Motorola Commercial Series Service Information page 18

3-2
Symptom
No Audio 1. Synthesizer out of lock
2. Defective IFIC
3. RX audio buffer fault
4. ASFIC fault
5. Audio PA fault
6. Defective speaker
No Receive 1. No first injection
(squelch noise
present)
2. No 5R source.
3. Harmonic filter or
antenna switch fault
4. Back end fault
5. No second injection
Possible Causes Procedure
Verify U201-4 is at 3V dc.
Verify audio is present at U51-8.
Verify audio is present at U451-2.
Verify audio is present at U451-41.
Verify U451-14 is high.
Verify U490-1 is <0.2V dc.
Verify audio is present at U490-5
and 8.
Verify audio is present at speaker
terminals.
Check that RF level at T42-6 is
approx +6 dBm.
Check that RF level at U251-8 is at
least -8 dBm.
Verify U401-49 is high in RX.
Verify Q311 gate is 0V dc in RX
Verify Q311 drain is 5V dc in RX.
Apply on-channel 100 mV RF signal
at antenna port. Verify RF level at jct.
C1/C2 per schematic.
Apply on-channel 100 mV RF signal
at antenna port. Measure RF levels
from FL51 through U51.
Measure RF level at U51-3, verify
approx. 280 mV rms.
TROUBLESHOOTING TABLES
Corrective Action
Troubleshoot synthesizer/VCO
circuits.
Check Q70, Y70, U51.
Check U510 and associated
parts.
Check squelch setting, PL/DPL
programming. Troubleshoot/
replace U451.
Check Q490.
Check/replace U490.
If not, check continuity of J471-2
and 3. Check J491. If yes,
replace speaker.
Check injection filter C40-44,
L40-41.
If yes, check Q280 and associ-
ated parts. If no, check U251
and components on pins 5 and
6.
Check/replace U401
Check/replace Q313.
Check for shorts, check/replace
Q311.
Check TX harmonic filter, D120-
121. Should be 0V dc on D120-
121.
Check components prior to
loss-of-signal point.
If dc voltages at U51-3 and 4
are OK, check Y51 and
associated parts. If not replace
U51.