Shure U2 Service Manual page 20

Bench Checks
Dc Problems
Rf Problems
25B1022 (AG)
Shure U2 Hand-Held UHF Transmitter
n Verify the battery voltage is between 2 and 3.5 V.
n Check for proper Vcc and +5 V rf at Q208 (make all dc measure-
ments with respect to rf ground unless otherwise specified). if
incorrect voltage is present, check the biasing resistors values.
n Trace the dc back to the dc to dc converter.
n Check the power switch.
n Look 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.
n Verify rf OFF signal changes from +5V at power up/power down
to 0V during normal operation. Trace the signal to the micropro-
cessor. Q208 should be saturated during normal operation.
n Verify that battery voltage appears at U208, pin 1. Check for
reversed polarity capacitors C239 and C247.
n Verify Vds of Q204 is approximately –.025 V. Q204 should be
saturated.
n Verify 5V dc/dc converter output at U208, pin 8. Check for open
coil L205, reversed polarity or bad diodes D201 and D202, or
bad converter U208.
n If the carrier is out of specification by less then 40 kHz and
cannot be corrected by adjusting VC201, check the values of
VC201, C224 and Y203. If these values are correct, replace
Y203. As a last resort, replace synthesizer U205.
n If the carrier frequency is not between 782 and 810 MHz, or is
unstable, the loop is unlocked. Check the solder connections at
the, head board, VCO, microprocessor (traces LE, D, and CK)
and synthesizer U205.
n Check for 4 MHz oscillation at pins 1 and 2 of U205. if not
replace the crystal Y203.
n Check the values and polarity of the loop filter: R226, C231,
R251, C237, R243, C257 U210, R227, and R270.
n Check the bias voltage of U205 pins 3 and 4 and U210, pin 8.
n As a last resort, replace synthesizer U205. The tuning voltage
of the VCO should be about 3 Vdc when the transmitter is pro-
grammed for 800 MHz operation.
19
Bench Checks