03-06-2019, 01:00 PM
Q705 is in the negative power supply to the solenoids and is turned on by Q704.
The base drive to Q704 comes from an independent rectifier D708 connected directly to the transformer winding via a biasing circuit that prevents the solenoids chattering. Disconnect C705 and listen to the solenoids chatter.
In the event of a power failure it drops the solenoids so preventing the tape un-spooling if the power fails.
If you shorted Q705 and pulled the power with the machine running the tape is likely to go all over the place due to the inertia in the motors and the charge in the main capacitors holding the solenoids in while the motors are running down to a halt.
It is a well engineered machine and quite a bit of thought has gone into the design.
The base drive to Q704 comes from an independent rectifier D708 connected directly to the transformer winding via a biasing circuit that prevents the solenoids chattering. Disconnect C705 and listen to the solenoids chatter.
In the event of a power failure it drops the solenoids so preventing the tape un-spooling if the power fails.
If you shorted Q705 and pulled the power with the machine running the tape is likely to go all over the place due to the inertia in the motors and the charge in the main capacitors holding the solenoids in while the motors are running down to a halt.
It is a well engineered machine and quite a bit of thought has gone into the design.
