I'm rebuilding a 1931 Knabe Ampico B, and have come across a concern
regarding the governor resistor (the resistor in the drawer, by the
roll-drive motor, not the large resistor under the piano).

The 1929 manual says that the resistance should be 360 ohms.  I'm
getting a reading of 200 ohms on my ohmmeter.

Does this mean that the resistor is bad, or perhaps there was a change
in design from 1929 to 1931?  If the resistor were bad wouldn't it
either show infinite resistance (open) or none (shorted)?  If it needs
to be replaced, is anything still available that would mount in the
same manner, without modification?

Thank you,

Craig Roothoff
Escondido, California

 [ Editor's comments:
 [
 [ It's the governor shunt resistor, called "governor resistance" in
 [ Ampico B literature.  A schematic diagram of the roll motor is at
 [ http://mmd.foxtail.com/Tech/Ampico/index.html and the MMD articles
 [ are indexed at http://mmd.foxtail.com/Archives/KWIC/A/ampico.html
 [ Use the "Find in page..." feature of your web browser to search
 [ for "motor" or "resistor" within the index of Ampico articles.
 [ These are some likely titles:
 [
 [   Repairing Ampico B Electric Spool Drive Motor
 [   Ampico B Spool Motor Governor Resistor
 [   Ampico B Roll Drive Motor
 [
 [ You're correct, a wire-wound resistor doesn't change value, it
 [ usually goes open circuit.  200 ohms is still a reasonable value,
 [ and it probably was installed originally.
 [
 [ I recommend keeping the 200 ohm resistor if the motor governs okay.
 [ The lower resistance (compared to 360 ohms) results in less "kick
 [ back" voltage across the points, hence the points run cooler and
 [ there's also less television interference.
 [
 [ However, the lower value resistor increases the motor torque when
 [ the points are open, which may prevent the motor from stopping when
 [ the Tempo lever is at zero.  This problem may go unnoticed until
 [ the piano is equipped with a PowerRoll or similar magnet valve
 [ device, and the roll motor must be shut off completely.
 [
 [ -- Robbie