JBL LSR28P, Technical & Service Manual

Page: 45 / 75

45
High Frequency Power Amplifier Test
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HF Amplifier pulls excessive current when board is powered up
If the output of the amplifier, pin 3 of U2, is centered (around ground potential)
check for a short from ground to the output pin, pin 3 of U2. If no shorting problem
is found replace U2.
If the output pin, pin 3, is offset to one of the rails the power consumption should
go to normal idle level (Less than 20W) when the 4-ohm load is removed. If it does
not check for shorting from the output, pin 3, to either the +36 V or -36V supplies.
If not shorted replace U2.
If the output is offset, and the power consumption goes down to normal with the
load removed, check the following:
Verify that the input to the amp, pin 10, is close to the ground potential. If not
close, check for shorts surroundings pin 10. If no shorts are found replace U2.
Verify that pin 9 reflects the output-offset polarity, i.e. the voltage across R36
should be insignificant. If not look for shorts surrounding pin 9 and verify values of
R36, r53 and C34. If no problem found replace U2.
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No output from the HF amplifier when driven.
Verify input to the amp on pin 10. If no signal is present verify that there is signal
at J3 pin 6. If signal is present at pin 6 of J3 verify values of C26, R51and R52.
Look for shorts at pin 10 of U2. If nothing is found in above testing replace U2.
The amplifier should be out of muting. Verify that U3A pin 2 is at -15 volts and
that U2 pin 8 is about -3V. If the muting pin is substantially lower, replace U2. If
the voltage goes higher than around -5V, and the turn on delay is operating properly
replace U2. If the voltage at U3A pin 2 is incorrect refer to “Amplifier Mute Test”
above.
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Is the gain of the amplifier low or high?
(The gain should be equal to x14).The gain from pin 10 to pin 3 should be around
x14 (23dB).