As for the pin 5 might wait tell someone else also posts but as long as pin 5 is the correct one cause I’m not sure I’d have to search the forum you just need to search for a 9 pin serial pin out and it’ll show how those pins are numbered. Take the wire to the best Earth ground (house ground) you can get to.
the “voltage float” in surprised it’s not on both transformers But I suspect that when you set a given channel it’s either with a load or without but when the load moves from one to another the channel that had no load now has a load and the transformer is power settings it’s not going to be like Cruise control and keep the same voltage. As the load increases the voltage will drop. On the other channel as the load leaves it has more available power so the voltage will rise. I don’t know if there’s anything that causes this to be more profound in a one Z4000 to another other people are more knowledgeable on that transformer than I
