Chiro C800, how do you work Zone 2?

I have an old Chiro c800 preamp and can't seem to find the manual. I press the zone button on the front panel and can switch which source goes to zone 2, but I can't seem to figure out how to change the volume for that zone. I've never used that feature, but would like to do so now. And, I can't find an online manual either.

If anybody knows the answer, I'd appreciate it.