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tial) check for: incorrect values of D1, D2, R15 or R16; for proper polarity on C10 and C15;
for excessive current being pulled (do U3 and U4 run hot?...etc.) and for base emitter
shorts on Q1 (2SB1186A) or Q2 (2SD1763A).
+/– 31V Supply Test
Look for open traces between main bridge, BR1, and main filter Capacitors, C41, C42 and
power input connector, J7. Verify that BR1 is not open. Return to "Main Test Loop" after
fault found.
G) Amplifier Muting Test
Power up board and measure volatage at junction of D8, R43, and C40. The voltage should
be about –28VDC. If voltage is positive check polarity of D12 and check values of R43,
R40, C40 and R42. Also check for any shorts around Q5 and D7.
H) High and Low Frequency Power Amp Test
Turn off Generator.
1. Amp pulls excessive power when board is powered.
If the output of the amplifier, pin 3 of U1 (LF) and of U2 (HF), is centered (ground potential)
check for short between ground and output pin, pin 3 of U1 and U2. If no shorting problem
is found replace U1 or U2 (Depending on which one failed).
If the output pin, pin 3 is offset to one of the power rails, the power consumption should go
to normal idle level (less than 40W) if the load is removed. If it does not, check for shorting
from the output pin, pin 3 to either the +31V or –31V supplies. If not shorted, replace U1 or
U2 (Depending on which one failed).
Verify that the input to the amp, pin 10 of both U2 or U1, is close to ground potential. If not
close, check for shorts surrounding pin 10. If no shorts are found replace U1 or U2 (De-
pending on which one failed).
2. No output from the amp when driven.
Verify input to the amp on pin 10, of U1 (LF), if no signal is present verify that there is
signal at pin 1 of U4A. If signal is present at U4A verify values of C13, R7, R6, and C7.
Verify input to the amp on pin 10 of U2 (HF). If no signal is present verify that there is
signal at pin 7 of U4B. If signal is present at pin 7 of U4B verify values of C16, R10, R11
and R14. Look for shorts at pin 10 of both U1 and U2. If nothing is found in above testing
replace U1 and U2 (Depending on which one failed).
The amp should be out of muting. Verify that pin 8 (Muting pin of U1 or U2) is equal to
about –3.4V. If the turn on delay is operating properly (Collector of Q6 at around –15V),
and the muting pin is substantially lower or higher than -0.5V, replace U1 or U2 (Depending
on which one failed).
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