In audio and retro computing circles, replacing the electrolytic capacitors of devices or “recapping”, is often sugggested as something that should be done to older devices as a matter of course. Statements such as “it’s 20 years old now - those caps will need replacing!” are made.
Unfortunately as a blanket statement this isn’t true, and recapping a device has some dangers. Here’s my checklist of things to consider when deciding whether to recap a device.
Is the device malfunctioning and you’ve diagnosed the fault to be due to faulty capacitors?
If your device is malfunctioning or underperforming, first do a proper fault finding diagnosis and pinpoint the cause of the problem. There can be many causes of problems and blindly replacing the caps will usually not fix the problem.
Testing capacitors in circuit is not really possible - lift one end of the cap so it can be tested in isolation. Note that the tolerances of electrolytic caps are quite large - usually ~20%, and circuits are designed with that in mind - exact specific values can’t be relied upon.
Testing of the capacitance can be done with many multimeters, however you may want to test the ESR, this can be tested with more specialised meters such as the Peak Electronics ESR meter
Are the capacitors showing signs of bulging or leaking?
If there are capacitors showing signs of leaking, e.g. residue on the PCB near the capacitor, possibly accompanied by corrosion of ajacent components or the PCB copper, or bulging or venting, then the capacitor should be replaced.
Capacitor electrolyte is corrosive, so can cause permenent damage to the PCB or surrounding components so these are best replaced when spotted.
Some devices are notorious for having capacitors that leak, e.g. the Commodore Amiga 600 and 1200 have early (and probably cheap) surface mount electrolytics that are very prone to leaking.
There is also the Capacitor Plague where a large number of faulty capacitors were manufactured between 1999 and 2007 that are very prone to leaking. These were used in many PC motherboards and power supplies.
Dangers of recapping
The biggest danger with recapping is that faults that did not already exist may be introduced. For example PCB traces may be damaged during the desoldering/resoldering process or ESD damage to other components on the board through handling.
Parameters for choosing replacement capacitors
- Capacitance - the capacitance should match. In some cases a higher capacitance can be used without issue, but the exact nature of how the capacitor is being used in the circuit would need to be known before doing this.
- Voltage - the voltage should be equal or higher. There is no problem picking higher voltage rated caps, howwever they may be physically larger
- ESR - some applications such as switching power supplies require low ESR capacitors. Check the schematic (if available) or markings on the capacitor to see if low ESR caps are required. Note that capacitor electrolyte formulations have improved, and “low ESR” may sometimes me met by normal spec capacitors now.
- Pin spacing - if the pin spacing matches the PCB holes the capacitor can be mounted flush to the board which is ideal for neatness.
- Diameter - is the replacement capacitor going to fit?
- Height - similar to diameter