Bill Hendry

Greetings! I am trying to fix a drill speed control from the 70's that employs an ST4 device presumably as a trigger for the SCR. I've replaced it with an ST2 diac, but there's no speed control (drill runs at constant speed irrespective of pot setting). I get a better result - although nothing like full range - when I short out the ST2. Reply appreciated......

Maybe an open circuit resistor. however we can't help much without a circuit for your drill.

-Ben