Hi again, Thanks for the tis on SMPS faults, I have now got it up and running. The first fault was a failed Y suppresion capacitor on the input - this had split open with a 'pop', easy to spot and replace. Then the PSU gave 1.1V on its 5V output, this was because the output was not regulated and went over volts, which triggered an SCR on the 12V rail. I removed the SCR and thought it was faulty, but the replacement had exactly the same resistance (80 ohm from gate to cathode in either polarity) so I fitted it anyway. I checked the voltage at the opto isolator and saw it vary as a changing voltage was applied to the 5V output, so this looked good. I tried drawing a circuit diagram based on the layout, but found this very difficult. However, I found two bad soldering joints when tracing the circuit. These were both at points when a trace on top of the PCB met a component then carried on underneath the PCB - ie the circuit relied on the solder forming a joint both under and on top of the PCB. I then applied varying DC to the input (up to 110V, this is a 230V supply). At around 80V, the board made a loud whistling noise, and the 5V and 12V outputs came up. However, as the input voltage was varied the output varied in step. I then checked the quad comparator chip (CA338, equivalent to LM338) and decided it was faulty. This was soldered in so replacing it with a socket was a pain. Anyway, with a new chip fitted the PSU came up fine. I still don't understand the whole circuit, there were around 5 transistors in an ossicaltor circuit which I couldn't get to grips with. I found the following sites helpful: http://www.sencore.com/custsup/techtips.htm - I used the 'dynamic testing' approach, looking at the SU in blocks. http://www.datasheetarchive.com/ Once back in the machine everything seemed normal again. I dnt seem to have a boot disk, but that is another story! Thanks, John _________________________________________________________________ Get Hotmail on your mobile. Text MSN to 63463 now! http://mobile.uk.msn.com/pc/mail.aspx