Shure U1 Service Manual page 22

Shure U1/U1L Body-Pack UHF Transmitter
Low Rf Output Power
Excessive Current Drain
Deviation Problems
Bench Checks
nCheck the VCO power output. If the power output is still low, the
VCO is faulty.
nVerify power across C261. Isolate the rest of the circuit by
removing L209.
nCheck R254, R255, R256, R257, C261, L209, and C248.
nVerify the dc bias of Q208. Verify that the rf mute signal changes
from +5 V at power on/off to 0 V during normal operation. If Q209
is not saturated during normal operation, rf power at the antenna
will be attenuated by approximately 45 dB.
nIsolate components, starting from the antenna and working back
to the rf power amplifier.
nCheck continuity from U207, pin 1, to the center conductor of J3.
nRemove microwave isolator U207 and check the low-pass filter
power output at LP201, pin 4. It should be approximately 10 dBm.
nRemove the low-pass filter and check the power output of the
amplifier at LP206, pin 1.
nCheck the output matching network: Q208 C258, L206, and
C256.
nCheck the bypass capacitors: C254, C253, C252, and C5.
nReplace Q208.
nReinstall L209 and U207.
nCheck continuity between pins 1 and 4 of LP201.
nIsolate different sections of the circuit (rf, audio, digital,
dc to dc converter).
nApply +3 V to the battery terminals and measure the current
drain, using a dc milliammeter. It should be less than 110 mA.
nIf R243 cannot be adjusted to obtain 45 kHz deviation, isolate the
problem to either the audio or rf section.
nCheck R243.
nCheck the rf section by setting the transmitter frequency to
800 MHz and verifying that the tuning voltage of the VCO is
approximately 3 Vdc.
20
25C1021 (BG)