A20112 Power Supply Schematic |link| Instant

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.

If you are assembling a supply based on this schematic, consider these best practices: a20112 power supply schematic

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