The Power Cab has no voltage adjustment. The standard track voltage for HO scale is 14v. for N scale it is 12v.The incoming Power Supply for the Power Cab is 13.8 VDC. The track output as measured directly on the back of the Power Cab Panel (PCP) should read about 13.8v AC. DCC track voltage is difficult to read correctly with ANY type of meter. It is a Digital PWM square wave and is closer to a computer signal than anything else.
Getting correct voltage readings on the track relies heavily on using the proper gauge wiring for your DCC track bus and track feeders. Your DCC track bus wire should be either 14 or 12ga. wire with track feeders every three feet. Track feeders should be either 20 or 18. ga if you are in HO scale. The wire supplied by Kato is not sufficient for reliable long term operation. Also you need to take into account that the rail joiners are only good for a PHSYICAL connection and rail alignment. They are horrible for track power and should not be relied upon for electrical connections. You can also solder some of the pieces together, that will help.
Go here for more info: http://wiringfordcc.com/