Reliance DC motor controller component info?

Have a failed Reliance "Jr. Pak-1000" 0.3KW DC motor drive. Note that this is
not Reliance USA, as in Rockwell, etc., but Reliance, Japan, which used to be
part of Reliance Int'l, but no more.
Symptom is that dc motor is not running smoothly at low speed setting (motor
"jerks"), and doesn't reach top speed. Swapping motors and controllers
indicates this controller is at fault.
It consists of 3 basic components:
- a PBF206 thyristor "H-bridge"
- a 25cm^^3 cube-shaped module
These two modules are in series with each other and with the load and the
230vac mains.
Controlling these modules is a PCB which accepts a small (0-8vdc) speed
control voltage and a speed-proportional feedback tachometer voltage
(0-100vdc), and outputs control signals (via small conductors/molex plug) to
both modules.
The "mystery cube" is marked with simply "MC-0005G". No source logo or icon.
I can't find any reference to such a part. Any help would be greatly
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You need to look at the field regulator, from Zero speed to base speed field should be max.
After reaching base speed field voltage should start falling off.
The tach signal of course should increase as speed increases, if there was a tach problem the motor would run ragged all the way to full speed.
I recently helped someone with an identical problem and it turned on the field regulator was allowing the voltage to change causing rough operation.
Regards Daveb
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Thus spake DaveB:
Field is fixed at ~100vdc. Output from common rect. bridge feeds fields of all 6 identical motors.
Not in this design; fixed field voltage.
Tach signal was, at first, suspicious; it looks sawtooth, while all other motors put out steady speed-relative dc voltage. I marked this up to the jerking operation of the motor, resulting in a sawtooth tach voltage.
Swapped motors and controllers, so this rules out: - motor - tachometer/generator (bolted onto motor) - wiring
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check for dried out electrolytic capacitors on the firing board.
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