What Is This transistor Circuit For?

[ms660]

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

Lawrence.

[dave cox]

Q704's base is driven from a 1/2 wave rectified 60V ac transformer tap with a potential divider and some smoothing. My first guess would be some kind of emergency stop if ac supply is lost since Q705 seems to be connected to a 'blake' (brake?) solenoid.

dc

[ms660]

Originally Posted by dave cox

Q704's base is driven from a 1/2 wave rectified 60V ac transformer tap with a potential divider and some smoothing. My first guess would be some kind of emergency stop if ac supply is lost since Q705 seems to be connected to a 'blake' (brake?) solenoid.

dc

So far as I can make out, if the AC supply was lost the brakes would be activated whether Q705 was there or not,

Lawrence.

[Marconi_MPT4]

By my estimation with power failure both the pinch roller and brake solenoids will stay engaged for as long as C810 (220uF 160V) holds the supply, maybe up to second or so.

In contrast, circuit Q704 and Q705 together with a fixed time constant of 1.5k and 47uF will disable the solenoids somewhere between 80 to 100ms (rough estimate). Probably this circuit may be of greater value when winding or rewinding where power loss to the AC motors will result in the spools continuing to spin unpredictably and potentially unwind several layers of tape before the brake activates.

Rich

[ms660]

I'm not up to speed with that.

For Q705 to cut off the solenoid(s) supply quickly from the charged C801, the common and NC contacts of the auto shut off switch S802 have to be open, when they are open Q705 might as well not be there so far as a quick solenoid(s) disconnection from C801 goes.

What have I missed?

Lawrence.

[John10b]

Interesting circuit, what is the function of D705, 706 I wonder?
Cheers
John

[ms660]

Originally Posted by John10b

Interesting circuit, what is the function of D705,706 I wonder?
Cheers
John

Back EMF limitation so far as I can make out.

EDIT: I'll try to do a simplified schematic in a bit and post it in this thread.

Lawrence.

[ms660]

Below is a simplified schematic showing the circuit in rewind mode but at a point where the common contact and the NO contact of the auto shut off switch S802 are open, the only time Q705's collector/emitter is in circuit is when those contacts are open.

Note: Earlier on I referred to the common and NC contacts being open, that was an error it should have been the common and NO contacts open.

Lawrence.

[dave cox]

I don't think that can be right, the brake solenoid is permanently activated!


dc

[ms660]

Originally Posted by dave cox

I don't think that can be right, the brake solenoid is permanently activated!


dc

It's a simplified schematic, it's correct so far as I know....shown as described in rewind mode, thus the brake solenoid is shown activated, I purposely left out the auto shut off solenoid the pinch roller solenoid and all the function switch switchery that appertains to everything otherwise it would not be a simplified schematic....

Lawrence.

[dave cox]

As far as I can see the only thing connected to Q705's collector are 2 solenoids and a diode ...

dc

[ms660]

Originally Posted by dave cox

As far as I can see the only thing connected to Q705's collector are 2 solenoids and a diode ...

dc

I can see two diodes connected to Q705's collector in the Sony schematic,
one's a Zener diode.

valves4amps has come up with the answer for Q704, Q705 etc over on GVR forum (Post#16):

https://golbornevintageradio.co.uk/f...id=7464&page=2

Lawrence.

[Trigon.]

Again simplified, this is the correct circuit:-



When the auto shutoff goes from NO to NC, the voltage on C801 is applied to the shut off solenoid.

As said earlier Q705, Q705 provide a fast emergency shutdown on loss of ac power.

Cheers

[dave cox]

yes, that looks more like it !

dc

[ms660]

Revised schemo on it's way in a bit, what I thought was a dot cross was a plain cross

Lawrence.

[ms660]

Redrawn simplified schematic showing rewind mode, function switch contacts etc included this time, hopefully panic over.

Lawrence.