FM broadcast transmitter repair

Thread Starter

Mooks

Joined Nov 27, 2016
10
I am trying to troubleshoot a Tugicom 25 watt FM broadcast transmitter. It's output has dropped to 1 watt. I replaced the
power transistor on the PA but it didn't make any difference. The voltage supply to the PA is what it is supposed to be ( 26 volts).
The PA is fed by a 1 watt driver. I am not sure what to measure/ do next and have not been able to find any other resources to
help me out. Attached is the schematic for the PA, I don't have a schematic for the driver board. The transistor on the PA is an MRF314, the two transistors on the driver board are 2N4427 and KSP10 . Any help would be appreciated.
Mike
Tatlayoko lake BC
 

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drc_567

Joined Dec 29, 2008
855
Check the DC voltage at the collector, base, and emitter of the power transistor. What do you get?

Edit ... emitter should be at ground. Also, how are you testing this transmitter? Typically, you should have a resistor of the correct resistance, and sufficient wattage, connected to the RF output, in place of the transmitting antenna, in order to avoid improper transmissions, ...e.g 50 ohms and 50 watts or whatever is appropriate.
 
Last edited:

Thread Starter

Mooks

Joined Nov 27, 2016
10
Check the DC voltage at the collector, base, and emitter of the power transistor. What do you get?

Edit ... emitter should be at ground. Also, how are you testing this transmitter? Typically, you should have a resistor of the correct resistance, and sufficient wattage, connected to the RF output, in place of the transmitting antenna, in order to avoid improper transmissions, ...e.g 50 ohms and 50 watts or whatever is appropriate.
 

Thread Starter

Mooks

Joined Nov 27, 2016
10
I have used an IFR 120B service monitor to measure the output and also a bird wattmeter into a 50 ohm dummy load.
When I measure the DC voltages, do I use a regular DVM with regular probes or do I have to use RF probes to measure the voltage?
Please let me know and I will check the voltages.
tnx
Mike
 

Thread Starter

Mooks

Joined Nov 27, 2016
10
A good 100MHz scope should be able to measure the output of the driver stage. What equipment are you using?
I have a 200mhz digital scope ( Owon) . Here are two snapshots of from the scope. One is the input to the PA and one is the output from the PA. Vk is the RMS voltage. Let me know if you would like other shots. thanks Mike
Input23Oct.JPG Output23Oct.JPG
 

drc_567

Joined Dec 29, 2008
855
I have used an IFR 120B service monitor to measure the output and also a bird wattmeter into a 50 ohm dummy load.
When I measure the DC voltages, do I use a regular DVM with regular probes or do I have to use RF probes to measure the voltage?
Please let me know and I will check the voltages.
tnx
Mike
... just use a dc voltmeter/dc probes for the transistor. The base to emitter voltage should give some indication that the transistor is at least turned on. The collector to emitter voltage might also give a useful value. The DC collector current would be helpful, but that would require a series current measurement, so don't bother.
... Does it look like the power transistor is getting warm? ... with the power supply turned off, check for heat from that component.
... basically, just trying to track down the fundamental problem.
 

Ylli

Joined Nov 13, 2015
965
This is a class C amplifier. The base-emitter voltage looks like it is set at 0.25 - 0.30 volts (470 ohm/4.7 ohm divider). With no drive, the MRF314 should remain off, and therefore cool.

Is this 25 W amp an add-on? Can you disconnect the driver from the power amp, load the driver with 50 ohms, and make that scope measurement? Are you sure Vk is the rms value of the waveform or perhaps the pk-pk value?
 

drc_567

Joined Dec 29, 2008
855
A serious low pass filter on the output regenerates the sine wave from the current pulses.
The waveform at the transistor collector should appear as a half wave ... or something of that nature, if everything is working correctly .. right?
 

Ylli

Joined Nov 13, 2015
965
The waveform at the transistor collector should appear as a half wave ... or something of that nature, if everything is working correctly .. right?
It may appear a bit more distorted, but it still is connected to that resonant filter on the output. The second half of the sine wave will ring back from the filter.
 

Willen

Joined Nov 13, 2015
294
As the Mooks saying, the small UHF signal transistor and a 2N4427 are as a driver stage. KSP10 has around 600MHz fT and maybe 50mA Ic rating. So how the tiny KSP10 can drive the 2N4427?

Maybe the KSP10 is there for different purpose there.
 

Thread Starter

Mooks

Joined Nov 27, 2016
10
This is a class C amplifier. The base-emitter voltage looks like it is set at 0.25 - 0.30 volts (470 ohm/4.7 ohm divider). With no drive, the MRF314 should remain off, and therefore cool.

Is this 25 W amp an add-on? Can you disconnect the driver from the power amp, load the driver with 50 ohms, and make that scope measurement? Are you sure Vk is the rms value of the waveform or perhaps the pk-pk value?
This is a class C amplifier. The base-emitter voltage looks like it is set at 0.25 - 0.30 volts (470 ohm/4.7 ohm divider). With no drive, the MRF314 should remain off, and therefore cool.

Is this 25 W amp an add-on? Can you disconnect the driver from the power amp, load the driver with 50 ohms, and make that scope measurement? Are you sure Vk is the rms value of the waveform or perhaps the pk-pk value?
The 25W PA is a separate board. It is fed from the driver board by some very thin coax that I don't have a connector for. Thus I could unsolder the coax but then I would have no way to properly feed it to a 50 ohm load. Any suggestions as to how to do that?
Tnx
Mike
 

Ylli

Joined Nov 13, 2015
965
The 2N4427 data sheet indicates it can provide an output power of 1W with an input power of 100 mW. I can see the KSP10 boosting the exciter output of perhaps 10 mW to the 100 mW driving the 2N4427.

(For the red trace) What's the vertical scale on that scope? Is it 2 V/div? Where is ground reference? Set the Scope to DC coupling so we can see the limits of the voltage excursions.

Using your DC voltmeter, with no RF drive, what is the DC voltage base to ground and collector to ground?
 

Thread Starter

Mooks

Joined Nov 27, 2016
10
The 2N4427 data sheet indicates it can provide an output power of 1W with an input power of 100 mW. I can see the KSP10 boosting the exciter output of perhaps 10 mW to the 100 mW driving the 2N4427.

(For the red trace) What's the vertical scale on that scope? Is it 2 V/div? Where is ground reference? Set the Scope to DC coupling so we can see the limits of the voltage excursions.

Using your DC voltmeter, with no RF drive, what is the DC voltage base to ground and collector to ground?
Which transistor would you like me to measure the DC voltage , the 2N4427, the KSP10 or the MRF314? The vertical scale of the scope is 5V per division.
 

drc_567

Joined Dec 29, 2008
855
Which transistor would you like me to measure the DC voltage , the 2N4427, the KSP10 or the MRF314? The vertical scale of the scope is 5V per division.
The KSP10 datasheet shows the base to emitter DC voltage is 0.95 volts, with Vce... collector to emitter of 10 volts. These values would indicate the KSP10 is turned on correctly.
... With the DC voltmeter, do you read anything close to these values?
 

Ylli

Joined Nov 13, 2015
965
Which transistor would you like me to measure the DC voltage , the 2N4427, the KSP10 or the MRF314? The vertical scale of the scope is 5V per division.
Yes :). I was thinking of the MRF314, but it wouldn't hurt if you posted the DC voltages (Vbe, Vce) of all three transistors. RF off is possible.
 
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