There are several user which didn't had the AGC issue and I started my own VHS conversion project with an ati based USB device and I wanted to analyze this problem via older and alternative Driver files.
This files should be the Firmware files of the drivers:
The ati 750 USB uses a Ati T507 Chip which I assume has the firmware file CTRLT507.s3. source: https://www.mail-archive.com/linux-m.../msg48605.html
I have several Driver Versions which have 2 different versions of this Firmware File and different Driver version. If someone wants to test all Driver version if the AGC issue affects all of them, he can download all versions but don't forget to uninstall old Driver with a restart. I although found a Firmware Update Tool for ATI TV wonder cards but couldn't test it until now but i will upload it too later.