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Which capacitors should be inspected and, if necessary, replaced before attempting to power up an original Mac (purchased January 1984, but "upgraded" to a 512Ke) that has been sitting unused for a few years.
Where can one find proper replacement capacitors, if needed? (I have soldering tools and know how to use them with the right parts.)
Are there any other components that should be inspected?
You can find a list of capacitors and other analog board components here: https://docs.google.com/spreadsheets/d/132RwzC8HM5ask-BdY_31txErOCwJDSkz099GY2XLpE0/edit?hl=en&hl=en#gid=0
This list is maintained by James Wages. The analog boards for the 128k, 512k, and Plus are compatible with 3 different revisions. Digikey part numbers for replacement parts are included. The first tab lists the capacitors most likely to need replacement, and the second tab lists all capacitors.
You're looking for capacitors that are bulging or leaking. A lot of people just replace all of them, or at least all of the first page ones, because the extent of the leaking often isn't revealed until you remove the capacitors and can see underneath them. For components besides the capacitors, you're generally looking for anything burnt or physically broken (such as diodes and transistors). Almost always you will want to re-solder the pins on the flyback transformer and all the connectors (J1, J2, and J3). These solder joints were notoriously bad.
The capacitors on the logic board have not been observed to leak. The logic board generally doesn't require attention unless the battery has caused significant corrosion. So if there is still a battery present, be sure to remove it.
The CRT and PSU circuitry contain high voltages. If the computer has been turned off for an extended period (years, as you indicate), the CRT capacitors have probably discharged. However, I always follow CRT discharge procedure when working on a compact Mac analog board. See https://archive.org/details/manuals-apple-safety for details. On the PSU side of things, there is a bleed resistor that will discharge the PSU capacitors within a few minutes of being turned off (but still plugged in).
The vertical sweep nonpolar capacitor (C1) is the most likely to fail. No inspection is complete without checking also the CRT yoke connector (plastic four-pin connector) for cracked solder joints, and perhaps the focus resistors (which fail by increasing resistance).
Those old CRT tubes are irreplaceable, and the connector on the neck will break the glass if you bump it. Pull that connector OFF as soon as the back is removed.
A few generic things to know when proactively replacing electrolytic capacitors:
There are at least two measuring devices you need to truly test electrolytic capacitors - a capacitance meter and an ESR meter, and some way to check for leakage current is also helpful (though a truly degraded, super leaky capacitor is unlikely to give a correct reading on a capacitance meter). Capacitors that have their capacitance within tolerance but have far too high ESR are commonly found in aged equipment.
Low ESR and "normal" electrolytic types are not interchangeable, especially not in switching power supplies ( a normal when used where a low-ESR is expected can cause malfunction and/or overheat; a low ESR type where unexpected can in rare cases cause stuff to oscillate by forming high-Q circuits, or worsen pulse loading on other components). BTW, if the negative marking band is golden or silver as opposed to white or black, that usually signifies a low ESR device).
Electrolytic capacitors degrade to some degree in unpowered storage - so will never-opened old stock! Most of that effect is reversible and automatically reverses when powered, but at the cost of much higher initial current draw (which can damage power supplies and the capacitors themselves).
Replacing electrolytics with types of a) a higher voltage rating, b) vastly smaller size, or c) different temperature grade can cause problems: Physically larger types sometimes have higher peak current ratings and better ESR specs, which the design might rely on. Higher voltage types can also be worse in that regard. 105°C types are more likely to have a flammable electrolyte inside, which might not fit some designer's fail-safe concept. Reading datasheets of original capacitor (if the type and manufacturer can be identified) and replacement is recommended.
Old teardrop shaped tantalum capacitors are a judgement call to replace or not - if they fail while connected to a high current capable source, they fail really catastrophically, but replacing them comes with some traps - in some cases, their better RF performance and lower ESR might be relied upon in the design. And theoretically, replacing with new tantalums needs meticulous care - most datasheets are exacting how they must be soldered (temperature and duration), with a risk of latent catastrophic failure if mishandled (this is how catastrophic failure looks https://www.youtube.com/watch?v=VknpLq7DkSM)
Also pay attention to the CMOS battery. I can't remember what Mac I had issues with but I remember it was pre-1993, a Quadra, beige. It had been unplugged for several years. If I remember right the battery looked like a small purple barrel, possibly with green markings. I thought the motherboard or PSU was shot because it wouldn't power on, but I left it plugged in for a few hours and came back and pressed the power button and it booted just fine.
All that said, the machine was kept in temperate storage and all capacitors were perfectly fine.
