CHECK EMITTER AND BASE RESISTORS WHILE DEVICES ARE REMOVED,
Each output transistor has an associatated pair of 0.47 ohm resistors in parallel.
Each BANK of output transistors has a 15 ohm resistor from base to rail (emitter bus).
Audio Outputs, Troubleshooting Current Limit
WEAK CURRENT LIMIT -- PREMATURE CLIPPING.
The usual symptom of weak output current is premature clipping of one or more peaks of the audio voltage.
This could be caused by missing step, weak current limit, or dead output section.
PREMATURE CLIPPING at 60% VOLTAGE, SIMILAR AT ALL IMPEDANCES:
This points to a step problem (2402, 3002 only). See Step Troubleshooting.
If the amplifier reaches full voltage at 8 ohms, but prematurely clips at 4 ohms or 2 ohms, we can assume
the step is OK but the output current is too low (see below).
NO OUTPUT AT ALL ON ONE POLARITY.
This indicates complete failure (open circuit) in the circuit leading to the dead output polarity. Check the
series components in the current splitter for missing or open:
Ch 1:
Positive,
Ql9,
R381,
Q20,
R70,
DIO,
D14. Negative:
Q22,
R382,
Q21,
R71,
Dl
I,
D12
Ch 2:
Positive,
Q64,
R383,
Q65,
Rl93,
D35,
D39. Negative:
Q67,
R384,
Q66,
Rl94,
D36,
D37
CONSTANT, PREMATURE CLIPPING, WORSE AT LOW IMPEDANCES.
First, check the clamping voltages on
C21
(Ch
I+),
C22 (Ch I-),
C56
(Ch
2+),
C57
(Ch
2-),
as shown in
table below. At idle, all four voltages should all be similar. If one is out, check parts according to the
following table
CLAMPING VOLTAGES ARE WRONG AT IDLE
CHANNEL-POLARITY
Measure voltage on:
3002 and 2402, about
6V.
1602, about
4,6V,
1202 about
4.9V.
Voltage too high: missing resistor:
or missing transistor:
Voltage
0-0.3V:
shorted transistor
or missing resistor
Voltage
0.7V,
missing resistor:
Voltage wrong: wrong value
CH
I+
CH
l-
c21
c22
R60
Q14
Ql4,24
R
51,
72
R59
R59,60
R61
Ql5
Ql5,
25
R53,
75
R62
R62,61
CH
2+
C56
RI83
Q59
Q59,69
Rl74,
195
RI82
Rl82,
183
CH
2-
c57
RI84
Q60
Q60,70
Rl76,
198
RI85
Rl85,184
The exact voltage varies with temperature. Look for the mismatching value on the weak cell.
A too-low voltage causes early clamping of that output section, as explained in the previous several pages.
If the voltage is correct and current is still low, also check for missing
-
unsoldered output device, or emitter
resistors.
15
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