General Advice

I’ve listed below some general advice that you might find helpful with vintage computer restorations, in case it may be of use to you too.

Electrolytic Capacitor Replacement

What are electrolytic capacitors?

Aluminium electrolytic capacitors are commonly used for filtering, smoothing, and decoupling in both high- and low-voltage electronics. They are quite cheap in comparison to their solid-electrolyte counterparts (such as tantalum and polymer electrolytics), so are very common in consumer electronics.

Why are electrolytic capacitors a potential problem?

Aluminium electrolytic capacitors typically comprise aluminium windings which are coated with a liquid electrolyte, which can dry out over time (negatively affecting the performance of the capacitor, often causing them to fail dead-short), or even leak out and cause corrosion to the PCB, pads, traces, and components.

Their useful lifetime is highly dependent on the specific application that they are used in (i.e. frequency, ripple current, steady-state voltage versus rating) and temperature, as well as the manufacturer and series of the specific component.

Electrolytic capacitor leakage in a Microsoft Xbox.

Any system can fall victim to electrolytic capacitor failure, particularly those manufactured during the capacitor plague, so these could be a cause of various issues. Certain series of capacitors are often more prone to failure due to manufacturing defects, SMD capacitors are often more prone to failure too due to a higher likelihood of seal breakdown, and supercapacitors are often more prone to failure due to using aerogel rather than a standard electrolyte.

Electrolytic capacitors can still leak in storage, not just during use.

Certain systems are well known for electrolytic capacitor failure, including:

How can I tell if a capacitor is bad?

Problems with a significant number of mains-powered devices (televisions, display monitors, amplifiers, network gear etc.) can be due to cheap or ageing electrolytic capacitors, it is not always possible to spot a bad electrolytic capacitor (or any other component for that matter) just by looking at it. Some failed capacitors bulge, leak, blow their gasket at the bottom, or split their case pressure vent at the top, but often you can only confirm that the capacitor is bad by testing it.

Electrolytic capacitors have several failure modes: they can go open or short circuit, their ESR can go out of spec, or they can suffer from a mechanical or chemical failure. Ideally, they should be tested out-of-circuit using a capacitor tester such as the Peak ESR70 to give a good indication of their actual health – if the capacitor is high-voltage it should be discharged before removal, and note that it might test OK with a low test voltage then fail at its working voltage.

Recapping a system is not a fix-all solution and should only be attempted if you have reasonable cause. Instead of “shotgunning” parts at something and potentially making things worse, first identify potentially problematic parts by physical inspection and/or using a schematic, and test them properly.

Replacing electrolytic capacitors

It can definitely be worth replacing capacitors in systems that require it, however it can be a fiddly job which is not easy to do properly, even with the experience and tools required for through-hole and SMD soldering/desoldering – as such, recapping a system should only be attempted if you know what you’re doing.

Choosing the proper parts

There are often several production variants of different systems, each of which may have different electrolytic capacitor values and locations, so take note of which you have. You can also often buy capacitor packs for common systems from sellers such as Console5, but I usually just make up my own by noting the specifications of all of the parts required and ordering a set of parts as required.

When substituting electrolytic capacitors, the capacitance needs to be the same, and the voltage rating can be the same or higher (within reason, as typically the higher the voltage rating of a capacitor, the higher its ESR).

Where possible, I like to use tantalum or polymer electrolytic capacitors for SMD capacitors and polymer electrolytic capacitors for through-hole capacitors – these are equivalents to standard aluminium electrolytic capacitors but use a solid electrolyte, meaning that they cannot physically leak. It’s common in some systems such as the Game Gear for people to use ceramic capacitors in place of SMD electrolytic capacitors, which don’t have an electrolyte and aren’t polarised – these are microphonic and their capacitance changes with the applied voltage, so they are not appropriate for use in some applications, and I try not to use them as replacement parts where possible.

When you’re going through all this effort to recap something, be sure to use high-quality replacements from known brands (such as Panasonic, Nichicon, Rubycon, KEMET, etc) and reliable suppliers (such as RS Components, Mouser, DigiKey, etc).

General notes on installation

When fitting new electrolytic capacitors, you must take care to ensure that the value, voltage rating, and orientation of the new capacitor are correct – electrolytic capacitors are polarised, so must be installed the correct way around, else they’ll get hot when powered on (and probably explode).

The polarity is marked on the case: for aluminium electrolytic capacitors, the negative side is usually shown by a white stripe (for through-hole) or a black bar (for SMD); for tantalum capacitors, the positive side is usually shown by an orange or white bar (for SMD). This catches a lot of people out!

Some PCBs don’t have polarity markings for capacitors on their silkscreen, and even if they do you can’t always trust them due to factory design errors (for example, the audio circuit on the Commodore CD32), so care must be taken to match the orientation of the new capacitor with the original. Make sure to take lots of “before” pictures for your reference, and double-check your work throughout the process.

I would recommend using a fume extractor when doing electronics rework, primarily because of the solder and flux fumes, but also because leaky SMD capacitors often pop and let the magic smoke out once they’ve heated up.

Installing the new capacitors

For SMD capacitors, I usually remove all of them at once using a hot air rework station (small nozzle, high flow, 300C) with kapton tape and aluminium foil to protect the surrounding areas, or by carefully twisting them off using needle-nose pliers whilst pushing downwards instead of pulling upwards – note that this technique may not be suitable if the pads are damaged, as they could delaminate from the board.

The pads can then be cleaned up by adding fresh solder to remove corrosion or oxidation, using desoldering braid or a vacuum desoldering station to remove all the solder, and cleaning the area thoroughly to remove any electrolyte and flux using 99% IPA and an ESD-safe brush. The new parts can then be installed – I usually apply some flux to the pads, hold the capacitor in place with tweezers, then tack both sides in place.

As for through-hole capacitors, I usually remove each one-by-one using a vacuum desoldering station (medium nozzle, 350C or higher depending on the thermal mass of the PCB), then immediately install its replacement part – this minimises the likelihood of getting things wrong. Again, the area should be thoroughly cleaned to remove any electrolyte and flux using 99% IPA and an ESD-safe brush.

Mains Filter Capacitors

A lot of equipment from the 1980s features RIFA-branded metallised paper mains filter capacitors, including the Acorn BBC Model B, which are known to fail short-circuit and release clouds of acrid smoke during operation. Other equipment, such as the Commodore PET 8032 and 8050 FDD, have mains filter capacitors built into the mains connector before the power switch, which can also fail in a similar quite explosive manner. Mains filter capacitors like this should be identified and removed or replaced (using suitable X/Y rated safety capacitors) as part of servicing vintage equipment before attempting power on.

Failed RIFA mains filter capacitor from an Acorn BBC Model B.

CRT Display Safety

A word of warning for working on equipment with CRTs: like all other mains-voltage electronics, CRTs can be dangerous to work on, so it is important to take care and use the proper precautions – if you don’t know what you’re doing, it’s probably best to leave it to someone who does.

The high voltage side of CRTs can contain extremely high voltages (up to 30KV for colour displays, less for monochrome), and the “low” voltage side has mains voltage and large line filter capacitors, all of which can give you a nasty shock and/or burns.

If you work on a CRT, you do so at your own risk – if possible, work on the CRT when it is unplugged and powered down, and safely discharge the CRT anode cap before starting work (preferably using a high-impedance lead so as not to damage the CRT interior coating). Take care to minimise the risk of shock across your chest (to help protect your heart), such as by keeping one arm behind your back.

More modern CRTs usually contain safety features to automatically discharge any stored charge (i.e. bleeder resistors), but even if these exist they can fail, so it’s always best to err on the side of caution.

Due to a phenomenon known as dielectric absorption, capacitors (including the CRT itself) can self-charge again even after being discharged – if you’re worried about this, keep any potential hazards shorted out while you’re working.

If you do need to work on the CRT while it’s powered up (for example, whilst measuring voltages or making control adjustments), use insulated tools and an isolation transformer, and take the same care required for working on any live equipment.