Thyristor = Silicon Controlled Rectifier (SCR). Another version is a TRIAC which is two SCR's back to back and used extensively in low cost light dimmers. They are switches, either conducting or not. That Ampex early MDA was designed at a time when cost effective high power transistors were not available. TheAmpex MDA is a PWM, Pulse Width Modulator varying the duty cycle of a square wave controlling the motor torque/speed. These SCRs can probably be easily substituted with modern parts as their function is rather generic. I keep telling you to get the red, first edition, Harry Kybett 'Video Tape Recorders' book. he explains this exact circuit in detail as well as many of the other vintage VTRs you own. It's a great complement to any service manual as service manuals typica;;y don't explain the design theory. There's two in that famous auction site right now. There is also a second edition that covers later broadcast helical machines in brief detail but I recommend the first edition.
Are these still germanium or already silicon transistors? In both cases the question arises whether the current carrying capacity was tested with a blocked scanner (sticky tape)
