Could be going into hiccup mode (short/overload protection) due to a fault on secondary side. Could be a bad capacitor in the primary side too, often a small cap providing the initial charge to fire up the SMPS IC is the culprit.
What does the SMPS use for a control IC?
Was the power supply ever used under heavy load for some time? I've heard of some of these having aluminum wire in the transformer, getting too hot under load and the insulation breaking down between turns, consequently blowing the switching transistors.
Checked the output rectifier diodes?
Well, this is totally bizarre. I've done a lot of troubleshooting today but ultimately, have arrived back at the same problem!
To answer your question, I would say it had a fair amount of load - up to 20A at 13.8VDC with a 50% duty cycle for a couple of hours at at time
I removed the dual Schottky rectifier packages and tested them - fine, not shorted, not open.
Removed the main transformer - fine, normal low DC resistance and not shorted between primary to secondary and neither to core
Removed ?base transformer - as above
Checked ESR of main filter caps, low. Capacitance in expected range given tolerances.
Checked all fusible resistors and diodes - none open, none shorted.
Checked the controller IC which is (probably fake) Fairchild KA7500B - this outputted a sine wave at 28kHz on pin 8 when 15VDC was supplied to the VCC pin.
Put it all back together, dim bulb tester. More odd fizzing - nothing. IC not receiving any voltage. Thought I had fried it. Turned out the second chopper transistor has failed short-circuit! I'm missing something here and can't for the life of me figure it out.
Excuse the condition of the board - it is getting tired now given the amount of soldering and desoldering that has occurred!