Task: Replace Electrolytic Capacitors to Prevent Circuit Board Damage on Retro Computers
Needed: Pliers, soldering station, solder, tweezers, new capacitors, 99% isopropyl alcohol
Time: 1-3 hours
Introduction
Electrolytic capacitors were commonly used on retro computer circuit boards. These components are fairly reliable on circuit boards from the 1970s and 1980s. However, cheaper variants used in the 1990s are highly prone to leaking acidic substances that can damage circuit board traces, contacts, and components. I have three classic Macintosh computers (Classic II, LC, and LC III) that all have leaking capacitors on the main logic board and the analog board of the power supplies. Fortunately, I was able to replace them all and there was no apparent damage to the circuits. This is a must complete project for anyone with retro computers from the 1990s. Otherwise, they are all ticking time bombs waiting to fail. Unfortunately, this does require some soldering skill and can be time consuming.
Instructions
First, source new capacitors for your particular computer model. A company called Console 5 sells kits with all the necessary capacitors for most models. For example, here is the kit I used to replace the capacitors on my Mac SE. I recommend double checking all capacitors as Console 5 does sometimes make mistakes. There are also generic capacitor kits that are good to have on hand for one-offs. I have this one from Amazon. These are available on eBay too.
Second, remove the logic board or analog to be repaired and inspect it for leakage and damage. Below is the logic board for my Mac LC (an LC II board) that had quite a bit of leakage.
Third, remove the faulty capacitors after taking photos to indicate location and type. There are several different methods that people use to do this without lifting the solder pads from the board which is a real risk that can complicate the process. I decided to go with the twist method. Here, you use some needly nose pliers to grasp the capacitor and twist it back and forth about 1/5 a turn each way while gently pressing down. This fatigues the legs of the capacitor and they eventually break off leaving the pad underneath intact. I have yet to lift a pad using this method. I recommend watching this video from Adrian’s Digital Basement for more information on the method. Once off, the board can be inspected more closely. The leakage is even more apparent. Note that the analog power supply boards tend have the bigger through-hole type of capacitor. These need to be desoldered using a desoldering iron.
Fourth, it is time to get the capacitor leakage cleaned off the board. Some people use vinegar to clean the juices off. I prefer to use soap and water given soap is a mild base that can neutralize the acid. I use a toothbrush to scrub the entire board with soapy water after removing the socketed ICs and any RAM sticks. This is followed by a water rinse. I then rinse the board and scrub some more with 99% isopropyl alcohol (IPA). This also serves to get any remaining acid off while also helping the board dry. Be sure and let it the board completely dry before any powering on so residual water doesn’t create a short. I use compressed air to blow residual water out from under chips or inside connectors.
Fifth, it is time to clean up the solder pads be removing residual capacitor legs and old solder. I first use the solder iron with a little fresh solder to loosen the residual legs and lift them off. I then use some solder wick with the soldering iron to clean the pads of any residual solder. Be careful not to apply any pressure so as not to damage or lift the pad. Once the solder is off, I clean the pads with 99% IPA to remove flux residue. Note that some pads can get a dark film over them from the acid damage. When this happens, I use the sharp point of some tweezers to gently scrape it off revealing the silver pad below. Below are the cleaned pads from the board shown above.
Sixth, it is time to solder in the replacement capacitors. I used tantalums in this one. You can also use new electrolytics if you choose. Note the polarity of the capacitor should match the + symbol printed on the circuit board. For tantalums, the dark orange stripe is the + side as shown in the image above. I first apply some solder to one pad and then lower in the tantalum with a pair of tweezers while heating the solder. Hold it in place until the solder cools and then apply a blob of solder to the other side. That is it! Double check all soldered components to make sure you didn’t miss any solder pads and that all capacitors are in the correct orientation.
Seventh, once the board is bone dry, it is time to assemble the computer and give it a test. Mine have all worked on the first try.
Comments
This is a bit intimidating the first time through. It is also a bit tedious checking and double checking to make sure everything is done correctly. The Mac LC boards I recapped above went much smoother than the Mac SE that served as my first run through. The analog boards that come with the power supplies are in some ways easier because they are larger through-hole capacitors. Below is a photo of the two power supply boards I recapped for these Mac LCs. The PSU on the right had quite a bit of leakage and was not working. It works great after the recap.
