JBL LSR28P, Техническое и сервисное руководство

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The amplifier will begin to clip at around 55 V P/P. If the amp prematurely clips, verify
that the input signal is not clipped. This can be measured at the junction of R48 and C23.
Upon reaching the 50 V P/P clipping plateau at the amp output, there should be around
3.2V P/P at the input to the amp. If one or more of the above tests fails, proceed to “Low
Frequency Power Amplifier Test.”
Adjust oscillator for 3.2V P/P at pin 1 of U1. Verify voltage at input to the filter, pin 1 of
J3, is about 2.4V P/P. If not proceed to “Low Frequency Filter Test.”
High Frequency Amplifier
Turn off the signal generator. Measure at high frequency amplifier output (+HF test point
J5 ref. J4) and confirm less than +/- 50mV of offset. Now, turn on the signal generator and
adjust frequency to 3KHz. Adjust oscillator amplitude to obtain 45VP/P at amplifier
output. The waveform should be a nice clean sine wave, with perhaps, a small hint of
clipping. The amplifier will begin to clip around 50V P/P; the amplifier input can be
measured at the junction of R52 and C26. With 45V P/P at the amplifier output there
should be around 3.1 V P/P at the input to the amp. If one or more of the above tests fail,
proceed to “High Frequency Power Amplifier Test.”
Note: The integrated power amplifier will begin to shutdown due to thermal limitations after
only a few seconds. Do not be misled that a fault condition exists if the output is not as
expected after several seconds of testing.
Excessive Line Current Test
Verify correct voltage levels, polarity and that the following parts are not shorted:
C16, C17 (main filters) and D11, D12 (low frequency power amp), D12-10, D14, C10,
C15, C6, C9 (+/- 70V) at the legs of the doubler capacitors). No section of D13 (Main
Bridge) should be shorted. Verify that the + 15V and -15V supplies are not shorted.
Return to “ Powerup" when the fault is fixed.
Measure across Q7 and Q8 emitters using DC Voltmeter. Slowly ramp up input voltage
never allowing more than 4 amps of current to be pulled from Variac. The voltage across
the emitters (TP3) should be less than 5mv. If the voltage is excessive, refer to “Low
Frequency Power Amplifier Test”.
Measure at +HF test (ref. J4) using DC voltmeter. Slowly ramp up input line voltage never
allowing more than 40W to be pulled from Variac. The voltage at the output pin, pin 3 of
U2, should be less than +/- 50mV. Monitor the temperature of U2, with your finger. The
temperature should be warm slightly but not excessively. The temperature should not rise
more than 10 degrees C after a few minutes of ‘idle” operation. If any of the above
voltages are incorrect refer to “High Frequency Power Amplifier Test.”