Hytera MD78X, Service Manual

Page: 253 / 519

UHF3 (350
–
400 MHz)
Troubleshooting Flow Chart
35
[4] The sensitivity at TP6101 is about -110 dBm.
[5] The received LO signal at TP6106 is about 18 dBm.
[6] The sensitivity at TP6104 is about -120 dBm.
[7] The sensitivity at TP6103 is -117 dBm.
6.5
Frequency Generation Unit (FGU)
Check FGU
U915 outputs 5 V and
U918 outputs 3.3 V?
Replace U915 or
U918
No
Q104 outputs 4.3 V?
Yes
Check Q104
No
Yes
VCO oscillates?[1]
Q108, Q112, Q101, and
Q105 work normally?
[2]
No
Replace Q106,
Q110,
Q100, and Q103
Yes
Check D107, D108, D109, D110,
D121, D122, D125, and D126
No
TP124 CV is normal?[4]
Yes
PLL U100 is normal?
[5]
Yes
No
Replace PLL U100
No
Check U101, U102,
R187, and R171
TP109 feedback is
normal?[6]
Yes
Yes
Check the feedback
circuit
Check L809, L813,
L810, M801 and
peripheral circuit
Yes
No
End
Q106, Q110, Q100, and
Q103 work normally?
[3]
No
Replace Q108,
Q112,
Q101, and Q105
Oscillates (19.2 MHz)?[7]
Yes
Check 19.2MHz
oscillator circuit
No
Description of normal situations:
[1] The oscillation frequency of RX TP123 and TX TP111 can be detected.
[2] Q101, Q105, Q108, and Q112 are turned on and output about 3.6 V.
[3] The emitter voltage of Q106, Q110, Q100 and Q103 is about 1.8 V.