Quad Electroacoustics QUAD 405 - FAULT FINDING

FAULT FINDING
6
Reproduced in pdf format
by Keith Snook
d.c.~daylight ltd. 2002
The following information may assist in locating faults occuring on the amplifier boards of a 405.
In each case only the faulty channel of the 405 is driven, as in the test procedure. The input should be
a sine wave of 0.5V RMS and the output should be applied to an 8„ load unless otherwise stated.
The numbers refer to the relavent test check.
*Board type M12368 only **Board type M12565 only.
Effect Cause
1.
R33 Burnt Collector-Base Tr10 o/c
R37 Burnt L1 o/c (solder joints)
R41 Burnt L3 o/c (solder joints)
R39 Burnt R20 or R21 o/c
R38 Burnt D5 or D6 o/c
2.
High Current Tr2 o/c, Tr3 o/c, Tr7 o/c Tr9 s/c
 Tr10 s/c, R7 o/c C8 s/c
* C3 s/c
** D2 o/c R8 o/c
Draws high current which drops to 0.1A
after approx 2 seconds R14 o/c
3.
No increase in a.c. supply current
for increase in signal R3 o/c, C1 o/c, R31 o/c
Signal is unstable and clips R6 o/c
100W output for 0.3V input R20 or R21 o/c
Waveform trace as in Fig. 3 Tr8 o/c, Tr6 s/c, R36 o/c, R30 o/c, C10 s/c
Waveform trace as in Fig. 4 L2 o/c (solder joints)
Approximately 4W output R16 o/c
4.
Second Harmonic Distortion IC1, Tr1, Tr2, Tr3, Tr4, R5, R6, R17, R18, R22, C1
Second Harmonic Distortion especially at 100Ó C2, C7, C8
and on o/c load
Third Harmonic Distortion especially at 100Ó R5
Third Harmonic Distortion L2, R3, R6, R16, R20, R21, C3
Hum and Noise C5 o/c
Hum* R37 o/c
Waveform trace as in Fig. 5* Tr3 s/c
Waveform trace as in Fig. 6* R23 o/c, R5 o/c  
Waveform trace as in Fig. 7 R33 s/c
Waveform trace as in Fig. 8* R8 o/c
Waveform trace as in Fig. 9 C5 s/c, R15 o/c, Tr1 o/c
6.
Distortion at 20Ï D5 or D6 s/c,
8.
Liimiting resistor R11 has no effect R10 s/c
9.
Square Wave trace as in Fig. 10 C6 o/c
10.
Noise especially at 100Ó R5
Noise with large spikes Tr1
Noise R12, R3, R4, Tr2, IC1 (change to topology!)
11.
Current limiting check with 1„ load
Waveform trace as in Fig. 12 R29 o/c, R28 s/c, R25 o/c
Waveform trace as in Fig. 13 D3 s/c, R27 o/c, R24 o/c, R26 s/c
Waveform trace as in Fig. 14 Tr6 o/c
Waveform trace as in Fig. 8 C11 s/c Tr5 o/c
13.
Draws high current with 6V d.c. supply T2 s/c
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