Audio Problems - Shure UC2 Service Manual

Audio Problems

25A1044 (RI)
No Tone Key
No tone key means that the 32.768 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. They should be, roughly, –23 dBc
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 devi-
ation, check part integrity and audio path continuity between
U201, pin 14, and the VCO.
If the spectrum analyzer shows a deviated carrier, probe for a
32 kHz signal at TPA6 and I209 and continue to the next step.
If there is a signal at I209 but not at TPA6, then the mute circuit
through Q211 is being activated. Verify that the logic signal at
Q211, pin 1, is logic low. If it is low, replace Q211. If it is not low,
check the line for a short to +5 V. If there is no short, the micro-
processor is defective and must be replaced.
If there is a 32 kHz signal at both I209 and TPA6, replace R236.
If there is not a 32 kHz signal at either I209 or TPA6, there is
something wrong with the tone key circuit or the tone key supply
voltage. If the tone key supply voltage (U200, pin 8) is less than
2 V, there is a problem with one or more of the following com-
ponents: R261, R264, R265, R266, C238, Q220, C237, R232,
or R260.
If the voltage at U200, pin 8, 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."
Verify +9 V, half-supply bias, and ground connections to U200.
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.
27
Shure UC2 Hand-Held UHF Transmitter
3 dB.
Bench Checks