This video details the repair of a faulty Senor PA881 stereo power analyzer, a sought-after vintage instrument featuring independent, isolated left/right channels, built-in loads up to 250W, and various measurement capabilities (RMS voltage, dBm). The unit presented with incorrect meter readings and non-functional auto-ranging. Troubleshooting identified two failed attenuation relays. The primary cause of the failure was traced to the use of an incorrect 15V power supply connected to the unit's 12V-only battery input, which bypassed the internal voltage regulator and overstressed the 12V rail and associated components, particularly the relays. A previous user modification that bypassed the original isolation on the oscilloscope outputs was also discovered and reversed. The repair involved replacing the damaged relays and ensuring the correct power input voltage via the regulated AC input. Post-repair testing demonstrated restored range switching, accurate meter readings across its dynamic range, and functional isolated outputs and internal loads, successfully bringing the instrument back to operational status.
Repairing a Senor PA881 Stereo Power Analyzer
0:00 Instrument Introduction: The video focuses on repairing a Senor PA881 stereo power analyzer, highlighting its features like independent, isolated left and right channels, internal 250W loads, meters (dBm, RMS volts), filters, and a built-in speaker.
0:00 Faulty Unit & Modification: The unit is faulty and expensive. It was previously modified, specifically bypassing the isolation on the oscilloscope output ports by tying them directly to the input loads.
0:01 Initial Power-up & Symptoms: When powered on, the fan briefly runs. The auto-ranging meters are unstable, settling on random values, and react erratically.
0:01 Input Testing: Applying a known 1 kHz, 1V RMS signal shows incorrect readings on both channels, indicating a fundamental measurement problem.
0:03 Auto-ranging Issue: Testing with 0.1V RMS confirms one channel hunts for the correct range while the other reads a value off by a factor of 10. Manual range selection also yields incorrect readings.
0:04 Relay Diagnosis: Manually cycling through ranges reveals that some relays are not clicking, suggesting a physical failure in the range selection or attenuation circuitry.
0:04 Schematic Analysis: Reviewing the block diagram and schematic shows independent channels with input relays, attenuation relays (AC and DC paths), RMS-to-DC conversion, and isolated outputs via opto-isolators. The focus shifts to the attenuation relays.
0:07 Reverting Modification: The previous user modification connecting the oscilloscope outputs to the load inputs is located on the board and reversed to restore the original isolation circuitry.
0:11 Relay Testing (LCR Meter): Using an LCR meter to measure the inductance of the relay coils reveals two attenuation relays (one on each channel's attenuation path) have no inductance, confirming they are internally broken (open coil).
0:13 Suspecting Overstress: The speaker decides to replace all eight relays as they are the same part number and show slight discoloration, indicating they may have been stressed.
0:13 Power Supply Architecture: The power supply converts a single input into multiple isolated power domains using oscillators, transformers, and regulators. Relays are powered by the 12V rail.
0:15 Root Cause Identification: The included 15V power adapter was incorrectly connected to the unit's 12V battery input, bypassing the internal voltage regulator. This resulted in the main 12V rail running at approximately 15V, overdriving components including the relays.
0:16 Power Supply Voltage Check: Measuring internal voltages confirms the isolated Β±5V supplies are correct, but the main 12V rail measures ~15V when powered incorrectly.
0:17 Power Adapter Correction: The plug on the power adapter is modified to fit the correct AC input socket, ensuring power goes through the internal regulator. The main 12V rail now measures the correct ~11.7V.
0:19 Component Replacement: All relays are replaced (with some difficulty finding good replacements), along with some degraded relay driver transistors. The main power supply capacitors are also replaced (recapped) for improved longevity.
0:20 Post-Repair Testing (Audio Line): Testing with the corrected power input and replaced components confirms that the auto-ranging now works correctly across the audio line input (1mV to 7V RMS), and the meters display accurate values.
0:22 Oscilloscope Output Check: The isolated oscilloscope outputs are functional, providing a significantly amplified signal proportional to the input (e.g., 1mV in yields 1V out). A minor difference in harmonic distortion (6dB 2nd harmonic) between channels is noted on the scope output.
0:24 DC Voltage Measurement Check: DC voltage measurement via external inputs works, though the right channel shows some non-linearity at low voltages.
0:25 Dummy Load Check: The internal dummy loads are confirmed functional and measure correctly (2, 4, 8, 16, 32 ohms) on both channels. The DC voltage detection feature for the load connection also works.
0:27 Calibration Points: The schematic shows numerous potentiometers for calibration (gain, offset), noted as tedious to adjust precisely.
0:28 Speaker Function: The built-in speakers are confirmed to work.
0:28 Conclusion: The repair was successful, restoring the instrument's functionality. The speaker plans to use it for future high-power amplifier testing.