Investigating Oberheim Matrix-6R LFO; got scope weirdness

Synth Repair
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My Matrix-6R is on the Bench of Investigation because I’m rearranging my rack case and while it’s out I’ve replaced the battery, dumped the patch memory, checked the power supply voltages, and made a trimpot adjustment to null out the DAC as per the service manual.

One thing that’s always bedeviled this synth is glitchy LFO - an audible hiccup in discontinuous (UPSAW, DNSAW, SQUARE) LFO waveforms). I was trying to scope the LFO input to the Curtis CEM3396 voice chip (I’m assuming the LFO signals are coming out of a DAC) but when I do, I get this ripply mess on pin 2 of the CEM3396 when playing a patch with LFO1 in UPSAW mode controlling DCO pitch:

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If I listen to the voice on which I’ve got the scope probe on pin 2 of the CEM3396, I can hear a “zipper” effect but only when the scope probe is touching the pin - pull it off and the zipper effect disappears. I’ve measured the frequency of that ripple at 66.7Hz - not power supply ripple.

In the photo you can see that downward spike at the sawtooth transient but before I try to investigate further I’d like to be scoping clean signals. I switched off my magnifier light on the bench to make sure it wasn’t inducing this ripple, so that’s not it. Any ideas what’s causing it?

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My guess is your scope probe is loading down the sample and hold cap. 66.7Hz is the DAC refreshing the cap every 15ms.

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Makes perfect sense. But the scope says it has 1M inputs - that’s a “load??”

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Technically it is always a load. The important factor is how the input impedance of the measuring instrument compares to the input impedance of the circuit under test. If it’s much greater, no big deal. On the other hand, for example, the input impedance of CMOS chips can be 10 megs. If you connect a 1 meg scope to that, it will most certainly affect what the circuit is doing.