[volt-nuts] 7081 AC buffer *again*

[Ed Breya]

I agree with JL - it's either picking up an available frequency, or 
making its own. Oscillation can happen with transistors, especially any 
common-base amplifiers or emitter followers that don't have enough 
degeneration - but that is usually in the VHF range. For MHz-type 
oscillations, look for opamp loops that went unstable for some reason - 
bad grounds or bypassing can do it.

An old trick for VHF is to poke around with the tip of a wood-sheathed 
lead pencil - the partially-conductive graphite core and the lossy 
wooden capacitance to an even lossier hand would sometimes damp 
oscillations and point directly to the problem, so to speak. For lower 
frequency you need brute force - try a small screwdriver, a meter probe 
lead, or tweezers held in the hand, and just poke around on various 
nodes, without shorting anything out. If that's not enough, a small RC 
series damper - say 1000 pF and a few hundred ohms - with a clip lead to 
ground should show some results. All you're looking for is some effect - 
it should either reduce or aggravate the problem when you are around the 
problem circuit. You will naturally be injecting all kinds of line 
frequency and RF interference into any high-Z circuits, so ignore that 
and just look for the effects at the target frequency.

Ed