564 AC ripple in beam intensity


Hi Folks -

I'm seeing an intermittent intensity modulation at line frequency on my 564, and have been unable to measure the source. I've tried both a 3B3 timebase, and a 3A72 in the horizontal slot with an external ramp to try to confirm that the problem is not from one of the plug-ins. In either case, I see the same thing. Given the depth of modulation I'm seeing, I'd expect there to be dozens of volts of ripple somewhere in the beam's path, but it's proving to be illusive.
There was some out-of-spec ripple in the low voltage supplies, but I've all but eliminated that by changing out some of the electrolytics (there's still a little more than spec on the -100V, but only by a few mV).
I can't measure any appreciable ripple on either side of the pair of neon bulbs between the cathode and the grid, and about half the time, when I touch my 1000x probe to those points, the problem disappears. On the storage targets of the cathodes of V913, I'm getting a ramp of about 400-500 mV, which, while I'm not sure what's up with that, it doesn't seem large enough (or the right shape) to be the source of this problem.
The external CRT cathode on the rear is grounded, and the switch has been cleaned and is in the chopped blanking position.

Any ideas?

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