For those with V3 A-138 mixers, I finished some simple and useful mods on mine.

I put in a DT switch at JP3 to let me select a + or minus normalized DC voltage. I noticed the manual calls it "JP4" possibly in error, but it's pretty obvious which one to use.

I also realized that JP2 is an inverted output. The top is the hot signal, the bottom is the ground, so I wired a jack to it.

I really wanted to add the switch and the jack to the A-138xb. I managed to do it... just barely, but it would have been easier I guess mounting the mods on another blank panel