A20112 Power Supply Schematic -

Measure voltage across C4 (VCC pin of PWM IC). It should rise to 12–18V. If it stays below 10V, replace C4 and check R5 (startup resistor). If still low, the PWM IC may be shorted.

Measure across C1. You should see ~300V DC (for 230V AC). If zero, check F1, MOV1, DB1. a20112 power supply schematic

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. Measure voltage across C4 (VCC pin of PWM IC)

Share your findings in the comments below. And remember: the schematic doesn’t lie—but component tolerances do. Always verify, measure twice, and power up safely. Keywords used: a20112 power supply schematic, a20112 circuit diagram, flyback converter, PWM controller, TL431 feedback, switching power supply repair, UC3842 application, 12V power supply troubleshooting. If still low, the PWM IC may be shorted