RF POWER AMPLIFIER MODULE REPAIR:
Transistor Substitution
his operation has to be made in a proper laboratory or site, equipped with all the necessary
instrumentation (Spectrum and Network analyser, TV peak sync wattmeter, generator,
dummy load, multimeter, tools, etc.;) and carried out from qualified personnel, skilled in this type
of RF work.
It must be born in mind that any misuse or operation of the equipment not in accordance with
accepted practice or with the manual (if not previously authorized by ABE Elettronica SpA) will
invalidate the Warranty.
First of all it has to be read carefully the manual of each piece of equipment composing the
transmitter and especially the chapters “General conditions of warranty”, “General safety
instructions” and “Equipment’s description” with particular attention to the section “Safety
instructions”.
The following instructions are therefore intended for use by qualified personnel, already expert in
the field of repair, test and adjustment of TV high power solid state transmitters and transposers.
Proceed as follows:
1) When it has been recognized that the amplifier module is not properly working (low gain, DC
short circuit, etc.), it has to be identified if the problem comes from one or more faulty power
semi-conductors (in most cases) or from other faulty components/reasons (couplers, capacitors,
DC bias circuit, bad soldering, etc.).
2) The simplest way to detect if a power semi-conductor works is to check the bias current. This
operation has to be made with no RF signal, with the correct power supply voltage and with the
amplifier module connected to suitable loads (input and output terminated with 50Ohm to avoid
auto-oscillation risks).
If the DC input supplies a single semi-conductor, it is enough to put an ammeter in series.
In many modules the DC input is only one and it supplies two or more semi-conductors.
If in series to the collector or drain of the semi-conductor there is a resistor (i.e. 0,1Ohm resistor
that can also be composed by 10 resistors 1Ohm in parallel), it is possible to check with a
multimeter the voltage drop across the resistor (after having checked the resistor value) to
calculate the current (i.e. voltage drop 30mV divided by resistor value 0,1Ohm = current 300mA).
If there is no resistor in series and the semi-conductors connected to the same power supply
point are two, another way is to adjust the bias of both semi-conductors to 0 (interdiction), to
check that the total current is 0 and, after that, to adjust the bias of each semi-conductor one after
the other to control that both are working (i.e. semi-conductor 1: interdicted; semi-conductor 2
bias: adjusted = total current: 300mA – semi-conductor 1: adjusted; semi-conductor 2: adjusted =
total current: 600mA).
If we are in the previous condition and the bias of the semi-conductor cannot be set to 0, the only
way is to break the circuit in series to the collector or drain and to connect an ammeter; it has to
be payed particular attention in this case because it is very easy, following this last procedure, to
damage semi-conductors.
3) Remember that the RF power semi-conductor bias current can be more or less constant
independently from the RF output power if it is polarized in class “A” (i.e. BFQ136: 500mA with or
without RF signal) or it can strongly vary if it is polarized in class “AB” (i.e. MRF373S: 300mA –
bias only, no RF signal; around 3A with RF output power).
© ABE Elettronica SpA 05/2001 All specifications contained in this document may be changed without prior notice.
4) When the faulty semi-conductor has been detected, it has to be substituted using all
precautions to avoid electrical shocks, mechanical stress, high temperatures during soldering.
Remember that the mechanical fixing is very important to have the proper power dissipation; in
many cases it may be needed to use a veil (but only a thin veil) of silicon thermal paste or
equivalent thermal compound.
5) After the substitution, the semi-conductor has to be polarized according to the indications of
the schematic diagram or of the manual: pay particular attention in this operation not to exceed
the maximum bias current indicated.
It is better, before switching on the module for polarizing a semiconductor, to adjust the bias
regulation to the minimum (normally potenziometer fully counterclockwise) and increasing slowly
and carefully the adjustement.
After that the module has to be re-tested (gain, matching, output power, linearity, etc.).
For some semi-conductor, and especially for MOS and LD-MOS polarized in “AB” class, the bias
current (no RF signal) can strongly affect the performances (i.e. gain, intermodulation, differential
gain, phase, etc.). In this case the bias current indicated in the schematic diagram can be a
starting indication, but it can be eventually varied (within the specified maximum range) to obtain
the best performances from the module (i.e. MRF373S: normal bias current 300mA but in some
cases can be set from 200 to 600mA).
Sometimes best bias current can be different according to the frequency.
Best testing has not to be made at low RF level: generally amplitude/frequency response has to
be made with an output power of around 30-50% of the nominal, but paying attention not to
exceed 70% of the nominal output power with CW signals!
Transistor Substitution
his operation has to be made in a proper laboratory or site, equipped with all the necessary
instrumentation (Spectrum and Network analyser, TV peak sync wattmeter, generator,
dummy load, multimeter, tools, etc.;) and carried out from qualified personnel, skilled in this type
of RF work.
It must be born in mind that any misuse or operation of the equipment not in accordance with
accepted practice or with the manual (if not previously authorized by ABE Elettronica SpA) will
invalidate the Warranty.
First of all it has to be read carefully the manual of each piece of equipment composing the
transmitter and especially the chapters “General conditions of warranty”, “General safety
instructions” and “Equipment’s description” with particular attention to the section “Safety
instructions”.
The following instructions are therefore intended for use by qualified personnel, already expert in
the field of repair, test and adjustment of TV high power solid state transmitters and transposers.
Proceed as follows:
1) When it has been recognized that the amplifier module is not properly working (low gain, DC
short circuit, etc.), it has to be identified if the problem comes from one or more faulty power
semi-conductors (in most cases) or from other faulty components/reasons (couplers, capacitors,
DC bias circuit, bad soldering, etc.).
2) The simplest way to detect if a power semi-conductor works is to check the bias current. This
operation has to be made with no RF signal, with the correct power supply voltage and with the
amplifier module connected to suitable loads (input and output terminated with 50Ohm to avoid
auto-oscillation risks).
If the DC input supplies a single semi-conductor, it is enough to put an ammeter in series.
In many modules the DC input is only one and it supplies two or more semi-conductors.
If in series to the collector or drain of the semi-conductor there is a resistor (i.e. 0,1Ohm resistor
that can also be composed by 10 resistors 1Ohm in parallel), it is possible to check with a
multimeter the voltage drop across the resistor (after having checked the resistor value) to
calculate the current (i.e. voltage drop 30mV divided by resistor value 0,1Ohm = current 300mA).
If there is no resistor in series and the semi-conductors connected to the same power supply
point are two, another way is to adjust the bias of both semi-conductors to 0 (interdiction), to
check that the total current is 0 and, after that, to adjust the bias of each semi-conductor one after
the other to control that both are working (i.e. semi-conductor 1: interdicted; semi-conductor 2
bias: adjusted = total current: 300mA – semi-conductor 1: adjusted; semi-conductor 2: adjusted =
total current: 600mA).
If we are in the previous condition and the bias of the semi-conductor cannot be set to 0, the only
way is to break the circuit in series to the collector or drain and to connect an ammeter; it has to
be payed particular attention in this case because it is very easy, following this last procedure, to
damage semi-conductors.
3) Remember that the RF power semi-conductor bias current can be more or less constant
independently from the RF output power if it is polarized in class “A” (i.e. BFQ136: 500mA with or
without RF signal) or it can strongly vary if it is polarized in class “AB” (i.e. MRF373S: 300mA –
bias only, no RF signal; around 3A with RF output power).
© ABE Elettronica SpA 05/2001 All specifications contained in this document may be changed without prior notice.
4) When the faulty semi-conductor has been detected, it has to be substituted using all
precautions to avoid electrical shocks, mechanical stress, high temperatures during soldering.
Remember that the mechanical fixing is very important to have the proper power dissipation; in
many cases it may be needed to use a veil (but only a thin veil) of silicon thermal paste or
equivalent thermal compound.
5) After the substitution, the semi-conductor has to be polarized according to the indications of
the schematic diagram or of the manual: pay particular attention in this operation not to exceed
the maximum bias current indicated.
It is better, before switching on the module for polarizing a semiconductor, to adjust the bias
regulation to the minimum (normally potenziometer fully counterclockwise) and increasing slowly
and carefully the adjustement.
After that the module has to be re-tested (gain, matching, output power, linearity, etc.).
For some semi-conductor, and especially for MOS and LD-MOS polarized in “AB” class, the bias
current (no RF signal) can strongly affect the performances (i.e. gain, intermodulation, differential
gain, phase, etc.). In this case the bias current indicated in the schematic diagram can be a
starting indication, but it can be eventually varied (within the specified maximum range) to obtain
the best performances from the module (i.e. MRF373S: normal bias current 300mA but in some
cases can be set from 200 to 600mA).
Sometimes best bias current can be different according to the frequency.
Best testing has not to be made at low RF level: generally amplitude/frequency response has to
be made with an output power of around 30-50% of the nominal, but paying attention not to
exceed 70% of the nominal output power with CW signals!
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