45
12. Troubleshooting Audio Protection Circuits
Over Current Protection Circuit
FIGURE 12-3 - OVER CURRENT PROTECT
11/28/01
12.3A124
TO PROTECT
IN
L = PROTECT
Q2
Q1
D1
D3
R3
C1
D2
Q3
C2
R2
R1
B-
CH2
CH3
CH4
CH5
47K
47K
47K
47K
CH3
CH2
CH5
CH4
47K
47K
47K
47K
TO
RELAY
47K
(-)
Q200
R200
0.22
Ω
R100
0.22
Ω
Q100
R6
1.5K
Ω
R5
6.2K
Ω
Q4
CH1
POWER AMP
47K
Since current in the Output transistors also flows through the 0.22 ohm emitter resistors, the voltage-drop across
these resistors will be proportionate to the current in the Output transistors. The Over Current Protection (OCP)
circuit monitors the voltage-drop across the 0.22 ohm resistors (R100 and R200) in the Power Amplifier.
R5 and R6 are in parallel with R100 and R200. Since R100 and R200 are very low ohm values as compared to
R5 and R6, the current through R5 and R6 will be insignificant to the Power amplifier’s normal operation. As
current increases through Q100 and Q200, the voltage-drop across the resistor pairs (R100/R200 and R5/R6)
will also increase. When the voltage-drop across R6 reaches 0.6 volts, Q4 will turn “ON”. When Q4 turns “ON”,
current flowing through Q4 E/C junction, R4 (the 15K ohm resistor), D3 and R3 will “pull up” Q3’s base and Q3
will turn “ON”. When Q3 turns “ON”, its collector will go LOW, activating the protection circuit.
Troubleshooting Method
When D3 is conducting (0.6 voltage-drop), check the B/E junction of Q4 for a 0.6 voltage-drop. If you find 0.6
volts, the problem is in Channel 1’s Power Amplifier. A similar OCP transistor will be found in each Channel.
Check each Channel’s OCP transistor for 0.6 B/E voltage-drop. The one with 0.6 volts is the Channel with the
problem. If you find 0.3 volts across the B/E junction of any of the OCP transistors, that transistor has become
leaky (replace).
Over Current Protection problems are usually caused by a defect in the Bias Network circuit in the Power Amplifier.
Shorted Output transistors may also cause OCP. (It is assumed that you have checked the output transistors
with an Ohmmeter long before reaching this point.)
Summary of Contents for STR-DE845
Page 59: ...APPENDIX ...
