Page 34 of the manual in the link below shows the circuit for a Sony TC-640a tape recorder, the question is what is the function of Q704 and Q705 in the system control/power supply section? I'm ok with the rest of the circuit in that section.
The circuit for the TC640 appears to be the same except that those transistors and their circuit aren't fitted.
I might be suffering from wood for the trees on this one:
http://sportsbil.com/sony/TC/TC-640A.pdf
Also posted on UKVRRR.
Lawrence.
As far as I can see they drive solenoids for the pinch roller and brake. PM802, PM801 on the diagram. They might be part of a circuit that gives a higher voltage to a solenoid to get it going which then reduces when it has pulled in.
(03-06-2019, 10:53 AM)ppppenguin Wrote: [ -> ]As far as I can see they drive solenoids for the pinch roller and brake. PM802, PM801 on the diagram. They might be part of a circuit that gives a higher voltage to a solenoid to get it going which then reduces when it has pulled in.
The AC supply to the brake and pinch roller solenoids supply rectifier is via S801, 100v to actuate and 60v to hold, it switches to 60v hold once the brake solenoid plunger is more or less home.
With the tape threaded up as normal, the collector and emitter of Q705 are shorted out due to S802, so at that stage Q705 doesn't supply or drive anything per se.
So far as I can make out, Q705 is in circuit the instant that the pole of S802 breaks from the NO contact, at that point Q705 is in series with the supply and take up reel motors in their 0v side, as soon as S802 makes NC the reservoir capacitor for the brake and pinch wheel solenoid supply is connected across the auto shut off solenoid enabling that solenoids actuation.
Lawrence.
Q704 looks like an AC voltage detector leading to Q705 pull down.
It looks like braking for the spools with an added emergency stop if there is a power cut.
(03-06-2019, 11:53 AM)Refugee Wrote: [ -> ]Q704 looks like an AC voltage detector leading to Q705 pull down.
It looks like braking for the spools with an added emergency stop if there is a power cut.
How would Q705 actuate braking or an emergency stop due to power failure?
Lawrence.
Q704 has a diode and resistor connected to the rectifier. Once the AC stops it switches to add braking.
The remaining switches have diode gated connections to Q705 to add normal service braking.
(03-06-2019, 12:30 PM)Refugee Wrote: [ -> ]Q704 has a diode and resistor connected to the rectifier. Once the AC stops it switches to add braking.
The remaining switches have diode gated connections to Q705 to add normal service braking.
But how? Talk me through the circuit and process.
Lawrence.
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.
In play, ff or rew mode Q705's collector/emitter is shorted out, pull the AC plug and it's still shorted out.
I keep looking at the circuit, it's bugging me but I now have another take on it but my knowledge on motors is a bit lacking.
Is this feasible.... If a transistor is connected in series with an AC motor and the transistor is turned on and can cope with the reverse CE voltage would the motor try to rotate? If so my new theory might fit but if not then my new theory is out the window.
Lawrence.
Could it be something to do with reducing the take up/supply motors power when selecting the ff or rew function after a tape loop has been created due to unequal braking times when in ff or rew due to different spool weights? ie: to reduce the effect of tape "snapping" due to a loop, the only other way would be to select play first or manually rotate the reel to take up the loop.
Lawrence.