Motorola MTP700 Detailed Service Manual page 203

TRANSCEIVER 800 MHz
2nd LO Troubleshooting
Perform the following:
1. Remove the 2nd_LO shield.
2. Set the Unit to continuous
receive mode (Rxtdm).
3. Measure the 2nd_LO control
voltage between C739 & R734.
Is the control voltage
between 0.5 and 4.5Vdc?
Yes
Measure the RF power at
C751 at 219.3 MHz.
Is the power level
-10dBm ±1dB and stable
on frequency?
Yes
The 2nd_LO VCO/Synthesizer
is O.K.
MTP700 Portable Radio / Detailed Service Manual
Check for feedback problem.
1. Check soldering of C751,
L732, C750 and C748.
2. Measure the RF power level
at Pin 5 of the LMX at 219.3MHz.
No
Check Q730, R738 for
No
soldering problems and
for the right values.
(compare with BOM)
Check D730, C740,
L730, C741, C742,
R735, R737, C743,
C744, R736, Q730,
C745, R738, C746,
L732, C748, L731
and C750 for
soldering problems
and for the right
values.
Check DC supply for the
VCO:
1. Check for 4.5V on
L731, R738 and C746.
2. Check for
TETRA_2nd_LO_En on
Q731.
Does the VCO have
its DC Supply?
Yes
Check DC supply for the LMX.
1. Check for 5V_RF on C734, C735.
2. Check 2.775V_RF on C737.
Does the LMX have
its DC Supply?
Yes
Can the frequency
No
be set to
219.3MHz?
Yes
The VCO is working properly.
The problem is with the LMX.
Replace the LMX.
4.2 - 81
No
Check DC
Block
No
Check DC
Block