The wheel is assembled and all the spokes now have some tension. I just have to live the fact it's a bodge, to use the term of a well known YT channel.
I am hugely gratefull to those who took the time to offer their knowledge and help me in my quest.
If it makes you feel better, i bet the spokes you removed were in the wrong holes, and the wrong length from before! Then copying the spokes like for like made it basically impossible to fit them correctly.
Maybe, we may never know. It feels most probable that the spokes are the wrong length and to get past that problem i had to put the spokes in the wrong hole. As I saw it, the spoke holes are angled for entry. When i did this, i was able to tighten all the spokes. It seems that i did something right. Now i have about 98 pairs of wheels left to build before i can say i can build wheels(or is it 998?). Anyway i felt i was able to hand it back to my son with a reasonably clean concience. I gave it my best shot.
I'm inclined to believe that the spokes are a tad short for the hole they were meant to go in. In order to get some workable tension on all the spokes i had to put them in the "wrong hole," as I interpreted the spoke hole pattern and the fact that they were angled. It might have been built incorrect from the very beginning or my son may have measured them incorrectly. He wasn't aware of a thing called a spoke guage or ruler. However, I did manage to put it together, it's just quite annoying suspecting that I built it incorrectly. When I followed the elementry steps the spoke length wasn't working, even though i had the correct order for the valve opening, which you get when you build standard bike wheels with the correct spoke length. A lot of head and chin scratching has gone on since the Xmas hols.
Hopefully all will become clearer when I build the next pair of wheels for myself, hopefully in the not too distant future. You can call it a steep learning curve.
So, latest adventures: started riding the first wheel I built up a while ago - done shoved in the back of a bike with Sheldon’s instructions on the iPad. It was spinning beautifully, but when ridden, developed a regular slight spoke twang - like a twig caught in the mudguard acting like a spokey dokey. Get off the bike, no twig, no fouling, rolling fine, spinning true. Back on the bike, ting, pause, ting. Which led me to conclude that as it only happened with a man’s worth of weight, it had to be the build.
The perfect opportunity to try my (new to me) truing stand (more of which later). Wheel was laterally true, no loose spokes, no damaged nips or eyelets - but not round enough! There was enough of a radial ‘bulge’ to tension and un tension the spokes under load and make them ping slightly.
So I’ve loosened everything off and started again with a bit more care and knowledge gleaned from the internet and Musson. Round true and tight and quiet.
Learning:
Old wheels (especially it seems 80s Mavics that I’m fond of) eventually become the trueness limit: by which I mean slight variations in the surface, especially near the join mean you ain’t getting any truer. I’m not sweating this - tiny fractions of mm. (Presumably this is also true for non machined sidewalls?)
Because of the above, you might get different roundness readings on each rim lip; hence I’ve added a central ‘gauge’ (bent spoke taped on) that feels the centre line of the rim.
Kit.
£20 Minoura dishing tool is fab.
Having a dedicated truing stand is lovely. Workflow baby. This Xtools one is a Park copy that CRC we’re knocking out a few years ago. It’s very robust but they got some bad reviews because they weren’t calibrated out of the box; pretty sure that’s why previous owner sold it hardly used. They have masses of adjustment though, so 30 mins lubing, aligning and fettling worked wonders.
A plectrum (guitar pick) is excellent for gauging tension pitch. I’m going to invest in a cheap tensionometer to calibrate pitch to rough kg/f, but the pick is so much more fluid to use.