1980 vintage home computer power supply repair

This is a off-line switchmode supply delivering +12V, -12V and +5V. The fault was that it wouldn’t work and a couple of capacitors on the mains input were in a bad way.

SMPS power supply repair faulty components

Step one was to remove the faulty looking parts and clean the brown gunk off the board. Step 2, check the bridge rectifier and main fuse – OK. Applying current-limited mains power with a 0.5A load on the 5V output and nothing worked. A bit of probing with a multi-meter quickly showed that the inrush surge limiter (NTC) was open circuit.

SMPS power supply repair mains input filter

After replacing the inrush limiter, a couple of X-class and one y_class capacitor and it tested OK. Not knowing  the rated output currents, it was tested with about 1A load on the 5V output and 0.5A load on the 12V output. Ripple on all 3 rails was less than 50mV.

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