> > Can anyone explain the behaviour? > It's hard to know what to do from a distance but here's what I think I'd look at if I was faced with this problem.
I think failing under moderate load could be explained by one or more of the following possibilities and probably others I haven't thought of: - The power supply is not capable of producing sufficient current. Check how the voltage across the main input smoothing capacitors which have been replaced varies while the load is applied. If it dips severely, check input components such as filters, surge limiting devices, connectors and so on for breaking down under load. If you can measure the ripple here while changing the load, an increase may indicate that one side of a fullwave rectifier is going high impedance or open circuit under load. Also check for damage that might have occurred in the struggle to remove the capacitors. - Overcurrent sensing is kicking in too soon. Look for low value, moderate to high power resistors in the output current paths and check their values and how the voltage across them varies with applied load. If they seem good, check associated small components. - Regulation is not working correctly. Try to figure out how the regulation is supposed to work and take measurements to see how it is behaving in reality. Easy to say but may be difficult to do in practice. If the PSU uses a chip to provide regulation and drive to a chopper device, the data sheet for the chip may provide some guidance on how it is supposed to work. Be careful taking measurements as accidentally shorting something out could lead to big bangs. - The PSU may be looking for feedback from other parts of the machine in the form of remote voltage sensing or remote current sensing or inputs which cause particular supply lines to be switched on or off or come up in a particular order. If this is the case, the fault may be elsewhere in the machine or may be as result of operating the power supply without it being connected to the rest of the machine. Hope this helps. Regards, Peter Coghlan.
