Got HTTP status 404 Not Found for "http://john-chambers.us/~jc/music/book/1748/LondonMagazine.abc"
pngcmd="(./ps2png 512 512 /Users/jc/w/tmp/Tune-15536-LondonMagazine.ps) >>/Users/jc/w/tmp/Tune-15536-get.log 2>&1" returned status 256
http://john-chambers.us/~jc/music/book/1748/LondonMagazine.abc

Error exit status 256; here are some files to check:
[P=tuneget V=1/1 B=0 scale=0.70 512x512 PNG]
filesizedescription
Tune-15536-LondonMagazine.abc 151 ABC music file with the extracted tune(s)
Tune-15536-LondonMagazine.ps 6 PostScript
Tune-15536-LondonMagazine.txt 151 Plain-text file with the extracted tune(s)
Tune-15536-get.log 4167 Log file, useful mostly for debugging
These files should be available for 24 hours.