Many thanks for all the suggestions. I will go through them all, although it may take me a while....
On your last point, at first I thought that there isn't any feedback from the machine. However, apart from a DC OK (which will be an output), there is a wire labelled "Warning", which could possibly be an input, so I will look at that too. Regards Rob > -----Original Message----- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Peter > Coghlan via cctalk > Sent: 17 September 2017 21:26 > To: General Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> > Subject: Re: H7878 Fails Under Even Moderate Load > > > > > 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.
