The MP cap blew in the PSU. Before I order a new one, does it have to be a MP type cap? Or will any cap with the same values work?
Thanks!
Anonymous
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If you're referring to a Metallized Paper capacitor (the common brand is RIFA but there were others) used for line RFI suppression, then it is not necessary to replace it with the same type. These capacitors have such a poor reputation that it isn't even easy to find them today. What is important is that any replacement have the correct ratings.
Line RFI suppression comes in two forms: across-the-line or X type, and line-to-ground or Y type. You need to understand which type of circuit the failed capacitor is a part of. If the capacitor's legs are connected to the Live and Neutral wires from the socket, it is an across-the-line or X type suppression cap. A replacement must carry an X2 or better rating for safety reasons. It also must have the correct voltage rating and capacitance value, either the same as the failed cap, or chosen based on the schematic. If either of the capacitor's legs are connected to Ground (Protective Earth), it is a line-to-ground or Y type suppression cap. In this case a replacement must carry a Y2 or better rating for safety.
Most X2 or Y2 suppresion caps today do not use metallized paper construction. The most common materials are polypropylene film or polyester film.
C1 on the Apple IIe PSU is the X2 cap that sits between the line and the neutral. It must be replaced with a modern X2 rated cap with the same capacitance and whatever voltage is the mains voltage in your country. You can also remove it completely as it doesn’t affect the work of the PSU.
But you shouldn't.
Because...it's a noise suppression capacitor and it filters out a lot of noise on the mains in both directions.
IT filters out noise entering the power supply and it filters out noise that the power supply generates and injects into the household mains.
That capacitor was put there for a good reason.
So buy a good quality replacement part and install it.
Noise entering the power supply cannot affect it in any way. This capacitor is there only to stop the outward noise generated by the PSU. Filtering in both directions does happen (because it's a capacitor), but the inward filtering is not needed and the noise cannot propagate to the motherboard.