[sdiy] Minimoog tuning woes, Op-amp swaps?

[Eric Frampton]

Hi y'all -

I just finished installing a kit of matched resistors on my Mini's (s/n in the 1700's) oscillator board (the "old", actually middle, rev), and a kit of electrolytics, both from Kevin Lightner (I didn't want to take any chances on my cap selections). I also got a buffer board from him that I haven't installed yet - I doing all of these things a little at a time to see what changes, if any, I can make out.

So far I can't tell much. The filter seems to have brightened up a bit, but all the stability and tuning improvements I was hoping for haven't happened yet. In fact, they may be worse.

Two main problems:

First, I cant seem to get any of the three oscillators to track more than the 3 octaves on the keyboard in a given range - when I switch ranges (even leaving the other 2 oscillators untouched), the scaling goes to hell.

I realize that the buffer board affects the range switching between oscillators, so I'm not worried about the imperfect tuning yet. It's the scaling that's driving me nuts.

Second, I'm wondering what I can do to get the keybed itself linear. For instance, the tuning between high A and lowest A requires a different scaling than, say, the middle two C's. It's like it's a perfect 1V/Oct on the ends but not in the middle. Do I just have to go through and replace all the resistors on the keybed, or am I missing something else? The bus bar contacts are clean.

Also, is there a consensus on what op-amp to replace those 741's in the oscillators with? I tried TL071's and can't get the trimmer low enough to scale 'em (consistent with all 3 oscillators). I'm sure I"m missing something...

Thanks,
Eric