MicroVAX issues

Is the 12V rail present at the power supply connector? If not and the other rails are working, it is likely that only a very small number of components and could be responsible. If the problem is intermittent, it is likely to be due to a bad solder joint between one of these components and the power supply PCB or a bad contact in a connector. If you trace the wire carrying the 12V line into the power supply, it should be possible to find which rectifier and reservoir capacitor produce it from the chopper transformer. The only other components involved are likely to be a filter choke, a fuse or circuit protector of some kind and maybe a low value resistor to sense current draw. Identify the components involved and check that they are all securely soldered to the PCB. Try tapping the components involved and the PCB in their vicinity with something insulated while monitoring the 12V rail to see if the problem can be provoked. Be very careful to avoid touching or short circuiting anything on the live side of the chopper transformer and the mains input side of the power supply in general if you have to run it with the cover off. Regards, Peter Coghlan.