Posted here are three servo amplifier circuits popular in the 60's. Microtrol of England used the top circuit using low parts count among the circuits. It was for a power system with a center tap.(4 wire connectors). Heathkit (U.S.) also used a centr tap in the battery but is unique in using the AM radio tuner capacitor as the feedback adjuster to adjust the pulse width. The third circuit of O.S.(Japan) used a center tap as well but is the circuit that used the tap only in the final stage. As such, this circuit can be converted to a 3 wire system without a center tap in the battery system. The final stage can be converted into an H bridge circuit to run a motor rated at the supply voltage.

     They all employ the same principle of producing a local pulse of the opposite polarity to the incoming pulse. Also, the local pulse is initiated when an incoming pulse appears. Two transistors wired as flip flop produces the local pulse. A POT or the tuning CAP in Heathkit's circuit controls the pulse width. When an incoming signal appears, the local pulse is initiated. since they are of opposite polarities, they cancel one another if the widths are the same. It is the differential pulse that is detected by either the PNP or the NPN transistor, depending on the polarity of the net pulse, which sends signals to the driver and final transistors to run the motor CW or CCW. The motor suns a set of gears to produce torque. The final shaft is connected to the feedback POT/CAP to adjust the local pulse to match the incoming pulse at which point no differential pulse is produced.

     The author successfully duplicated all these circuits and the O.S. circuit modified to run an H bridge, eliminating the center tap.


     Tke note that any of these circuits can be used as a switch to turn on a relay (horns, lights, water pump etc.) Instead of running a motor, a relay is substituted.