Shure U2 Service Manual page 21

Shure U2 Hand-Held UHF Transmitter
Low Rf Output Power
Excessive Current Drain
Deviation Problems
Bench Checks
n Check the polarity of the microwave isolator (rf IN is marked
with a dot).
n Check the polarity of the low-pass filter (rf IN is marked
with a dash).
n Check the polarity of Q203.
n Check for missing ground connections.
n Verify VCO output power.
n Check dc level at VCO Vcc pin.
n Isolate the VCO from the rest of the circuit by removing R255,
R256 and C207. The carrier frequency will no longer be locked,
but the output power from the VCO should be approximately
2 dBm into a 50Ω load.
n Check the VCO power output. If it is still low, the VCO is faulty.
n Verify power across C270. Isolate the rest of the circuit by
removing L202.
n Check the values of R255, R256, R257, R258, and C217.
n Verify dc bias of Q203. Verify rf Mute signal changes from +5V
at power up/down to 0V during normal operation. If Q209 is not
saturated during normal operation, the rf power at the antenna
port will be attenuated by approximately 45 dB.
n Isolate components, starting from the antenna port and working
back to the rf power amplifier.
n Check continuity from U207, pin 4, to the center conductor of
J201.
n Remove microwave isolator U207 and check the power output
of the low-pass filter at LP201, pin 4.
n Remove the low-pass filter and check the power output of the
amplifier at LP201, pin 1.
n Check the values of input and output matching networks Q203,
C270, L202, C217, L207, C225, and L210. Look for open coils.
n Check the values of bypass capacitors C259, C283, C242,
C218, and C276.
n Replace Q203.
n Isolate different sections of the circuit (rf, Audio, Digital, dc/dc
Converter). Look for capacitors with reversed polarity and im-
proper resistor values.
n If R227 cannot be adjusted to obtain 45 kHz deviation, isolate the
problem in the Audio or rf section.
n Check the value of R227.
20
25B1022 (AG)