hywlen wrote:When I ground the negative and probe the end pin of vr4, I get no signal but without the ground, I get the signal I have described.
If this happens with an ungrounded probe (the ground doesn't have to be obtained directly; sometimes it's obtained indirectly over the earth connections) you're probably NOT getting the filter signal here; I've experienced this myself quite often.
I suggest at this point tracing the signal through the filter ladder. Please keep in mind that the signal is going to be VERY weak compared to the signal you've found at the waveform switch.
Perhaps you could start at C17, then continue through C18, C19, C24, C26.