two motors different electrically?

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No, it's your basic permanent magnet DC motor.

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I believe to have seen a pot to adjust the motor speed when I repaired my HP67 card reader, but the 65 doesn't seem to have one?

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Yes, late model 67's had the pot, early 67's and the 97 did not.

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Is there a way to adjust the motor speed on the HP65 card reader?

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Yes, you change the value of the resistor between pins 2 and 3 of the sense amplifier IC. The 97 manual shows values between 2.87k to 9.09k as the range, all are 1/4 watt, 1% tolerance.

The procedure is documented in the 97 service manual. Speed is set such that the tracks written to the card meet a min/max track length shown by dipping the card in "Magnasee", colloidal iron suspended in a solvent.

Edited: 20 May 2012, 7:40 a.m.

[Harald]

Thanks, I will try changing that resistor and see what happens.

I know the motor is a permanent magnet motor. It might still have different windings though. There must be some reason why it is slower than the original motor.

Edit: Thanks for the info. That did help.

Edited: 20 May 2012, 12:12 p.m.

[Randy]

FWIW, I have disassembled both types of motors and the only difference I noted was the ability of the later motors to have the brush neutral position adjusted as part of the manufacturing process and then having that point locked into place. This lowered the overall operating current, sometimes as much as 20 ma.

The original HP65 motors with the red, molded in place brush holder were not adjustable, they were just molded into a fixed, coarse adjustment position. As a result, the operating current had a much higher variation from motor to motor. This no doubt let to differences in speed as the motor was used at much lower voltage levels than it was designed for, most likely in the 10 to 15 volt range. The card readers ran them at an average of 3 volts. These variations, along with the variation in capacitor values in the sense amp die, necessitated the need for the "trimming" resistor to properly set the motor speed for the application.

Edited: 20 May 2012, 7:46 p.m.

[Dave Shaffer (Arizona)]

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2.87k to 9.09k as the range, all are 1/4 watt, 1% tolerance.

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If that's the range, hardly seems to me that you need 1% tolerance!?