> Thanks, Timothy. I will try those values. I also appreciate that

you

> explained the math behind your conclusions.

Which just goes to show one shouldn't attempt these things late at
night - I briefly considered the loading effect of the input resistor
on the op amp summing the CVs, but dismissed it as negligible. This
is a mistake, because as it is of the same order of magnitude as the
others, it has a considerable effect!

So, attempt no. 2: at the max pot setting, the 100k input resistor
will appear in parallel with (all of) the 50k of the pot, therefore
their total resistance is 33.3k (=50*100/(50+100)k). Thus as before
the required resistance is R, where 33.3*12/(33.3+R)=5, which gives
R=46.6k. If you use 47k, you'll get just under 5V, and 43k will give
5.2V. (I leave it as an exercise to calculate what a 68k would in
fact give...)

> Can you suggest the best way to cut the PCB tracks (I've done this
> before, but never on purpose.) Is an exacto knife a good idea

If an exacto knife is what I suspect it is, it'll probably be OK as
long as it has a sharp, sturdy point on it. I use a penknife (aka
pocketknife), and rather than trying to literally cut the track, I
repeatedly scratch the copper with the point, until gradually it is
worn down and a gap is made in the track (patience is the key!).

If you need any help identifying precisely which tracks to cut, let
me know, but I shall be away for two or three days (family duty and
all, being that time of year!)

Tim