Protection Circuit - Oscillator Stop 3 (Current) - Sony XM-1502SX Training Manual

1. Car Power Amplifier
Operation
Refer to Figure 1-11. The offset protection circuit consists of: an imbalance circuit (Q803-804), a latch circuit
(Q810-811), the LED indicator (D810) and the oscillator stop circuit inside IC901 (via pin 1). Both left and right
audio output channels are sampled at the speaker output via R153 and R253. This audio voltage should be 0V
+0.2Vdc with no audio input. That should also be the average dc voltage of the audio in normal operation as well,
proving there is no thermal leakage in one of the output transistors or MOSFETs.
C803 averages out the audio that may be present on the speaker lines and the resultant DC is applied to Q803
and Q804/base. If the average voltage rises to +0.6Vdc, one of the two transistors is turned ON. The one that
is turned ON momentarily grounds Q811/base and activates the latch. The latch inverts this LOW input, producing
a constant HIGH output Q811/collector. The HIGH output is used to light the Offset LED (D810) and stop the
power supply oscillator (IC901/pin 1) via D803.
Testing
The offset light means that either noise spikes are being input to the audio input then passed out, or there is a DC
imbalance in one of the output channels (leaky output transistor).
Shutdown after power ON - Since the positive or negative offset cause is not stored, your best clue to the
imbalanced channel is the DC offset voltage to the speaker before the protection circuit activates and removes
power (latches Off).
Once you have identified the defective channel, remove all the output transistors from that channel and re-
measure the speaker voltage (without speakers connected). If the voltage is now 0Vdc, the differential amplifier
part of the output stage is balanced and working and one of the output transistors you removed was defective
(see Audio Processing – Output Troubleshooting).
Shutdown when hot - Note the speaker DC voltage cold, then hot. If either channel is above 0.2Vdc, or increases
considerably when hot, that channel is suspect. The trip point is +0.6Vdc at the speaker terminals.
Protection Circuit – Oscillator Stop 3 (Current)
Concept
Six protection transistors monitor the current drawn by six pairs of + output transistors. If the average current to
the speaker from these output transistor pairs exceeds the threshold for that model, latch –2 (Figure 1-12) will
turn OFF the power supply oscillator (IC901).
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