The reasons and maintenance of the FM Transmitter power amplifier module fauty

The reasons and maintenance of the FM Transmitter power amplifier module fauty

One or more amplifiers in the amplifier unit are overloaded and the amplifier is overloaded.
1, power amplifier overload protectionSymptom:

Failure analysis:

(1), the current amplifier to protect its actual detection point of the reflected power, resulting in protection, not the actual standing wave ratio of large protection.
(2), the amplifier load changes, the output power is constant, the reflected power display is too large
(3), the amplifier appears protection, the amplifier's MRF148A voltage drop is negative voltage, gain down, the amplifier to achieve the purpose of protection, because the protection circuit is designed to be unrecoverable circuit, overload protection elimination method only in the case of power amplifier to achieve

Single amplifier overload protection:

(1), power amplifier output RF cable (synthesizer - power amplifier plug) Weld, slightly need to re-welding, serious need to replace.
(2), the amplifier used to detect the reflected power of the coupler is not good.
Use the sweeter to test the directionality of the directional coupler (greater than 20dB), check the resistance (50 ohms), and replace it.
(3), the synthesizer standing wave is not good, causing the reflection to be too large.
Use the sweeter to test the standing wave of the input of the synthesizer (power amplifier is less than -25dB), check the absorption load resistance (100 ohms * 2) and the index (standing wave less than 1.1), bad replacement or adjustment.
(4), poor contact between the power plug and the hot plug.
Check the appearance of the reed, such as the lack of flexibility or damage, to replace the hot plug.
(5), the output power of the amplifier becomes larger.
Check the amplifier frequency response and gain (drive level 20 dB, the final stage 17dB) and the current level, and adjust (normal output 500W, I1, I2 current 8A, I3 is 0.5A).
(6), the amplifier output of the standing wave is not good.
Use the sweeter to test the output standing wave of the amplifier, normally within the use channel is less than 1.15.
(7), the detection circuit for the reflected power and the control circuit device are damaged.
Check the circuit, replace the damaged device or adjust the potentiometer P1

Multiple power amplifier unit overload protection processing:

(1), low-pass filter internal device changes, resulting in poor low-pass filter indicators.
(2), antenna, feeder index is not good.
(3), coaxial switch internal poor contact, switching is not in place, causing the launch
(4), the frequency adjustment of the exciter is not in the set channel.
(5), RF connectors, feeders and other damage

Power amplifier unit over-excitation display light, the whole output power down.


2, amplifier appears to be stimulated protectionSymptom:

Failure analysis:

(1), set the incentive protection is to protect the power amplifier tube in the input power increases caused by damage to the power tube.
(2), power amplifier over excitation protection detection point is the input power detection voltage size, generally about 125% of the normal input power.
(3), over-excitation protection circuit is activated, push the gate voltage into a negative voltage, gain reduction, power amplifier protection, because the over-excitation protection circuit is designed to return to the circuit, so as long as the lower excitation power, over-excitation can be eliminated.

A single amplifier has overrun protection:

(1), the input impedance of the amplifier changes, resulting in over-excitation detection voltage rise. Use the sweep meter to check the input standing wave and the booster MRF148A device for the amplifier (using the pointer table).
(2), the detection circuit or the control circuit components damage. Check the control board and the detector circuit to replace the damaged device.
Can be properly adjusted P2 point voltage
(3), the output impedance changes, resulting in amplifier input impedance changes, the input detection voltage increases

Multiple amplifier appears to have the incentive to deal with:

(1), loop control out of control caused by the driver input power increases. Check the loop control line.
(2), where the individual power amplifier does not work or work is not normal, under the action of the large ring ALC, the exciter output power becomes larger.
In the main control unit to check the power amplifier unit work.
(3), a switching power supply does not work, in the current flow, the other switching power supply voltage drop, the final amplifier gain decreased, the exciter in the large ring ALC under the output power increases, resulting in power amplifier over excitation protection

3, power amplifier overheat protectionSymptom

:(1), one or more power amplifier panel overheat protection indicator light.
(2), the main control unit shows PA-HEATHOT, then the driver output power decreases, the machine power down, troubleshooting, the driver slowly increase the power to reach the normal full power takes about 30 minutes or so.
If you need to quickly return to the rated power, you need to restart the machine

Failure analysis:

(1), the amplifier unit temperature exceeds 67 degrees when the overheat protection circuit starts.
(2), power amplifier protection circuit will start after the level of MRF148A gate voltage into a negative voltage, to protect the role.
(3), the amplifier protection circuit is activated, converted into digital signal generator control unit, the program according to set the processing method for processing.

Amplifier overheat protection treatment:

1, cooling system problem

(1), fan damage caused by power amplifier overheating, need to replace the fan.
(2), the fan speed slows down, resulting in power amplifier overheating, need to check whether the fan phase, the fan leaves are deformed.
(3), fan reversal, the normal should be the back door into the wind, the front door of the wind.
(4), the duct is not formed, the main check whether the back door is closed, whether the amplifier does not insert the wind distribution caused by uneven.
(5) fan filter network for a long time without cleaning, resulting in air volume is not enough.
(6), the hot air does not use the duct drain to the outdoor or duct clogging, need to clean the duct, conditional installation of the right amount of air conditioning to ensure that the engine room ambient temperature at 30 degrees
(7), need to regularly check the air duct and fan operation

2, RF access problems

(1), the amplifier inside the synthesizer is damaged, the power can not be normal synthesis, resulting in the radiator temperature rise. Check the internal power synthesizer of the amplifier.
(2), the power amplifier in the RF channel components damage, the efficiency of the entire power amplifier down, the temperature rise.
It is necessary to check the current of each power tube when the amplifier is running
(4 * MRF151G) 65%; 3KW (12 * MRF151G) at 60%; 5KW (20 * MRF151G) in the case of a single tube (MRF151G) efficiency of 75%; double tube is 70%; 1KW (4 * MRF151G)
55%; 10KW (40 * MRF151G) at about 50%

3, other parts

(1), for the protection of 68 degrees thermal relay (67F070) damage, resulting in power failure protection. Replace the thermal relay.
(2), control circuit components damaged. Check the control board and replace the damaged device.

4, power amplifier unit in a power amplifier output power of the large processing

(1), the power amplifier in the push level or the final level of the static current becomes larger. Adjust the potentiometer (P3, P6, P7) of the gate voltage in the bias circuit so that the boost current is 0.3A and the last quiescent current is 0A (gate voltage is 1.8V)
(2), the boost level of the booster stage becomes high, resulting in a larger output power.
Check the capacitor C66 in the configuration circuit of the MRF148A and replace it,

5, the amplifier in the two current imbalance processing

(1), the power amplifier in the synthesizer of the standing wave is not good. Check the internal composition of the two synthesizers (each port standing wave less than 1.15), generally the absorption resistance (BED / TC250 / 50) and the damage caused by ceramic capacitors.
(2), the power amplifier circuit in the amplifier circuit is damaged.
Check the components in the sampling circuit (MAX4080) and replace them.

3, summary

Amplifier with a circuit board hot-swappable structure, such as the emergence of a power amplifier failure, you can not directly under the conditions of unplugging, replacement and maintenance, each two power amplifier module constitutes a hot-swappable amplifier unit. But in the actual work, before plugging the module must first reduce the transmitter power to avoid the process of plugging the module board.
Through the maintenance of the FM transmitter in recent years, the CMT 700W power amplifier module is also gradually understand the vast majority of power amplifier failures are the above several cases, through the above summary, deepened the familiarity of the machine equipment, convenient future Better maintenance of the transmitter to ensure safe broadcast work.


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