I have to admit that building this multimeter was quite a fun. I only used parts that were available at home and this design constraint really forced me to use imagination and creativity, even more because designing 6-digit multimeter isn't trivial exercise.
This isn't my first excursion to precision test gear, I've done it before here and there, but this one was a bit different, because of the "do it from leftovers" part. This lead to some non-usual design choices, like using germanium transistors or quite old DSP, parts recycled from decades old e-waste and new old stock, gathering dust for years with no use.
I believe I managed to put it together at the end, with firmware needing some polishing, hardware needing to add USB-serial converter for easier logging or even Ethernet-serial converter so that the instrument could be used in more modern way.
What is still lacking is documentation. Since the aforementioned reasons, most of the design was done on the fly, with no schematics drawn beforehand and majority of documentation looking like this, including coffee stain
Though I believe all the information is there (my design notes, as well as sources and mechanical files are in files section), I'll try to redraw it into more useful form.