Motorola CDM1250, Basic Service Manual

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Maintenance 2-7
device may have been over-stressed as well. Handle the transceiver board per the procedure for
“Safe Handling of CMOS and LDMOS Devices”, Section 2.3.
2. If removing the transceiver board, the thermal pad material under the two Final stage devices
(Q1402 and Q1403) on the radio chassis has been pulled up such that the radio chassis is
exposed through the thermal pad, then the defective thermal pads must be removed and
replaced. This is only true for the case when the Final stage devices are being replaced. To
remove the thermal pads, use a plastic scraper to prevent scratching of the radio chassis. Use a
0.5% detergent-water solution to remove any residue left on the radio chassis.
3. Remove the defective Final stage LDMOS devices from the transceiver board with a heat gun.
Care should be taken not to cause displacement of any other parts.
4. Clean any excess solder from the device pads so that new devices can sit flush with the board.
5. Hand solder on a replacement LDMOS device, with a suitable size soldering iron, by applying sol-
der to one lead and pad. Press the replacement LDMOS device with a dismantling tool from the
top and reheat the lead and pad to cause the solder to reflow. Applying slight pressure on the
device with the dismantling tool will avoid gaps between the lead and the board. Continue this
approach and solder the remaining leads on one side of the LDMOS device. Once one side of the
device leads are soldered continue with the other side while pressing the LDMOS device to the
board with the dismantling tool. Finish soldering all leads taking care not to cause any shorts to
nearby components or shorts across the device leads.
6. This step is necessary only if Step (2) from above was required, that is, if the thermal pad was
required to be scraped off the radio chassis. If so, replace the thermal pad(s) from a 0.9 inch wide
roll of Bergquist “Softface” replacement material, 3280384M34. Cut the material to a 0.25 inch
length.
Remove the clear plastic backing from the replacement pad and stick the side from which the
backing was removed to the bottom side of the Final device(s) requiring a new pad. Press the
pad firmly to the bottom of the device making sure that it properly adheres to the device.
7. Reassemble the radio per Section 2.7.3.1, LowBand Radio Chassis and Transceiver Board
Assembly.
8. Using the radio’s Customer Programming Software (CPS), reset the Bias Voltage for each of the
Final devices and retune the radio’s power output.
2.6.4 CDM750 Control Head Disassembly
1. To dismount the control head housing from the back housing, insert the dismantling tool in the
groove between the two housings as shown in Figure 2-6.
Note: DO NOT use a heat gun to solder a replacement LDMOS device to the board. This
may cause damage to the LDMOS device.
Note: Exceeding the 0.25 inch length may result in an electrical short of any component
that comes in contact with the thermal pad.