Curve tracer / Rail splitter sourced current / Horizontal adjustment

[Zielsziek]

Hello,

I have built and been playing with this curve tracer. So far so good, I am enjoying it. I have three questions about it.

1. I find that I cannot adjust the trace to be horizontal when input is open. I suspect that this is because of the values for the adjustment section R21 and RV2 and RV3. I changed R21 from 100K to 50K, but the improvement was mediocre.

2. When I short the leads I am expecting a vertical trace, which I am obtaining, but that trace is roughly 1.25 divisions long. This, I believe to be because of the rail splitter which cannot supply enough current. Should  I switch to a rail splitter with two transistors? Or maybe a different op-amp. I know that normally a center tapped transformer would be used here, but I wanted to avoid AC. I have had issues before here, where C3 and C4 were causing oscillations of the split supply and I removed them.

3. Was it a bad ideea to switch mechanically between the frequencies? I find that if I touch the switch, which is metal, my oscillator starts behaving somewhat erratically. Should I have used something like a HEF4051B mux/demux ic?

Thank you.