Pull a capacitor off a vintage board and you are immediately faced with a choice the original designer made decades ago, and may have got wrong. Aluminium electrolytic, tantalum, or one of the modern polymer parts? They are not interchangeable. Each has a distinct failure mode, and on hardware this old the failure mode is the single most important characteristic. A part that fails by quietly drying out is a very different proposition to one that fails by shorting and catching fire next to an irreplaceable custom chip.
~50%
Tantalum derating
Common rule: run at roughly half rated voltage
Decades
Electrolytic lifespan
Then ESR rises and they vent or leak
Short
Tantalum failure mode
Dead short, not open, can ignite
Aluminium electrolytic: capacity for the money
Aluminium electrolytics are the workhorse of vintage hardware. They offer a great deal of capacitance per dollar and per square centimetre, which is exactly why they appear everywhere from power supply filtering to audio coupling. They are polarised, so they must go in the correct orientation, and they rely on a liquid or wet electrolyte to function.
That electrolyte is also their undoing. Over decades it dries out or escapes, and the consequences arrive in a predictable order:
- ESR rises gradually: equivalent series resistance creeps up as the electrolyte degrades, so the cap filters ripple less and less effectively long before it looks faulty.
- Capacitance drops: the part can no longer hold its rated value, causing instability, noise on power rails and erratic behaviour.
- Venting and leaking: eventually the cap vents, releasing electrolyte that is corrosive and will eat through traces, pads and nearby components if left.
The leaked electrolyte from surface-mount electrolytics is one of the most destructive problems we see, because it spreads silently under the can and corrodes the board beneath before anyone notices.
Tantalum: stable, compact, and quietly dangerous
Tantalum capacitors solve several of the electrolytic's weaknesses. They are very stable over temperature and time, extremely compact for their capacitance, offer low ESR, and have an excellent shelf life because there is no wet electrolyte to dry out. On paper they are the better part, and that is precisely why so many vintage designers reached for them.
Plenty of classic boards used tantalum beads from the factory. Various Amiga, Macintosh and Atari boards are littered with the little orange or yellow blobs, often on power rails and around decoupling positions. The catch is their failure mode. Unlike an electrolytic, which tends to fail open or high-ESR, a tantalum fails as a dead short. Once it shorts it can draw enormous current, overheat, and on a live rail it can scorch the board or catch fire outright. Failed and shorted tantalums are one of the most common faults we find on these machines, frequently presenting as a board that will not power up or that pulls a rail straight to ground.
⚠️ Tantalum fails short, and can ignite
Polymer and hybrid: the modern default
Polymer capacitors, and hybrid polymer types, have changed the calculus for recapping. They use a solid conductive polymer rather than a liquid electrolyte, which means there is nothing to dry out and nothing corrosive to leak. They offer very low ESR and very long service life. For most positions where an electrolytic originally sat, a quality polymer part is now the better choice, you get the bulk capacitance and the low ESR without signing the board up for another round of electrolyte damage in twenty years.
For small decoupling values, multi-layer ceramic capacitors (MLCCs) are the sensible modern replacement. They are non-polarised, cheap, reliable and have no electrolyte at all, making them ideal for the many small-value bypass positions that once used ceramics or small tantalums.
💡 When polymer is the better choice
Electrolytic vs tantalum at a glance
| Aluminium electrolytic | Tantalum |
|---|---|
| Fails open / high-ESR, then vents and leaks corrosive electrolyte | Fails as a dead short, can overheat and ignite |
| Higher ESR, rises further as it ages | Low, stable ESR throughout life |
| Bulkier for a given capacitance | Very compact for its capacitance |
| Cheap, best capacitance per dollar | More expensive per microfarad |
| Polarised; degrades over decades | Polarised; must be heavily voltage-derated |
| Risk: corrosion damage to the board | Risk: fire and shorted rails if abused |
Choosing the right type per position
There is no single right answer for a whole board. We pick per position, based on what the location demands:
- Bulk filtering and power rails: polymer or a good-quality low-ESR aluminium electrolytic. Polymer for longevity, electrolytic where you need a large value cheaply.
- Original tantalum positions: we will often replace a suspect tantalum with a modern part, and where a tantalum is retained it is fitted with heavy voltage derating. On vulnerable rails a polymer or MLCC is frequently the safer substitute.
- Small decoupling values: MLCC ceramics. Non-polarised, robust, and no electrolyte to worry about.
- Audio and timing-sensitive paths: type and value chosen to preserve the original circuit behaviour, not just to clear a fault.
The guiding principle is simple: respect the failure mode. A capacitor that dries out gives you warning and time. One that shorts and burns does not. Matching the part to both the electrical job and the safety profile of its position is the difference between a repair that lasts another forty years and one that creates a new fault.
Recapping done properly
If your Amiga, Macintosh, Atari, console or vintage PC is overdue for a recap, or you have already found a leaked electrolytic or a shorted tantalum, we can help. RetroRevive offers mail-in restoration Australia-wide. We assess each board position by position, choose the correct capacitor type and rating for the job, clean up any electrolyte or corrosion damage, and return the machine working and stable. Pack it up and post it in, and we will give you an honest assessment of exactly what it needs.