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