Shure U1 Service Manual page 21

Bench Checks
Dc Problems
Rf Problems
25C1021 (BG)
Shure U1/U1L Body-Pack UHF Transmitter
nMeasure the dc voltage at pin 1 of C104. This is the output of the
dc to dc converter. It should be 5V ± 0.2 Vdc.
nMeasure the dc voltage at C113, pin 1, and C105, pin 1. It should
be 5V ± 0.2 V.
nVerify the battery voltage is between 2 V and 3.5 V.
nCheck for proper Vcc and +5 V rf at Q207 (make all dc measure-
ments with respect to rf ground unless otherwise specified). If
incorrect voltage is present, check the values of the biasing
resistors.
nTrace the dc back to the dc to dc converter.
nCheck the power switch.
nLook for open coils, cracked parts, reversed polarity capacitors,
solder shorts. If there is a short to ground from 5 V, isolate differ-
ent parts of the circuit.
nVerify rf OFF signal changes from +5 V at power on/power off to
0 V during normal operation. Trace the signal to the microproces-
sor. Q207 should be saturated during normal operation.
nVerify that the battery voltage appears at U105 pin 1.
nVerify that the Vds (drain to source voltage) of Q101 is approxi-
mately –0.025 V. Q101 should be saturated.
nVerify that the dc to dc converter output is 5 V at pin 8 on U105.
nIf the carrier is out of specification by less than 40 kHz and
cannot be corrected by adjusting C239, check the values of
C239, C238, and Y201. If these values are correct, replace
Y201. As a last resort, replace synthesizer U204.
nIf the carrier frequency is not within the allocated UHF band-
width, or if it is unstable, the loop is unlocked. Check the solder
connections at the head board, VCO, microprocessor (traces LE,
D, and CK), and synthesizer U204.
nCheck for 4 MHz frequency at pins 1 and 2 of U204. If frequency
other than 4 MHz is detected, replace crystal Y201.
nCheck the loop filter: R238, C245, R240, C266, C267, R239,
C247 U208, R243, and R285.
nCheck U204, pins 3 and 4, and U208, pin 8.
nReplace synthesizer U204. The tuning voltage of the VCO
should be approximately 3 Vdc when the transmitter is pro-
grammed for the center of the frequency range.
19 Bench Checks