How to convert SPC files to ASCII without original spectroscopy software?

If you don't mind Python on the command line, there's an spc converter on Github.

While the trial version of the GRAMS Suite 9.2 is available with a 30 days license, it seemingly doesn't work with any data but the samples provided with it and therefore isn't suitable for your purpose :(

If you use R for data analysis, take a look at the R package hyperSpec. Example:

install.packages('hyperSpec')
library('hyperSpec')
spectrum <- read.spc('<your spc file>')
...
plot(spectrum)

You can also use Sys.glob() function for wildcard search of your files, then read all of them with read.spc(), and call collapse() to build a single hyperSpec class with all data points together.

A quick search turned up a number of useful tools. Wikipedia suggests:

• Essential FTIR - which advertises batch conversion of SPC to other formats.
• SPC2CSV - from AnalyzeIQ to convert to ASCII CSV format
• Spekwin32 - general spectra software, including batch conversion
• EPA's ShowSPC - not clear if it allows saving (or if it even runs on more recent OS)