Audio Problems - Shure UC1L Service Manual

Audio Problems

25A1043 (RI)
No Tone Key
No tone key means that the 32 kHz tone key sidebands are not visible
when the carrier is viewed on a spectrum analyzer. If tone key levels are
there but are too low, the deviation is off and needs to be recalibrated.
Apply a 1 kHz tone at a level of 0 dBu to the audio input with
the audio gain pot at minimum.
If the carrier on the spectrum analyzer does not show any
deviation, check part integrity and audio path continuity between
U201, pin 1, and the VCO.
If the spectrum analyzer shows a deviated carrier, probe for a
32 kHz signal at TP4 and I246 and continue to the next step.
If there is a signal at I246 but not at TP4, then the mute circuit
through Q4 or Q211 is being activated. Verify that the logic sig-
nal at Q4 and Q211, pin 1, is logic low. If it is low, replace Q4 or
Q211. If it is not low, check the line for a short to +5 V. If there is
no short, the microprocessor is defective and must be replaced.
If there is a 32 kHz signal at both I246 and TP4, replace R248.
If there is not a 32 kHz signal at either I246 or TP4, there is
something wrong with the tone key circuit or the tone key supply
voltage. If the tone key supply voltage (U201, pin 7) is less than
2 V, there is a problem with one or more of the following com-
ponents: R284, R287, R285, R286, C289, Q222, C222. R228.
or R229..
If the voltage at U201, pin 7, is greater than 2 V, the problem is in
the tone key circuit itself. Check for shorts and opens. Replace
parts in this circuit beginning with the crystal until the 32 kHz
oscillation returns.
No Audio
View the carrier on the spectrum analyzer. If there are not
32 kHz tone key sidebands around the carrier, follow the steps
above for "No Tone Key."
If there are tone key sidebands around the carrier, verify that the
signal generator is properly outputting signal.
Verify +9 V, half-supply bias, and ground connections to U2.
Verify +9 V, half-supply bias, and ground connections to U201.
Follow the audio path from where audio is applied until the point
of discontinuity is reached and replace the appropriate part.
25
Shure UC1 Body-Pack UHF Transmitter
Bench Checks