A20112 Power Supply — Schematic

He desoldered the two old, tired electrolytic capacitors and soldered in new, high-quality replacements, ensuring the polarity was correct (minus to ground).

4.5 Protection: Overcurrent, Overvoltage, Thermal

Apply an external 12V (current-limited) to the output. Measure TL431 Ref pin – should be 2.5V. If not, check R12/R13. Then check optocoupler LED voltage (should be ~1.1V). If both are fine, the optocoupler transistor side on the primary may be open.

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a20112 power supply schematica20112 power supply schematica20112 power supply schematica20112 power supply schematica20112 power supply schematic
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