This was an elusive beast: getting 80 column display on the commodore 128.
From GBS 8200 to CGA–>VGA converters, VGA to HDMI, CGA - SCART, you name it, I tried it. Dumping numerous $$$ into the quest, only to fail. Then one day, a year or so later, I had an epiphany (or something) and cracked open the 128. There I had thought I was all cool, upgrading my 128 VDC to the 64K version for those precious bytes of available memory. I pulled that out and replaced it back with the factory norm of 16KVDC. BOOM… 80 column works, and in just about every configuration that I thought would work previously.
Learn from my mistake folks and save a few dollars (though that money does support great development of solutions for us Commodore weenies stuck in a new era of hardware configurations)!
My current config is strictly a CGA to VGA converter driving the monitor.
Below is a snapshot of it - which I got of Etsy. Currently it shows unavailable, but I’m sure it’s out there.