Vintage amp hum or hiss when warm: recap symptoms

If you've pulled a 1978 Marantz 2238 or a Sansui AU-717 out of a loft and plugged it in, you're listening to electrolytic capacitors that have been quietly drying out since Jimmy Carter was in office. The aluminium cans on the PSU rail, the smaller radials sat next to the output transistors, the bipolar couplers between gain stages — they were specified for maybe 2,000 to 5,000 hours at rated temperature, and they've been sat in a warm chassis for forty years. The electrolyte has migrated out through the rubber bung. The dielectric oxide layer has thinned. ESR has crept up from milliohms into ohms.

What you hear is the symptom. The cause is almost always the same handful of parts.

This article is the checklist we run through on the bench when somebody ships in an integrated amp or receiver that hums, hisses, distorts on bass, cuts a channel when warm, or makes that distinctive "frying bacon" noise from the speaker on power-up. Most of these are recap jobs. A few of them aren't, and we'll be honest about which.

What "recap" actually means

An electrolytic capacitor is a sandwich of two aluminium foils separated by a paper soaked in conductive electrolyte, rolled up and sealed in a can with a rubber bung at one end. The dielectric — the actual insulating layer that lets it store charge — is a few-nanometre-thick oxide grown on the anode foil. That oxide is maintained by the electrolyte. Take the electrolyte away and the dielectric degrades; the cap loses capacitance and gains equivalent series resistance (ESR).

ESR is the bit non-technical owners never hear about, and it's usually what's killing the sound. A 10,000 µF PSU reservoir cap that measures 9,200 µF on a meter looks "fine" to anyone reading capacitance alone. Measure its ESR and you might find 0.8 Ω instead of the 0.02 Ω it had when new. That high-ESR cap can't deliver current fast enough on bass transients, can't filter ripple properly, and gets hot doing it — which dries it out faster. They fail progressively, then suddenly.

Electrolytics in a vintage amp are time-bombs. Not "might want looking at." Time-bombs. The only questions are which ones have gone first, and whether the failure mode has already taken anything else with it.

The five symptoms that scream 'recap me'

1. Mains hum that gets worse with volume, or sits there at idle and won't go away. Almost always the main PSU reservoir caps — the big 10,000 µF / 63 V cans bolted to the chassis. Their job is to smooth the rectified mains into clean DC rails. When ESR climbs, ripple at 100 Hz (UK mains is 50 Hz, full-wave-rectified = 100 Hz) leaks straight through into the audio path.

2. Hiss, fizz or "frying bacon" from one speaker, often worse when warm. Smaller electrolytics around the driver and output stages — typically 47 µF to 470 µF bypass and decoupling caps on the amplifier PCB. When these go leaky they inject noise directly into a high-gain node.

3. Channel drops out after 20-40 minutes, comes back if you tap the chassis or leave it off overnight. Classic intermittent — usually a bias-network or coupling electrolytic whose ESR is temperature-dependent. The cap "works" cold, fails hot. Can also be a dry solder joint, which is why we reflow suspect joints during a recap.

4. Bass sounds flabby, woolly, or just absent. Transients are mushy. Coupling caps between stages have lost capacitance — the low-frequency rolloff has crept up from 5 Hz to 50 Hz. Or the PSU reservoirs are sagging under load, so the rails collapse on every kick drum.

5. Loud DC thump on power-on, or the protection relay refuses to click in. DC offset on the output, usually because a coupling cap in the feedback or input network has become leaky and is passing DC where it shouldn't. The protection circuit is doing its job. Don't bypass it.

If your amp does any two of those, it needs a recap. If it does three or more, it needed one ten years ago.

What we do on the bench

First job is always a visual. Caps with domed tops, brown crust around the base, or visible electrolyte on the PCB are condemned on sight — no measurement needed. We photograph the board before anything is touched, so polarity and orientation are recoverable.

Then capacitance and ESR on every electrolytic in the signal and power path, in-circuit where the topology allows, out-of-circuit where it doesn't. We use an ESR meter that works at 100 kHz — meaningful for switching and audio-band ESR, not the 120 Hz nonsense you get on cheap multimeters. Anything more than 2× the datasheet ESR or below 80% of rated capacitance gets replaced.

Order of replacement matters when budget is tight. PSU rail caps first — they're upstream of everything and a bad reservoir cap can mask other faults. Then decoupling and bias caps on the amp board. Coupling caps last, because they're the most expensive (we use film where space allows, decent audio-grade electrolytics where it doesn't) and the least likely to be catastrophically bad. A staged recap over two visits is an option for big receivers.

After the recap goes a thermal soak — amp on the bench, dummy load on the outputs, scope on the rails, two hours minimum. Then a listen test on real speakers. Then DC offset and idle bias measured and adjusted to the service-manual spec.

What a recap cannot fix: a transformer with shorted primary turns (you'll smell it), output transistors that have already let the smoke out, scratchy potentiometers (separate job — DeoxIT and sometimes a strip-down), worn switches, a blown dial-lamp, or a knackered tape head. We'll tell you up-front during inspection if any of those are also in play.

What it costs

Workshop minimum is £40. Soldering bench rate is £80/hour, parts at cost.

A small integrated amp — think NAD 3020, Cambridge Audio A1, Rotel RA-820 — with a straightforward board and 15-25 electrolytics is typically £80-150 all-in, including parts and the listen test. Half a day of bench time.

A 1970s receiver — Marantz 22xx series, Sansui AU/G-series, Pioneer SX — has 40-80 electrolytics across multiple boards, often with through-the-chassis wiring loomed to phenolic boards that don't enjoy being heated. £200-350 is realistic, sometimes more for the big Marantz 2270/2325-class receivers. We quote after inspection, not before.

Vintage gear is shipped in mail-in only — pack the amp in its original box if you've still got it, double-box if not, and tell us the symptoms in writing so we can correlate what we measure on the bench with what you actually hear at home. Soldering and rework details, including how to send it in, are on the soldering-repair page.

If the amp is currently working but sounds tired, don't wait for a cap to fail short and take an output transistor with it — that's when an £80 recap becomes a £180 recap-plus-output-stage rebuild.

See also

• When recapping won’t save your vintage gear
• Capacitor plague: spot a swollen or leaking cap