Power supply and protection circuit (b)

Power supply and protection circuit.
    VT301 and VT302 form a midpoint DC offset protection circuit with a sensitivity of 10 seconds at ±3V and 4
seconds at ±6V. The left and right channels are connected to ‰l and R302 and ‰l and R3.3 form a partial pressure low-pass filter network. During normal signal output
, this filter can filter out most of the normal output AC signal voltage, and the output DC voltage is lower than ±o,5V.
When the midpoint voltage offset exceeds the fixed value, the main part of the low-pass filter output voltage is the offset voltage value, which will
greatly exceed ±o,5Vo. At this time, the relay will be released and the speaker circuit will be cut off. The action process is: when the voltage exceeds
+0.5v, it triggers VT302 to turn on; when the voltage exceeds -0.5v, it triggers VT3;01 to turn on. Regardless of whether VT:}oz turns on or
VT301 turns on, VT3Q5 can be turned on. The base voltage drops and the emitter current drops, which causes the internal resistance of VT307 to increase and the collector current
to drop. After reaching a certain level, the relay is released.
    VT303, P301 and VD303 form an overheating protection circuit. ‰l is a thermistor with a positive temperature coefficient, which is
installed close to the radiator. The thermistor starting temperature is adjusted to 80℃~100℃. When P301 reaches this temperature value, the resistance
increases sharply and the voltage drop increases. When >1,1V, VT3Q3 is turned on, and the relay is disconnected through VT30s and VT307.
    VT116, R14l, R142, Rl", etc. form an overload and short-circuit protection circuit. The emitter resistor
Ri of the output tube VT113 is used as a detection element. The starting control current is set to 12Ao. Once overloaded or short-circuited, VT116 is turned on and passes through VD30i Turn
on VT306, which then triggers island 1 to turn on, causing the current of VT307 to decrease and the relay to release.
    Protect the display system: when the protection circuit acts, the display circuit will work. The multivibrator composed of VTvjs and VT406
starts to oscillate. Once The protection is released, "VI" is turned on, and rD401 is turned on, and the oscillation and display stop.