A friend has a partially-failed baseball pitching machine, an older Jugs Jr. Pitching Machine. AC line powered, no transformer, DC motor. The controller has one LM723, two 2N6399 SCR's, three power diodes, one large white ceramic box resistor (no value yet), and three TO-92 devices. No big electrolytics. My guess is some kind of full-wave phase control similar to a TRIAC dimmer, possible based on motor current rather than output voltage, but I have not found a schematic.

The symptom is that the motor runs at or very near to its slowest speed throughout the entire adjustment range. With the motor disconnected, the voltage at the motor terminals varies from 22 to 110 V on a DC scale DMM.

A real schematic would be great, but if anyone has a 723+SCR circuit that is close, that would be a big help.

Thanks.

ak

 

Sounds like the SCR bridge controllers made by KB drives for DC motors, many use them for T.M. motor controllers, in fact there are quite a few post here with the popular MC-60 T.M. schematic.
I have a schematics for both KB and MC-60 if needed.
Some simple ones have use the Triac 'Dimmer' followed by a bridge versions.
Max.

 

Here is the controller. Two diodes and two SCR's form a phase-control bridge, and the 3rd diode is back-EMF suppression. Between the two SCR cathodes and the two big diode anodes, the big white resistor is in series with the motor.

Basic motor specs are 90 Vdc and 3 A max.

If you have a schematic that you think is close (at least in concept), please post it.

Thanks.

ak

 

That is what the common SCR bridge consists of in these drives, the bridge = 2 diodes, 2 SCR's and the reverse biased one in the bridge.
Max.

 

I traced out that part and updated post #3. Now, about that gate drive...

 

The schematics I have are not really that close to the apparent configuration of the one shown.
There doesn't appear to be that many components involved, it may be worth doing a little reverse engineering to get the commutation method used.
Is there any maker's ID on it anywhere?
Max.

 

Commutation is brushes in the motor. Part of the pc board circuit is almost identical to the upper half of this schematic:

http://www.next.gr/automations/motor-control-circuits/High-torque-motor-speed-control-l12852.html

but instead of the UJT there is an LM723 and three TO-92 devices.

ak

 

I meant the commutation of the SCR's
That link shows a series motor, is this one a wound field or a P.M. motor?
Max.