Dell GX280 \ LiteOn PS-5161-7DS power supply issues

I'm trying to troubleshoot a LiteOn PS-5161-7DS power supply in a Dell GX280. 

The system will power on but will not start. The power LED flashes orange. It will however run with a known good standard ATX PSU. Unfortunately said PSU cannot be used as it is not an SFF type.

I have checked the PS-5161-7DS and found two failed output capacitors, C203 and C510 which I replaced. This however did not solve the problem.

I have found that a few seconds after power on, the PSU seems to go into a cycling loop and starts making a ticking noise and restarting. This happens on both the GX280 motherboard and when trying to power a generic P4 system. At this point the power LED on the GX280 starts flashing in time with the PSU ticking.

The PWR_OK line also starts pulsing in time with this ticking noise. So far as I have checked, it does not appear to go high prior to this.

I also discovered I cannot start the PSU manually on the bench by pulling the PS_ON line low, which seems odd. In another forum, is was shown that pulling the PWR_OK signal high in addition to this started the PSU. This does not seem normal, as a standard ATX PSU is supposed to output this signal, not start because of it.

Are these known faults with this PSU model or (considering Dell's history with non-standard PSUs) part of normal operation? Is the PWR_OK signal different on this model for some reason? Is the inability to manually start it normal?