JBL LSR28P, Technical & Service Manual

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B) Power Supply Test
Verify that +15V and -15V supplies are at the correct levels.
C) Power Switching Test
Use an ohmmeter or continuity tester to ensure that both the power switch SW3 and line
voltage selector switch SW2 operate correctly by metering between IEC connector S1
active pin and connector P2 pins 2 and 4 and between IEC connector S1 neutral pin and
connector P2 pins 1 and 3 with the voltage selector SW2 at 115V. Check continuity
between connector P2 pins 2 and 3 with the voltage selector SW2 at 230V.
D) Quick Signal Tests
Note: In the majority of cases faults that occur which are not merely failures of
electromechanical components (see above) will be faults with semiconductor components. In
this case that means FET Q1 or I.C. op. amps. U1-8. Loss of output or high noise or
distortion can usually be easily isolated to a particular integrated circuit:
•
For the low frequency path apply a sine wave signal at 440 Hz and check signal integrity at
U1 pin 1, U1 pin 7, U5 pin 7, U6 pin 7 and U7 pin 1. The first test point in this sequence
where signal degrades or disappears locates the faulty IC.
•
For the high frequency path apply a sine wave signal at 4,400 Hz and check signal integrity
at U2 pin 7, U2 pin 1 and U4 pin 1. The first test point in this sequence where signal
degrades or disappears locates the faulty IC.
•
In a case where no output is present for either signal path check signal integrity at U3 pin 1
and U3 pin 7. If signal is absent, noisy or distorted at either of these test points change U3.
The following section discusses frequency response details enabling in depth troubleshooting of
out of specification system performance.