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5. The Explanation of Protection Circuit
1. Steady function
While the B voltage is supplied to the protection circuit as the power source switch is turned on, it is so designed
as to allow a lag of almost 4 seconds in the time required for the relay to be turned ON, by means of the time-constant
circuit set to motion by R 716 and 706 so that a drop in the output may be prevented of the main circuit in the
unsteady state.
In the steady state, transistors of Q 701 to Q 705 are cut off, so much so that the voltage at both ends of C 706
is caused to register a value of 6 by the function of R 720 and D 709, causing the transistor of Q 706 and the delay
to be turned ON, bringing forth the output.
Again, D 709 is designed so as to keep both ends of the relay coil from being brought under the pressure of abnormal
voltage.
In addition, 1S 1554
between
the base collectors for respective transistors is designed to thwart inverse
current.
2. Detection of abnormal voltage
As the equivalent of 2 or more is generated at the center line of the main amplifier, there is a mixing between the
right side and the left side at R 701 and 702, with the alternate current portion being cut off at C 701 and 702.
In the case that the direct current generated here is negative, the current, passing through D 701, is broken up into
partial pressure, acting on the base of the transistor at Q 702, which, in turn, causes Q 702 to open up allowing
the current to pass through in the sequence of
D 706-— R 707 —— R 708.
As a result, the portion of voltage generated by R 708 is caused to be reinforced to Q 705, rendering the transistor
to be turned ON.
This, in turn, causes the voltage of the collector at Q 705 to fall, thus making the base potential
of Q 706 go deeper into the negative side and turning the
Q 706 OFF.
This in turn causes the relay to be turned
OFF, and also, the output circuit to be cut off.
In the event the base potential is on the positive side, the similar function takes place within the loop of Q 721
—— D 202
—R 707—— R 708.
By eliminating a cause or causes for DC generation at the center line, the relay is caused to turn ON by the function
staged in reverse order of the description given, causing the proper function to start automatically.
3. Detection of abnormal
current
When the abnormal current is generated on the driver stage (power stage) of the main amplifier, the voltage is detected
by the collector resistance and added subsequently to semi-solid resistance R 537 and 637 which are designed for
current detection in the protection circuit.
(The semi-solid resistance is set to function with the adequate flow
of current causing the protection circuit to start operating.
Refer to the Chapter dealing with Adjustment.)
By
the voltage reaching Q 704 by way of D 536 and D 636, Q 704 is caused to be turned ON while R 705 witnesses
a voltage drop, resulting in Q 703 being turned ON.
As a result, the current flows in the sequence of
Q 703-—R 711—D
707-—R 708, and by the voltage generated
at R 708, @ 705 is turned ON while Q 706 is turned OFF, causing the relay, in turn, to be turned OFF, thereby
cutting off the output circuit.
By the voltage added to the base of Q 704 through R 713, on the other hand,
Q 704 is turned ON
and on account
of this, the lock circuit is set to work by Q 704 and Q 703, causing the realy
to be kept in the OFF state.
In the event a large current flows into the transistor of the main circuit, the said circuit is caused to start functioning,
locking up the amplifier output in the OFF state.
For this reason, it is necessary for the power source to be cut
off before setting out to remove the cause for abnormal current.
Turning the power source back on will set off
the normal function.
It should be noted that turning the lock circuit on immediately after the power source was
turned OFF will not lead to a spontaneous release of the lock-up, and therefore, it must be assured that the lock
circuit is turned ON a few seconds after the power source has been turned OFF.
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