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12. Troubleshooting Audio Protection Circuits
(+/-) Offset Protection Circuit
FIGURE 12-2 - +/- OFFSET PROTECT
11/20/01
12.2A124
TO PROTECT
IN
L = PROTECT
CH1
CH2
CH3
CH4
CH5
OCP
47K
47K
47K
47K
47K
Q2
Q1
D1
D3
R3
C1
D2
Q3
C2
R2
R1
B-
Q1 and Q2 detect DC voltages at the Power Amplifier Outputs (Ch1 through Ch5). Both transistors have their
emitters grounded. +0.6 volts at the bases will turn Q1 “ON”. –0.6 volts will turn Q2 “ON”.
The +DC-offset voltage is easiest so it goes first. If there is a positive DC voltage at any Output (Ch1 through
Ch5), the base of Q1 will be “Pulled up” through one of the 47K ohm resistors. When the voltage at Q1 base
r0.6 volts, Q1 will turn “ON”. When Q1 is “ON”, its collector will go LOW (ground), activating the
Protection circuit. Positive voltage at the base of Q2 will keep it “OFF”.
A –DC-offset voltage at any of the Outputs will pull the base of Q2 LOW through one of the 47K ohm resistors.
When the voltage at the base of Q2 reaches –0.6 volts, Q2 will turn “ON”. When Q2 turns “ON”, Q3’s base will
be “pulled up to ground” through Q2 E/C, D1 and R3. Q3’s emitter is “pulled down” by the negative (B-) supply
voltage through R1. When current flows through Q3’s E/B junction, Q3 turns “ON”. When Q3 turns “ON”, its
collector will go LOW, activating the Protection circuit.
NOTE: The output transistors used in most receivers are Darlington pairs (in a single package). The voltage
measurement across the B-E junctions for a Darlington pair will be approximately 1.2V.
Troubleshooting method
Note: reference Figure 12-2
If either Q1 or Q3 turns “ON”, the Protection circuit will activate. The best place to start is to check the B/E
junctions for 0.6 voltage drop. If Q1’s B/E junction reads 0.6 volts, check for a voltage drop across each of the
47K ohm resistors. The one with a voltage drop will point to the bad channel. If Q3’s B/E junction reads 0.6
volts, check D1 and D2 for 0.6 voltage drop. If you measure 0.6 volts across D1, check Q2 B/E junction. If you
measure 0.3 volts on Q2’s B/E junction, Q2 is probably leaky (common with PNP transistors). If you measure 0.6
volts on Q2’s B/E junction, check for a voltage-drop across each of the 47K ohm resistors. Again, the one with
a voltage-drop will point to the bad channel. If you find 0.6 volts across D3, you have a problem in one of the
Over Current protect circuits, which will be discussed next.
Summary of Contents for STR-DE845
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