Protection Circuit; Muting Operation; Relay Driver Circuit; Block Diagram Of Protection Circuit - Pioneer SA-8500II Service Manual

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5 . 4 P R O T E C T I O N C I R C U I T
This circuit protects the power transistors in case
of overload, the speakers in case of power amplifier
malfunction, and also performs a muting function
when the power supply is tumed ON or OFF. The
protection circuit is composed of three sections
( F i s . 7 ) .
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Fig. 7. Block diagram of protection circuit
1 . R e l a y D r i v e r C i r c u i t ( F i g . 8 l
The relay which connects the output circuits is
driven by this circuit. It also performs a muting
function to prevent unpleasant noise during ON-
OFF operation of the power supply as well as
opening the output circuit on command from the
detector circuits.
0 4
Fig. 6. Schematic diagram of power amplifier
Muting Operation
When the power supply is tumed .ON, Qu base is
reverse biased, through D6 and R26, turning Qo
OFF. Q? base potential rises as Ca charges through
R22 & Rz3, and Q? turns ON several seconds later.
The collector current of Q7 then flows through the
relay coil, operating the relay to tum on the power
amplifrer output circuit. The reverse bias of eu base
from Du & R2o disappears when the power supply
is set from ON to OFF. Qu remains ON however,
due to the residual power supply voltage. Ca very
rapidly discharges, Q, base potential drops and
Q? tums OFF. The relay releases and the power
amplifier output circuit turns OFF.
Note:
Q" is normally
OFF due to base bias and does not
participate in the muting operction.
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F i g . 8 . S c h e m a t i c d i a g r a m o f r e l a y d r i v e r c i r c u i t