FAQ
TL;DR: Electrolytics age in storage; “electrolytics typically have a skewed tolerance like −10/+50%.” Replace older caps or carefully re‑form before use. [Elektroda, Anonymous, post #21679263]
Why it matters: This FAQ helps hobbyists, restorers, and engineers diagnose capacitance loss, decide replace vs. re‑form, and avoid avoidable failures in vintage and stored gear.
Quick Facts
- Rule of thumb: replace electrolytics older than ~25 years in critical audio/synth gear. [Elektroda, Anonymous, post #21679267]
- Re‑forming method: slowly raise voltage while monitoring current to restore dielectric. [Elektroda, Anonymous, post #21679257]
- Tantalum bead electrolytics can short after long storage; watch for power‑supply stress. [Elektroda, Anonymous, post #21679260]
- Paper caps in old radios often go leaky; preemptive swaps prevent tube damage. [Elektroda, Anonymous, post #21679265]
- Electrolytic tolerance is typically skewed (≈ −10%/+50%), so “over‑value” readings can be normal. [Elektroda, Anonymous, post #21679263]
Do electrolytic capacitors age when not used?
Yes. The electrolyte and oxide deteriorate with time, even in storage. Expect increased leakage, ESR changes, and reduced effective capacitance. As one expert put it, “Electrolytic capacitors definitely do age.” If reliability matters, plan inspection or replacement before power‑up. [Elektroda, Anonymous, post #21679257]
What tolerance should I expect, and why do some measure high?
Large can electrolytics often ship with asymmetric tolerance, commonly around −10%/+50%. A 15,000 µF part may legitimately measure near 22,500 µF and still meet spec. That skew helps ensure minimum filtering under load. So over‑nominal readings on good meters are not unusual. [Elektroda, Anonymous, post #21679263]
Should I replace all electrolytics in a 30‑year‑old project?
Yes for mission‑critical or audio gear. Many restorers replace electrolytics older than roughly 25 years to avoid leakage, noise, and sudden failure. This proactive approach saves troubleshooting time on vintage synths and similar equipment. [Elektroda, Anonymous, post #21679267]
How do I safely re‑form an electrolytic capacitor?
Follow a gentle three‑step process: 1. Apply a low DC voltage and watch current. 2. Increase voltage in steps as current falls. 3. Stop if current won’t decline or the can warms. This can partially restore oxide and reduce leakage. [Elektroda, Anonymous, post #21679257]
Why did my 15,000 µF Sprague read 19,000–20,000 µF?
That falls within typical skewed tolerance for electrolytics. Premium brands may measure above nominal when new or healthy. The key is acceptable leakage and ESR under working voltage rather than exact capacitance alone. [Elektroda, Anonymous, post #21679263]
My big caps first measured 2–3× value, then settled. What happened?
They charged slowly on the first run, so the meter’s algorithm interpreted a larger effective capacitance. After initial conditioning, subsequent measurements reflected closer‑to‑nominal behavior. Pre‑charging or re‑forming often stabilizes readings. [Elektroda, Anonymous, post #21679266]
Do tantalum capacitors fail after long storage?
Yes. Older bead‑style tantalums can short when re‑energized after years offline, sometimes stressing transformers or supplies. Replace suspect parts or power up through a current‑limited source to mitigate risk. [Elektroda, Anonymous, post #21679260]
Which capacitor types age the least in storage?
Non‑polarized film types—polyester, Mylar, mica—tend to remain stable for decades. They rarely show the electrolyte‑driven failures seen in aluminum or tantalum electrolytics. Use them where practical for long‑term reliability. [Elektroda, Anonymous, post #21679264]
Are paper or oil‑filled capacitors risky in restorations?
Yes. Paper caps often become leaky and can damage tubes or coils. Some sealed oil‑filled types pose another risk: physical leakage of PCB fluid, which is hazardous. Replace on sight in vintage radios. [Elektroda, Anonymous, post #21679265]
How does aging affect RC timing circuits?
Aging electrolytics can change capacitance and leakage, shifting time constants and behavior. You may see partial recovery after use, but replacement restores predictable timing. That’s common in battery‑powered displays and timers. [Elektroda, Anonymous, post #21679261]
Is it better to replace or try to restore old electrolytics?
If time and cost allow, replacement yields the most reliable outcome. Otherwise, re‑form cautiously and monitor current. “If replacing all...is an option...that is probably the right thing to do.” [Elektroda, Anonymous, post #21679257]
What is an electrolytic capacitor reformer?
A reformer is a controlled supply that steps voltage while monitoring current to rebuild the oxide layer. EPE documented a DIY reformer in its August–September 2012 issues for audio work. [Elektroda, Anonymous, post #21679258]
Any quick field test for bad electrolytics in synths?
One restorer heats the leads with a soldering iron; a fishy smell suggests electrolyte issues. It’s a crude screen, not a replacement for ESR/leakage tests, but it flags replacements fast. [Elektroda, Anonymous, post #21679267]
Can a cap seem fine yet still be a failure risk?
Yes. An electrolytic can measure near‑nominal capacitance but have high leakage or ESR under voltage, leading to hum, heat, or regulator stress. Always check behavior at working voltage. [Elektroda, Anonymous, post #21679257]
Why do restorers preemptively swap radio coupling caps?
Leaky paper coupling caps bias stages incorrectly and can destroy hard‑to‑replace parts. Many restorers replace these on principle before first power‑up to protect tubes and coils. [Elektroda, Anonymous, post #21679265]
