Installation Instructions for Motorola, Toshiba, Mitsubishi and similar RF Power Transistors and Modules
NOTE: These instructions were prepared by RF Parts Company, 435 South Pacific Street, San Marcos, CA 92069. Phone: (619) 744-0750, (800) 737-2787. rfp@rfparts.com Copyright 1996 by RF Parts Company.
Please read these installation notes for RF Power Transistors & Modules. We have found that most failures in RF components can be traced to improper installation or tune-up procedures.
Module Mounting and Handling
(1) Before new part installation, clean all old thermal compound or contaminates from the heatsink. The surface should be smooth and clean, small burrs on mounting holes or bits of solder or metal will prevent proper heat transfer.
(2) Make sure there is adequate clearance between the components and the circuit board. Inadequate clearance may cause cover or substrate to crack or pop off transistor base.
(3) Spread a thin layer of fresh thermal joint compound on the new part. The compound provides heat conduction across otherwise small voids existing between the two surfaces. (An excessive amount of compound will actually reduce heat conductivity.) We recommend the compound “Thermalcote”. (It is available from us and other electronics distributors.)
(4) Care should be taken to avoid dropping part onto a hard surface.
Soldering
(1) Solder leads to the circuit board. Avoid excessive bending of leads.
(2) The recommended solvent for cleaning the residual flux is Ethyl Alcohol.
Module Tune-up tips
(1) Inspect the transmitter circuit before turn on. As an example, in some VHF/UHF transceivers we have found defective PIN antenna switching diodes (as high as 20%). This indicates that a defective antenna may have caused the antenna switching circuit to fail, thus damaging the module. We suggest checking each diode in the switching circuit for leakage and high-forward/low-reverse resistance. This is easily done by lifting one end of diode under test and connecting a conventional analog Ohmmeter (X 10 scale). Look for the usual high reverse/low-forward resistance readings.
(2) Check the power control circuit for proper operation. PIN Diode failure may be caused by a leaky or insufficiently turned-on driver transistor. If any doubt, replace.
(3) To avoid damage to the new component during tune-up phase, the bias circuit should be checked for proper operation and setting.
(4) Initial test should be at low power; if all appears normal, proceed to next step.
(5) Refer to your equipment manual for power adjustment. As the new module may have higher gain than the original, you should reduce the drive level before high power test.
(6) Most modules will exceed the specified power rating, it is suggested, however, not to exceed the module or equipment rating as reliability rapidly decreases with increasing power.
(7) Should the new module fail in the transmitter, it would be reasonable to assume there is a problem remaining which has not been found, i.e.: (a) Intermittent antenna or cable; (b) Intermittently defective PIN diode or associated control circuit, cold solder joint, etc.; (c) Power setting above rated transmitter output.
Transistor Tune-up tips
Matched Transistors are available from RF Parts Company. Transistors are selected into sets based on various criteria, including selection for low Icbo leakage, uniform power gain and linearity, similar d.c. Beta, and normal curve-tracer readings. The RF Power Test does not represent maximum power, only an output measurement for a selected drive level. Example: We test each MRF454 in a test fixture at 3 watts input drive. The corresponding output is measured. A typical test output of 80 watts is about 75% of the transistor’s peak capability. Transistors are then grouped by Beta & Power Gain. Thus, we are able to supply matched pairs, sets of 4, 8, 16, 32, etc., with very similar characteristics. You should save the transistor envelope, as it gives test information on the parts inside. In the event you are required to replace a transistor at a later time, you can order a similar replacement from the test data.
(1) Transistor installation is essentially the same as RF Power Modules. Care should be taken not to over tighten transistors having “studs”, i.e. MRF1946A/MRF454 455A, etc., as the screw is made of copper.
(2) Transistors & RF Power Modules can be damaged by one or combination of the following factors: Excessive case temperature, excessive voltage or current, excessive drive power, oscillations in the circuit, excessive VSWR, etc.
(3) Some amplifiers are basically unstable and care should be taken to avoid excessive bias. Optimum bias voltage in CLASS AB1 circuits is a collector current (at zero RF drive) less than 0.5% of the normal maximum collector current. Good linearity will be achieved without pushing the transistors into CLASS A (which often causes oscillation).
(4) The technician should first test at low power (low drive or low power setting on amplifier). You may increase drive power if the circuit remains stable and within normal current levels. When the power peak is reached, increased drive will not significantly increase output. At that point, output tuning should be adjusted to slightly less capacitance. Example, in a broad band amplifier, the tuning capacitor across the output winding should be set at slightly less capacitance than at peak power setting.
(5) MOS type parts require special care in regards to static electricity (similar to MOS IC’s)
Courtesy of RF Parts Co.
