Shure UC2 Service Manual page 26

Shure UC2 Hand-Held UHF Transmitter
Excessive Current Drain
Deviation Problems
Bench Checks
Low Rf Output Power
Check the polarity/soldering of the output low-pass filter, U1.
(Rf IN is marked with a dot.)
Check for opens or shorts on output coil L308.
Check the polarity of Q304.
Verify the dc bias of Q302 and Q304. Check L303, L304, and
L306, if no bias. Verify that the rf MUTE signal changes from
0 Vdc at power up and power down to +5 Vdc during normal
operation. If Q303 and Q305 are not in a low impedance state
during normal operation, the rf power at the antenna port will be
attenuated by about 45 dB.
Check for missing ground connections.
Check the dc level at the VCO Vcc pin. If no dc power,
check L302.
Verify the VCO output power.
Partially isolate the VCO from the rest of the circuit by removing
C323. The carrier frequency will no longer be locked, but the
output power from the VCO should be around –1
50 Ω load.
If the power out of the VCO is still low, the problem is in the VCO.
Check the values of R322, R323, R324, R325, and C329.
Isolate components starting from the antenna and working back
to the rf power amp.
Check the values/continuity of the input and output matching net-
works for Q302, Q304 (C329, C338, L305, C339, C340, L307).
Check the values of the bypass capacitors C357, C2, C332,
C333, C334, C335, C360, C341, C342, C349, C350, C355,
C356.
As a last resort, replace Q302 or Q304.
Try isolating different sections of the circuit: rf, audio, digital.
Look for reversed polarity capacitors or wrong resistor values.
Check the voltage regulators.
If R320 cannot be adjusted to obtain the deviation reference
voltage (see the Service Procedures section), try to isolate the
problem to the audio or rf section. Check the value of R320. To
check the rf section, set the transmitter frequency (see Table 2),
and verify the tuning voltage of the VCO.
Check the audio section by applying audio to the mic input. With
an oscilloscope, make sure audio is present at TPA7. If not,
work backwards towards the input.
26
2 dBm into a
25A1044 (RI)