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Abstract:
This video details the testing and debugging process of an aircraft radio receiver. The presenter first identifies that the unit's frequency selector uses a non-standard binary coding instead of the expected ARINC 410 (2-of-5) standard, necessitating the creation of a custom test selector. Frequency selection functionality is verified, requiring a minor repair to a faulty switch. Audio output quality is then assessed using a signal generator, revealing significant distortion above 40% modulation depth. Troubleshooting using schematics and oscilloscope measurements reveals that the audio saturation originates at the squelch gate. A critical finding is that the actual squelch circuit differs from the provided schematic. Reverse engineering the true circuit identifies the cause of distortion as the audio signal level exceeding the squelch diode's bias voltage, leading to clamping. A modification involving adding a parallel resistor to lower the bias voltage successfully resolves the saturation issue, achieving acceptable distortion levels (~2.4% at 85% modulation depth) after gain adjustments. Finally, the video begins the process of testing and tuning a custom-built ground plane antenna for the aviation band using a spectrum analyzer and directional coupler, measuring cable loss and initial antenna resonance.
Testing and Repair of an Aircraft Radio Receiver
0:00:04 Non-Standard Selector: The radio's frequency selector uses a binary coding system, not the standard ARINC 410 (2-of-5 code).
0:00:33 Custom Test Setup: A crude selector board with individual switches was built to manually input the binary codes for testing frequency selection.
0:00:53 Frequency Selection Test: The auto-positioner system is tested by inputting binary codes corresponding to known frequencies (e.g., 131.20 MHz).
0:03:01 Initial Failure & Fix: The frequency selection initially fails due to a faulty microswitch on the custom selector board. The switch is replaced (0:03:15).
0:03:24 Successful Frequency Selection: After the fix, the frequency selection test (e.g., setting 121.15 MHz) works correctly.
0:03:31 Audio Output Test Setup: The receiver's audio output is tested using a synthesizer generating a 1 kHz AM signal (initially 300 Β΅V, 30% modulation depth).
0:04:04 Audio Distortion Identified: Oscilloscope observation shows audio signal distortion appearing when modulation depth exceeds approximately 40%.
0:04:38 AGC Performance: The Automatic Gain Control (AGC) is observed to be functioning, maintaining a relatively constant audio output level despite input signal level changes.
0:05:16 Troubleshooting Audio Saturation: Investigation begins to find the source of the audio saturation/distortion.
0:06:09 Gain Adjustment Impact: Adjusting the internal audio gain (R118) improves but does not fully resolve the distortion, especially at higher modulation depths.
0:07:07 Locating the Saturation Point: Signal probing reveals the audio is clean before the squelch gate diode (CR10) but saturated after it (0:08:00).
0:08:32 Schematic Discrepancy: It's discovered that the actual squelch circuit wiring on the board differs significantly from the provided schematic diagram.
0:08:48 Reverse Engineered Circuit: The actual schematic, determined via reverse engineering, shows the audio signal path and bias arrangement for the squelch gate diode.
0:09:22 Cause of Saturation Explained: In the actual circuit, the audio signal voltage can exceed the squelch diode's bias voltage (~12V), causing the diode to turn off intermittently and clamp (saturate) the signal.
0:10:05 Implementing the Fix: A resistor decade box is connected in parallel with a bias resistor (R115) in the squelch circuit to lower the bias voltage.
0:10:36 Fix Confirmed: Adding a 20k resistor in parallel immediately resolves the saturation issue, resulting in a clean audio waveform even at high modulation depths (85%).
0:11:16 Final Audio Adjustments & Measurement: The audio gain is readjusted for optimal performance, and distortion is measured using a distortiometer, achieving ~2.4% THD at 85% modulation depth (0:12:05).
0:12:14 Antenna System Testing: The focus shifts to testing the antenna system, starting with measuring the loss of a 15m RG58 cable (~2dB at 120 MHz).
0:13:20 Custom Ground Plane Antenna: A homemade ground plane antenna with adjustable element length is introduced for testing.
0:13:45 Antenna Resonance Measurement Setup: A spectrum analyzer with a tracking generator and a directional coupler is used to measure the antenna's return loss / resonant frequency.
0:15:04 Initial Resonance Measurement: The antenna's initial resonant frequency is measured at 110 MHz, which is below the desired aviation band (118-137 MHz).
0:15:13 Antenna Tuning Required: The antenna element needs to be shortened to raise its resonant frequency into the target band. The video ends as this adjustment begins.