JBL LSR28P, Technical & Service Manual

Page: 34 / 75

34
TROUBLESHOOTING MAIN AMPLIFIER
Setup: The quickest and most efficient way to operate the LSR28P Main amplifier board is to
use a known, good LSR28P input PCB. Connect the boards together using an 8-pin
connecting cable assembly. Attach the secondary of the main power transformer (JBL#
562-00032-00) to J6 on the main amplifier and the primary to P2 on the input PCB.
Connect the low frequency output to an 8-ohm load and the high frequency output to a
4-ohm load. Verify the signal generator is turned off and connect the output of the
generator using an XLR connector to the input jack of the LSR28P, located on the input
board assembly. A representative bench setup is shown on page 11.
Note: all voltages used within this manual are measured from chassis ground to the
discussed test point if not specified.
Powerup: Apply ac line power to the power transformer through a Variac so as to maintain control
over this voltage if abnormal problems exist. Make sure that the transformer is
configured for the proper voltage applied via SW2. Monitor the current, +15V supply,
and the input voltage.
Note: If excessive current is drawn while the input voltage is in the range of 20V, power
down the unit and proceed to the "Excessive line current test."
If the current drawn stays within 1 to 2 amps and the +15 volt supply stays within
regulation, verify other voltages. Main rectifier voltage should be about +/- 36v (D13)
at TP1 and TP6, respectively. The bootstrap voltage, at TP5, should be about -70v and,
TP2, about +70v. If these voltages are non-existent or abnormal, proceed to "Main
Voltage Tests."
If the voltage checks okay, verify that the bias diodes, D5 and D6, are illuminating.
This indicates the bias circuit is working. Next, verify if the mute circuit is working by
measuring the voltage at pin 6 of U1 and cathode of D19 is around -15 V, and then
remove power to transformer. The mute line, pin 6 of U1 should toggle and go high to
+15v immediately (200ms). Then, re-power the board and verify that the initial voltage
is high and then toggles to a low in about 2 seconds. If abnormal, proceed to "Amplifier
muting tests."
Low Frequency Amplifier
Measure across emitters of Q7 and Q8 (TP3), for 4mVDC +/- 300uV (20mA). If required adjust
bias pot, R16. Measure at low frequency amp output (+LF test point TP4) and confirm less than
+/- 50mV of offset. Turn on generator A and adjust frequency to 900Hz. Adjust oscillator
amplitude to obtain 50V P/P at amp output. The waveform should be a nice sine wave, with
perhaps, a small hint of clipping.