Sony XM-1502SX Training Manual page 13

Prerequisites:
•
The output transistors need not be connected. It is best to not have them connected unless you know they
are all good. If one were leaky, it will cause an imbalance in the entire stage so you could not tell if the
imbalance was in the outputs or input. Remove them to properly test the early differential amp and drivers.
•
No load should be connected to the amplifier.
•
The protection circuit may be disabled to allow the power supply to operate for troubleshooting. The input
current must be limited to 1 amp if the protection is disabled. See Protection Circuits for the location
of D803 / D807 to disable.
Procedure:
1. With the output transistors removed from that channel, power up the amplifier. The normal speaker voltage
(at the junction of R139 and R140) will be 0Vdc + 0.1Vdc if the remaining circuitry is OK. If the normal 0Vdc
is now present, one or more of the removed output transistors is defective.
2. If the amplifier still shuts down (if you did not remove D803 / D807) or 0Vdc is not present at the junction of
R139 and R140, there is a problem with the differential amplifier or driver transistors. Disable the protection
circuit (see protection circuit) to allow the amplifier to power up and continue.
3. Disconnect the feedback resistor (R143 in Figure 1-8). The speaker output voltage will be normally unbalanced
to about +15Vdc with a +50Vdc supply voltage (even on a working amplifier) without this resistor but your
unbalanced voltage may be as high as +50Vdc or –50Vdc because of the defect.
Information – In the next steps you will first check balance and then check bias voltage. With the output transistors
removed and + power applied to the output stage, all the remaining transistors should be on the verge of conduction
(0.6Vdc at each base – emitter junction) and the differential amp reference should be 0Vdc.
4. Check the differential amp reference voltage first. Measure the voltage at B1 with respect to audio ground.
It should be 0Vdc if R133, R134, Q103, Q104 and C123 are OK. These parts form a voltage divider between
+50V and –50V yielding exactly 0V at the middle of the divider (B1).
5. Measure the base – emitter bias voltage of each transistor and note, which ones are below 0.59Vdc.
6. When you find a transistor below bias, look at the simplified schematic (Figure 1-8, left inset) and find the
current path of the transistors that are not biased.
7. Test the parts in the path.
For example, if both Q104 and Q106 base – emitter voltages were below 0.59Vdc, the path of components to
test would be Q104 (part 2), D103, Q106 and R138.
8. Reconnect feedback the resistor (R143).
9. After repairs to the differential amp / driver transistors, preset the bias control* for +0.55Vdc or +1Vdc (see
below) before installing the output devices.
•
Bipolar transistors = 0.55Vdc
•
MOSFET (Q109, Q108) = +1Vdc at the gate of the output
* If the bias control has not been damaged when the output devices failed, the voltage will change gradually
as the control is turned. If the control is unstable, the output devices will fail prematurely.
10. Install the output transistors and power up the amplifier.
11. Adjust the bias control as recommended in the service manual for that model.
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1. Car Power Amplifier