Diagnosing and Testing No RF Power Output in the PA Chain on a V3 Circuit

If your power amplifier (PA) chain is failing to output RF power, follow these steps to diagnose and troubleshoot the issue.

Note: This article may contain inaccurate information. Use caution.

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Quick and Dirty MOSFET Diagnosis

Shorted Pins Check

• A dead giveaway to shorted IRF510 MOSFET is excessive heat and current draw on the input power supply (12v). If you are seeing high current draw when transmitting at the lowest power setting, then the IRF510 is likely shorted.
• Remove external power source from the sBitx to perform this step!
• Set your multimeter to the continuity or diode test mode.
• Check for shorts between each pair of pins (Gate-Drain, Gate-Source, Drain-Source).
• There should be no continuity between any of the pairs. A beep or low resistance reading indicates a short circuit, and likely a damaged IRF510.

Resistance Check

• Set the multimeter to resistance mode.
• Connect the multimeter leads to the Drain and Source pins of the IRF510.
• Without applying any voltage to the Gate, the IRF510 should be "OFF", showing high resistance (ideally, it should appear as an open circuit).

Internal Damage

• A Damaged IRF510 that is destroyed by heat will not be able to amplify the signal. It may not get hot or draw excessive current.
• Remove external power source from the sBitx to perform this step!
• Set the multimeter to Diode Test mode.
• Connect the red lead to the source and the black lead to the gate of the IRF510.
• Record the voltage drop on the meter (source to gate).
• Reverse the multimeter leads and take another reading. A good MOSFET should show a high resistance or OL (open loop).

Visual Inspection

Before using any tools, visually inspect the circuit for:

• Burn marks on IRF510 MOSFETs and transistors
• Cracks or bulges on capacitors
• Discoloration or burn marks on resistors or other components
• Damaged or loose transformer windings
• Disconnected or cold solder joints

If you notice any of these signs, repair or replace the affected components.

Multimeter Testing

Use a digital multimeter to check key components below.

MOSFET and Transistor Tests

1. Source-Drain Continuity (IRF510s):

   • Set your multimeter to continuity or diode test mode.
   • Remove external power source from the sBitx to perform this step!
   • Connect the negative lead to the source and the positive lead to the drain.
   • There should be no continuity (no beep). If there is continuity, the IRF510 is shorted.

2. Gate-Source and Gate-Drain Continuity:

   • Remove external power source from the sBitx to perform this step!
   • There should be no continuity between the gate and source or gate and drain.
   • If there is continuity, the IRF510 is damaged.

3. Transistor Biasing Check (2N2219A & BFR106):

   • Measure the base-emitter voltage of Q11, Q12, Q13, Q14, and Q15. It should be around 0.6V to 0.7V.
   • If the voltage is too low or high, check surrounding resistors and capacitors for faults.

TR Switch Verification

1. Check Q9 (IRF4905)
   • Check if Q9 is switching correctly when TX is active. There should be approx. 12 volts at the drain (metal tab).
   • Measure the voltage at gate of Q9. If Q9 is always on, it might be shorted. If Q9 is always off, check if Q10 is functioning correctly.
2. Check Q10 (2N7002)
   • When TX is active, Q10 turns on, pulling the gate of Q9 to ground (~0V). This results in V_GS ≈ -12V, turning Q9 on and allowing +12V to pass to the PA circuit.
3. Check Q101 (2N7002)
   • Q101 works in conjunction with Q10 and Q9 to control when the power amplifier (PA) section receives +12V.
   • If the PA does not power ON in TX mode, check the following:
     • The Gate voltage should be HIGH (~12V) when TX is OFF and LOW (~0V) when TX is ON.
     • The drain Voltage should be LOW when TX is OFF and LOW when TX is ON. If it stays HIGH, Q101 may be faulty.
     • The Source should always be at ground (0V).

Power Supply Issues

1. Check the +12V Rail:

   • Measure voltage at key points in the PA circuit, especially at the drains of Q16 and Q17.
   • If voltage is too low, inspect the power supply and check for short circuits.

2. Inspect the HV Rail:

   • The HV rail powers critical stages of the amplifier. Measure its voltage and verify it is within expected limits.
   • Check D20-D25, C201, and associated components for faults.
   • Look for excessive voltage drop indicating a faulty component in the HV supply path.

Oscilloscope Measurements

Using an oscilloscope, check for proper signal flow:

RF Signal Input Verification

1. Measure RF at the Input (Q11, BFR106):

   • Use an RF probe to verify signal presence at Q11.
   • If absent, check the preceding stage for signal loss.

2. Check Signal at the Predriver (Q12 - Q15, 2N2219A):

   • The signal should be amplified at this stage.
   • If missing, suspect transistor failure or incorrect biasing.

Driver and Final Stage (IRF510 MOSFETs)

1. Check PA Bias Voltage:

   • Measure PA_BIAS1 at R75, R77, and D6 (Zener diode 5.6V).
   • Verify that D6 has 5.6 volts on the banded side when the TX is active.
   • If incorrect, check or replace these components.

2. MOSFET Gate Drive Test:

   • Verify that the gate of Q16 and Q17 receives a proper amplified signal.
   • If the drive signal is weak or missing, check the driver circuit and transformer T5.

3. Final Output Transformer (T7) Check:

   • Measure the output of T7 using an oscilloscope.
   • If there is no output, check transformer windings for continuity.

Final Output Stage Checks

1. Check Low-Pass Filter (L31-L41, C301-C308):

   • Verify that inductors and capacitors are intact and properly soldered.
   • If there is signal loss after this stage, suspect a faulty inductor or capacitor.

2. Antenna and Matching Network Check:

   • Ensure the antenna connection and matching network components (L402, L403) are intact.
   • A broken or mismatched antenna load can prevent proper RF output.

If you confirm a MOSFET, transistor, power supply, or HV rail failure, replace the faulty component and verify proper operation before applying full power.