I took my dear old 531A out of the cupboard and found out that the trace exhibited a very nice 100 Hz ripple, with an amplitude independent of the V/cm setting. I looked at the schematics and immediately suspected the 100 V supply. After checking (with a more trustable scope), it appeared that the 100 V, 225 V and 350 V supplies all showed ~1Vpp ripple. I replaced the triple 10 µF 450 V C679A/B/C with three brand new electrolytic capacitors but that didn't seem to fix the problem.
I'm currently checking my solders, which look good, and I wonder what else can be wrong. Any clue ?