Hum in a 1970s receiver: it was the filter caps, but not the ones I replaced first

2026-08-16 · Marantz-era solid state receiver, unknown service history

symptom

Loud hum on both channels, present with the volume at zero, worse when warm. Volume-independent hum points at everything downstream of the volume control, which is the power supply until proven otherwise.

what I measured

rail, no load ........ 42.1 V   (spec 41 V, fine)
ripple on rail ....... 780 mV pk-pk   <-- should be tens of mV
ripple after regulator  610 mV pk-pk
hum frequency ........ 120 Hz  => full-wave rectified, supply side

120 Hz rather than 60 Hz matters: it rules out a ground loop picking up mains directly and points at the rectifier and the smoothing after it.

the mistake

I replaced the two large can capacitors first because they are the obvious suspects and they looked terrible. Ripple went from 780 mV to 610 mV. Better, not fixed — which should have told me immediately that the mains-side smoothing was not the main problem.

The actual fault was a 470 uF electrolytic on the preamp supply board, physically small, sitting next to a heatsink that had been cooking it for forty years. ESR meter read 11 ohms on a part that should read under one. Replaced it, ripple dropped to 18 mV, hum gone.

lesson kept

Measure ripple at every stage before buying anything. The capacitor that fails is the one that lives next to something hot, not the one that looks the oldest.

after repair:
rail ripple ..........  22 mV pk-pk
preamp rail ..........  18 mV pk-pk
audible hum ..........  none at full volume, ear at grille