I guess there are a number of possibly causes. I too would have gone for caps - did you change them all (electrolytics)?

The fact it is generating some voltages suggests the PSU is basically good and the FETS are driving the transformer plus there are no short circuit rectifier diodes. But the PWM controller is throttled back to a low duty cycle.

Generally the PSU would monitor one of the output rails (5V probably) and use an opto isolator to send a feedback signal to the PWM controller. In my experience the opto isolator can fail and cause this sort of problem. ALso would get the datasheet for the PWN controller IC and you will probably find some example application circuits for that and your PSU is likely to be a very similar design.

Have a look for resistors in the feedback circuit (either side of the opto isolator) changing value and going high resistance. Also small ceramic capacitors around the opto isolator going leaky.

Possible faulty PWN controller IC or some reistors/capacitors near to it. Possible bad reference voltage (zener diode or 431 IC)

It's also possible that the PSU has active PFC and the PFC circuit isn't working so the voltage to the power FETs driving the switch mode transformer are running at too low a voltage.

Look for any large polyester capacitor (like these ones https://www.edgefx.in/how-to-find-the-value-o … ster-capacitor/ ) near the FETs/transformer on the primary side, typical 450V 330nF or larger. If you see one pull it from the circuit and check for low capacitance with a test meter.

Possible shorted turn somewhere in the switch mode transformer, but really I would expect that to cause it to shut down, pulse, or the drive FETs to go bang.

Those are at least a few ideas to check.