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Recycling Andrew Corporation 900MHz Power Amplifier modules for 23cms
MRF9045 module
Remove all the existing components except the bias decoupling
capacitors and the output capacitor and cut track and add copper tapes so
that the board looks like below.
After much tweaking, I ended up with the following layout and performance. You might need to add tuning tabs on the output and input lines as per the MRF9060 design below.
Peeling off the existing copper microstrips is relatively easy with a sharp scalpel, but take care not to rip off too much insulation under the copper.
MRF9060 module
Figure 2 unmodified module
This is trickier. It is impossible to fit this matching network for two devices on the existing board, so I decided to go for a more drastic method. I considered that 60 Watts would be more than adequate and simpler, so I decide to recycle the output modules as single device 60 Watt stages removing one of the transistors as a spare.
Now here’s where it gets really scary.
The LDMOS devices are soldered down to the module’s copper heat spreader plate! Reading the Applications note from Freescale gave very strict guidelines on temperatures for soldering these devices but I thought “what the heck” I can’t afford a soldering oven, but I can afford to trash a couple of devices if it doesn’t work. So I waited until the XYL was out and dropped the module down on her ceramic hob, turned it on and waited for the solder to melt. It duly did, so I slid the module off the heat, grabbed the devices with the tweezers and pulled. They came away easily. At the same time I swept off all the other components having first taken careful note of their positions, and also lifted the screen off to allow better access to the board.
This left me with a bag of bits, a spare MRF9060 and a (very hot!) blank PCB still glued to the heat spreader. I realised then that the PCB must be fixed with silver epoxy, not soldered as I expected.