5-36 Troubleshooting Charts: RX RF Failure
RX RF Failure – Page 3
VHF: input side of C345 & output
side of C357.
UHF1/UHF2: Q1922 at the base
(pin 2 or 5), collector (pin 3)
& emitter (pin 4).
2
Remove shield, SH16.
Measure RF levels at first
preselector filter and compute
filter loss.
UHF1/UHF2: Input side of C1935 &
output side of C1939.
Loss < 3 dB?
Check components
and replace defective
parts. Autotune the radio
No
Measure RF levels at LNA and
compute LNA gain.
VHF: Input side of C339 and
output side of C360.
Gain about
18 dB?
Check LNA:
Check DC bias voltages of
VHF: U304 at the base
(pin 1) &
collector (pin 3).
No
Is base voltage =
0.787 V? Collector
voltage = 1.56 V?
Replace
VHF: U304
No
Yes
Is base voltage = 1.022 V?
Collector voltage =
0.787 V? Emitter
voltage = 1.561 V?
Replace
UHF1/UHF2: Q1922
Check DC input line voltages
and visually inspect and
replace defective
components if needed.
No
Yes
Measure RF levels at second
preselector filter and compute
filter loss.
UHF1/UHF2: input side of C1959 &
output side of C1961.
Loss < 4 dB?
Check components
and replace defective
parts. Autotune the radio.
No
Yes
5
Yes
Yes
Check DC bias voltages of
VHF: Q303 at the base
(pin 2 or 5), collector (pin 3)
& emitter (pin 4).
Note: RF Test frequency used:
VHF: 154.275MHz
UHF1: 424.975MHz
UHF2: 485.075MHz
700: 769.0625MHz
800: 860.0625MHz
900: 938.0625MHz
Measured with a High Frequency Probe for relative comparisons and troubleshooting only.
Actual S21 gain or loss may differ if the test point is not 50 ohms.
VHF: Input side of C330 &
output side of C339.
UHF1/UHF2: Input side of C1931
and output side of C1936.
UHF1/UHF2: U1932 at the base
(pin 1) &
collector (pin 3).
UHF1/UHF2: U1932
VHF: Q303
Note: Autotune is not available for VHF radio
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