Ecler PAM1400, Service Manual

Page: 15 / 183

12
DC OUT CIRCUIT
The circuit shown in the figure corresponds to the DC OUT of channel L. The goal of this circuit is protecting
the loudspeakers when, because of a module fail, there is some DC appearing at the output. The voltages indicated in
the figure correspond to rest state and they are given by the dividers made of R320-R322 and R332-R323.
The resistances R323-R322 are linked by their extreme to the leg 7(Q) of the monostable IC310 (4538), which
has +10V at rest state. On the other hand R320-R321 are linked by their extreme to the L output, which, in these
conditions, has 0V respect to ground. If we apply Ohm's Law to these dividers we will obtain the above mentioned
voltages.
Let's remember briefly the function of a NOR gate like the HEF4001B.
A B C
0 0 1
0 1 0
1 0 0
1 1 0
Let's suppose there is a continuous voltage appearing at the module output, because of any malfunction.
This makes the voltage dividers lose balance, no matter if the above mentioned voltage is positive or negative,
the gate goes to 0V, the base Q302 loses the current stream and, as a consequence, the relay K301 opens. The aim
of the zeners D309 and D310 is protecting the gates, avoiding the voltage in them to be higher than 8.2V when the
voltage is positive and lower than -0.6V when it is negative; as you can see, the zener plays the role of a diode.