Hi-Fi Output Troubleshooting Overview - Sony STR-DE845 Training Manual

Hi-Fi Output Troubleshooting Overview

Troubleshooting Audio Power Amplifiers is frequently a matter of simply changing a pair of output transistors.
Other times the protection light just will not go off. In the worst-case scenario, the newly replaced transistors are
destroyed as power is applied. In this document we will examine troubleshooting methods for the more difficult
.
problems We will break down our problems into two types:
•
Excessive Current Draw
•
Constant Protection Light Indication
We start with the following assumption: you have changed two or more output transistors. Because of the
possibility that the new transistors will be damaged, it is wise to take a cautious approach when applying power
to the unit. A "Variac" will be used to slowly increase the AC power to the unit while watching for excessive
current draw. Before power can be applied in this way, the power relay must be bypassed. Of course, you can
locate the power relay and simply add jumpers to the relay contacts. There is a quicker way to bypass the power
relay, but it requires a very dangerous tool. This tool (shown in Figure 10-1) is like an extension cord only it has
two male ends. If this cord is ever plugged in to the AC outlet first, the other end will be live and exposed. Keep
this tool safely away from anyone not familiar with electricity.
SWITCHED AC OUTLET(s)
(Outlet shape and position varies
according to destination)
By FIRST plugging this cord into the "SWITCHED" outlet on the rear panel of the receiver, then SECONDLY
plugging it into the AC outlet of a Variac, you will bypass the power relay. Now, slowly increase the AC voltage
while watching the current meter on the Variac for a sharp increase in current. If the current raises rapidly, STOP,
lower the voltage back to 0 volts and turn to the "Excessive Current Draw" section of this document. If the unit
does not draw excessive current by the time the AC voltage reaches 100 volts, turn the Variac voltage back to 0
volts. Then remove the bypass cord and connect the unit to an AC outlet in the normal manner and press the
power button.
The Power Amplifier can be broken into three sections (shown in Figure 10-2):
•
Output stage
•
Bias network
•
Driver Amplifier
10. Hi-Fi Output Troubleshooting Overview
FIGURE 10-1
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