Hammond B-3 — Recapping the AO-28 Power Supply (120 Hz Hum)

Symptom

Steady low 120 Hz tone sitting under everything coming out of the organ — not a buzz, more of a hum riding right into the output. The tell was that it didn't move with the volume. Pull the expression pedal all the way down and the tone was still there, which placed it after the volume control — down in the output stage or its supply, not up in the signal path.

Diagnosis

Worked it from the outside in before opening anything up. Swapped the rectifier, checked the grounds, went through the tubes and the sockets for anything loose, swapped tubes around, then swapped the Leslie and the cables to rule out everything downstream. The tone never budged. That left the power supply filtering. The two large can caps in the AO-28 dry out with age and stop smoothing the rectifier ripple — and because the 6X4 rectifier is full-wave, that ripple lands right at 120 Hz, which is exactly the tone I was chasing. A cap test on the meter confirmed one of them was well out of spec.

Fix

Replaced both can caps with the AO-28 kit from bborgan.com rather than leave the second one to fail right behind the first. With fresh caps in, I put the scope on the output and the 120 Hz was gone. One more thing turned up on the way out: with the rail back up to spec, the 12BH7 output tube got touchy. It had been coasting on the sagging voltage, and once it saw proper B+ it showed its age. Dropped in a new 12BH7 and it settled right down. Dead quiet, back to the customer.

What I Took Away

Hum that doesn't change with the volume control is downstream of it — look in the output stage or its supply, not in the signal path. The frequency is the other half of the diagnosis: a full-wave rectifier puts its ripple at 120 Hz, so a 120 Hz tone points straight at tired filter caps, where 60 Hz would have me chasing a ground or heater problem instead. Always replace the can caps as a pair — the second one is the same age as the first and will fail right behind it. And check the output tube after a recap: a tube that's been running on a sagging rail can show its age the moment you give it proper B+ again.