DB ELETTRONICA TELECOMUNICAZIONI S.p.A.
PM 1000 – 1 kW FM Broadcasting Transmitter - USER’S MANUAL
100 /101
12.2 APPENDIX B - MOSFET REPLACEMENT
(see next figure)
1. Disconnect the supply voltage from the output board (faston connection).
2. Unscrew the damaged MOSFET and the output matching board.
3. Unsolder the MOSFET.
4. Clean the input and output boards to prevent short-circuits before replacing the MOSFET.
5. Locate properly the white teflon sheets between the output circuit and the heat sink: affix the
circuit using the two screws.
6. Put on the thermally conductive compound between the MOSFET case and the heat sink.
7. Fix the MOSFET to the heat sink (screws must be strongly fixed) and solder all the four pins of
the MOSFET: careful attention must be paid to the output board soldering since an
excessive quantity of tin can cause a short circuit between different board layers. Reconnect
also the 2 GND interconnections, indicated, and check the right soldering of the input
transformer and gate resistor.
8. Verify that the supply voltage is 45V (if it is not, there is a problem in the switching power
supply boards). Reconnect the supply voltage to the output board (faston connection), placing an
amperometer in series (minimum full scale deflection 10A).
9. Control the output power level (it should be 1000W) and the absence of spurious emissions (to
check it, use a spectrum analyser and an output RF monitor sufficiently attenuated). Pay
attention to the amperometer: the MOSFET must not drain more than 9A.
10. In case the replaced MOSFET doesn’t reach the nominal output power or in case of spurious
emissions you need to improve the input matching of the MOSFET: to do this compress or
expand manually the input circuit coil (see ) to maximise the output power.
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