Worried that switching on your old computer will finish it off? Here is how to restore a vintage computer safely: don’t power it on, open it, remove the battery, and check the capacitors first.
This guide is part of the site’s wider coverage of retro computing.
| Item | Value |
|---|---|
| Apple Lisa picture tube voltage | Over 15,000 volts DC |
| Apple Lisa clock battery pack | 4.8 V NiCd |
| Lisa pack hold time when unplugged | Up to 10 hours |
| Compressed-air limit when cleaning | 60 PSI or under |
| Humidity below which antistatic care is wise | Under 40% relative humidity |
The figures come from the Apple Lisa capacitor guide and a Low End Mac column. They can change, so check the source before you rely on them.
Inspect first, power on later
One published Macintosh Classic build log puts the rule bluntly: don’t turn it on, take it apart. Its author argues that with old power supplies, RIFA capacitors, leaky batteries and electrolytic capacitors, opening the case first can save a lot of trouble, as Jeff Geerling’s build log of August 2025 explains.
That log names two primary killers of a compact Mac: leaky batteries and leaky capacitors. A 2007 Low End Mac column gives a longer list of causes for why older Macs die, as Low End Mac reports:
- Dust and dirt
- Oxidized electrical connections
- Failing electrolytic capacitors
- Fractured solder joints, mostly in the original compact Macs
A sensible order of work follows from that list:
- Remove the battery.
- Deal with stored high voltage.
- Clean the machine and its contacts.
- Test the capacitors before replacing any.
- Check for shorts, then power up carefully.
If you are still choosing a machine, the guide to vintage computer collecting is a companion read. The comparison of 8-bit vs 16-bit computers adds background on how generations differ.
Remove the battery before anything else
On most old Macs the PRAM battery is soldered or socketed to the motherboard, and a leak damages everything around and below it, according to the same build log. That is why the battery comes out before any other step.
The Apple Lisa guide says to remove the I/O board battery pack before anything else, because NiCd packs of that age leak. Its advice is that the alkaline residue is neutralised, the area cleaned with isopropyl alcohol, and continuity checked on every trace in the affected area.
Chemistry matters for the cleanup. The Macintosh Classic guide notes that the Classic’s cell is lithium and that its residue is neutralised with bicarbonate of soda, not vinegar.
For a replacement, one Classic restorer fitted a coin cell in a MacBatt battery adapter, as the build log describes.
Treat the stored high voltage with respect
Apple’s service manual states that the Lisa picture tube “runs at over 15,000 volts DC and contains a high vacuum”, as the Lisa guide quotes. The anode keeps that charge after unplugging and can recover charge after it has been discharged.
The Classic build log adds that CRTs use deadly voltages and can hold a charge for days, weeks, or longer after being unplugged, per Jeff Geerling. It also warns that the anode clip must be pinched together to come out of the tube, not pulled straight up.
The mains side holds charge too. The Classic guide says 240 V mains rectifies to about 340 V DC, and the Lisa guide says its reservoir capacitors hold their charge after power-off: unplug, wait, and measure across each one before touching the primary side.
Follow a procedure written for your exact model. For the Classic, Apple’s order is to discharge the CRT before putting on a grounding wriststrap, per the Classic guide.
Clean the machine and its contacts
Cleaning is cheap and often decisive. The Low End Mac column calls opening and cleaning an older Mac’s power supply mandatory, because these are most often filled with dust and dirt, some of it partially conductive, per Low End Mac.
The same column describes how to keep static under control:
- Wrap aluminum foil around the vacuum nozzle and attach the foil to the chassis of the machine being cleaned.
- Add antistatic protection if the relative humidity is under 40%.
- Keep compressed air regulated to 60 PSI or under so small components are not blown away.
Contacts need attention as well. The author disconnects, cleans and reconnects every internal connector and plug-in item, and notes that Mac floppy drives act as air filters and often need serious cleaning.
Solder joints deserve a look. The build log reflowed a couple of joints with fresh solder and cleaned a few areas with isopropyl alcohol, according to Jeff Geerling.
Should you recap? Test before you replace
Aluminium electrolytic capacitors can dry out or leak and corrode the board, pads, traces and nearby components, and they can leak in storage as well as in use, per Adam’s general advice page. A bad one is not always visible, so testing is often the only confirmation.
The same page warns that recapping is not a fix-all and should only be attempted with reasonable cause. Identify suspect parts by inspection or a schematic, and test them properly instead of replacing parts at random.
The Classic guide lists what to look for:
- Dark haloes or dried crust at the base of the cans
- Dull or green solder on nearby IC pins
- Thinned or broken traces under a loupe
- Domed tops, crust at the leads and staining on through-hole parts
Choosing replacement parts
Do not borrow a list from a sibling model. The Classic guide counts eight capacitors on the Classic logic board but 13 or 17 surface-mount electrolytics on the Classic II board, depending on version, and the build log says to match your board’s part number because analog boards vary even within a model.
Adam’s page gives these substitution rules:
- The capacitance must be the same, and the voltage rating the same or higher, within reason.
- Polymer or tantalum parts use a solid electrolyte, so they cannot physically leak.
- Ceramic parts are microphonic and change capacitance with voltage, so they suit some circuits poorly.
Fitting them
Orientation is where many repairs go wrong. On aluminium electrolytics the negative side is usually a white stripe on through-hole parts or a black bar on SMD parts, while a tantalum stripe marks the positive terminal and a reversed tantalum fails short, according to the Classic guide.
Board markings are not always right: the Commodore CD32 audio circuit is one example of a factory error, per Adam’s page. He recommends photographing the original, double-checking your work, and using a fume extractor.
For SMD removal, that page describes a hot air station with a small nozzle, high flow and 300C, plus kapton tape and aluminium foil to protect the surroundings. This is rework for experienced hands, and the page says plainly that recapping should be attempted only if you know what you are doing.
Why slowly raising the voltage can backfire
A common idea is to bring a dead machine up slowly on a variable supply. The Low End Mac column says techniques that work on old radios are not safe for Macs, because they use switching power supplies that do not start until the input reaches a threshold of roughly 40 to 80 volts AC, per Low End Mac.
Once such a supply starts on low input, it sees a brownout and draws much more current to hold its rated output. The author considers that potentially harder on the electrolytics than switching on normally, and prefers to re-cap as needed.
Before the first power-up, the Classic guide says to inspect the joints, trim the leads and check each rail for shorts to ground. After that, check each rail at the logic board power connector against its nominal value.
Frequently asked questions
Can I plug in an old computer to see if it works?
Show answer
The sources reviewed advise against it. The build log says to take it apart first, since old batteries and capacitors may already have failed, as Jeff Geerling explains.
Should I raise the mains voltage slowly with a dimmer?
Show answer
For switching power supplies, as found in Macs, Low End Mac says it can be more stressful than switching on normally. A schematic and careful current monitoring would be needed for any workaround.
Do I have to replace every capacitor?
Show answer
No. Adam’s general advice says recapping is not a fix-all and should follow inspection and testing of the suspect parts.
What should I do about a leaking battery?
Show answer
Remove it first. Neutralise the residue to suit the chemistry, clean with isopropyl alcohol, and check continuity on every trace nearby, as the Lisa guide advises.
