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PROTECTION CICUIT
1.
OVER CURRENT PROTECTION
–
There are 7 digital amp ICs (IC 251/351/851/261/361/451/461) for 7 channel audio
amplification. Pin 5 of these ICs outputs a LOW state when the final output over
current is detected. Pin 5 is connected to base of 7 transistors
(Q251/351/851/261/361/451/461) respectively.
–
These 7 transistors are PNP type, and are turned on when base voltage goes LOW.
–
Any one of these 7 transistors turned on (when over current occurs) would turn off
Q281, then turn on Q286 and Q282. The emitter of Q282 becomes 0 volts, and the
LOW state would be sent to pin 31 of IC501 through R290 triggering the protection.
•
2. DC (to speaker) DETECT PROTECTION
–
There are 7 resistors connected to 7 channel speaker outputs (just after low pass
filter). The other side of these resistors are tied together and connected to the base
of Q284 and Q285.
–
If a DC voltage occurs at any audio speaker output line, a (+) will turn on Q285 or a(-)
turns on Q284 and Q283.
–
A LOW occurs at collector of Q285 when Q285 or Q283 are turned on. The LOW
state is sent to pin 31 of IC501 through R289 and triggers the protect function.
•
3. AC DETECT PROTECTION
–
The AC power from winding S1 of the power transformer is rectified by D1171,
producing a negative DC voltage which is applied through R1171 and R1172 to base
of Q1174, keeping it cut off, and keeping the collector HIGH (almost +5v).
–
When the AC power (S1) disappears, +0.7v is applied to the base of Q1174, turning it
on into saturation. A LOW is sent to IC501 pin 31 through R1178 triggering the
protect function.
5/8
NOTE
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