Shure LX4 Diversity Receiver
Bench Checks2125D1008 (BK)
Bench Checks
Power Input
n Make sure that 10.0 ± 0.35 Vdc is present at TP9 (U7, pin 3).
n If the voltage is lower than normal (but not zero), check for
approximately 15 Vdc at the input of U7 (pin 1), a reversed
electrolytic capacitor (C50), or a stage drawing excessive
current.
n If the voltage is zero, check for solder bridges or shorted foil
traces (defective pcb).
n If the voltage is higher than normal, or if other tests prove
negative, replace U7.
n Make sure that 5.0 ± 0.25 Vdc is present at TP8 (U9, pin 7).
Audio
n If there is no audio at the output of the receiver, make sure
that the channel is not squelched. Squelch may be defeated by
rotating R130 (R230) counter-clockwise until the wipers are
grounded.
n There should be 10 Vdc at TPA7 (TPB7), and the diversity A (B)
LED should be on. If not, there is a problem in the squelch cir-
cuitry; see the “Muting” section.
n Check pin 8 of U100 (U200) for an audio signal of approximately
–15 dBV. If there is no detected audio, check L110 (L210) and
the components connected to pins 9–12 of U100 (U200); also
check dc voltages at U100 (U200). Replace U100 (U200) if
no other problem is found.
n If audio is present at pin 8 of U100 (U200), but not at the output
of the receiver, trace the signal through signal amplifiers U1A
(U2A) and expander U9 (05B).
n Use a 1 k series resistor at the test point when checking the
audio at TPA5 (TPB5) to avoid loading the amplifier.
n Then check volume control R36 and the components going
to audio output jacks J3, J1. If no signal is present at J1, the
balanced LO-Z microphone level output, check U5C, U5D, and
associated components. Check dc voltages on any stages that
are not working properly; then look for wrong or missing compo-
nents. Replace IC(s) only if no other problems are found.
n If the audio level cannot be set correctly with R59 (R58), make
sure the other diversity channel is squelched. Only one channel
should be activated during this adjustment.
n If the audio level is correct for the individual channels, but not
when both are activated, check U6.
n If the audio level is correct, but the unit exhibits high THD,
increment the rf input frequency 20 kHz above and below the
operating frequency.
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