QSC1300 power amplifier principle and maintenance

The QSC1300 power amplifier has two playback channels, labeled CH1 and CH2 respectively. Each channel uses a separate circuit
board. The CH1 circuit board is fixed on the upper part of the chassis and is called the sub-board. The CH2 circuit board is fixed on the lower part of the chassis and is called the sub-board.
The power transformer, power switch, fan motor, playback mode, and selector switch on the main board are all connected to the main board
    . The working principle of the main plant is shown in Figure 6-36. The working principle diagram of the secondary plant is the same. The output stage is 8 pairs of amplifiers. The power tubes are connected in parallel
, and the emitter of each tube is connected with a current-sharing resistor (o. 47n/3W). The output tube model is MJ21193, MJ21194, and the parameters are
250V/16 A and z50w. Using this tube type makes the output tone cool and warm. Moderate, with an output of 650W.
    The front and pre-failure stages of this machine are conventional circuits, and their power supply is taken from ±80v, which is obtained by using resistors to reduce the voltage and diodes to stabilize the voltage.
    The power-on delay, speaker protection, and output tube overheating protection circuits are designed into one unit. When the DC
potential of the output terminal OUT (0 point in the figure) is different from the ground terminal potential, the potential of point A rises and VT19VT21 is cut off. , the relay Kl is released, cuts off the speaker channel, and short-circuit the speaker to realize the protection port of the speaker. When the VTZO is in normal operation, the potential of point A is sufficient to conduct VT19 and VT21. Relay Kl absorbs the platform, and the output is connected to the speaker. When the output stage overheats at the phase interface , the resistance of the thermal element RT fixed on the heat sink becomes low, causing the potential of point A to rise and the relay to release, protecting the speaker and output stage.