SCR Motor Speed Control made from salvaged parts

[Paul Jones]

Christophe,
I wish I had kept the schematic but I am pretty sure it is "phase angle control" using thyristors for controlling the phase angle or trigger angle thus allowing the output RMS voltage of the load to be varied. I think a more efficient method would use TRIACs instead of thyristors but I don't think these were available in the early 70's.
Thanks for asking, Paul