82104A card reader magnetic head


I'm trying to repair a 82104a card reader. I measured the two track magnetic head impedance. I used a Fluke 77 multimeter to do that. The instrument read ~51Ohms in one track and ~75Ohms in the other. Can somebody comfirm that? is unusual a magnetic head with diferent impedance in his tracks. Any information is welcomed. Thanks, Carlos.


Did you disconnect the head from the read/write PCB before you measured its resistance? It's possible the sense amplifier chip measures slightly different on the 2 channels.

Otherwise, I can see no reason why the 2 windings of the head would be different. It's possible (although unlikely) you have shorted turns in one winding which would lower the DC resistance.


I measured with the the head disconnected. The measure is the coil resistance without any active circuit connected. If somebody has a 82104a open and can mesure the coil resistance I will confirm (or not) the difference on coils resistance. Thanks


I measured three that I have open. All 6 coils are between 46 and 48 ohms. Meter = Fluke 179 on manual 600 ohm range.

Edited: 8 Aug 2005, 9:49 p.m.


Thanks for answer. The magnetic head has only two coils, one with red and yellow terminals and the other with blue and orange terminals, the black wire is a ground shield for minimize noise. I supouse that one coil is for timing porpouse an the other for data. Please take a impedance measure in the two coils terminals above descripted. Thanks again.


3 each 82104 = 3 heads x 2 windings each = 6 coils. Just as you asked. All the same resistance since my Fluke doesn't have an impedance setting :^)

I have only seen one bad head ever, on a 97. It was open on one winding. I don't know if a 75 ohm winding would cause a problem. It's much more common for the card detect switches to be the issue as they are easily bent while handling.

Edited: 10 Aug 2005, 12:16 a.m.


Thanks Randy. I supouse my 82104A head has some kind of problem because your heads coils has impedance consistent in ~50Ohms.


It's not clock and data. A pulse on one track is a '0', a pulse on the other track is a '1'. I don't know if simulateous pulses on both tracks are used for anything like a byte marker.


If this is rigth the two coils has the same porpouse and then they must have the same impedance. Thanks Tony

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