I have a tablet case with a keyboard in it. It came with a large USB type A plug and a large Y-adapter to USB micro-B. It has always worked. The tablet has a micro-B type connector and it works fine. I wanted to replace the A plug and adapter with a micro B plug, so I bought one on e-Bay. I searched about to find the pinout, and confirmed the A plug is -, data+, data-, + with the data+ wire blue coming out of the keyboard. When I put the new micro-B plug in the tablet I could not measure the 5V. I soldered the wires on and it doesn't work. When I connect the power and gnd (+ & -) I still don't measure the 5V. The tablet USB port still works and I can charge the tablet. Any ideas? In the photo the wires are (left to right) -, n/c, data+, data-, +.
Your phone is an "OTG device", meaning that it can assume both roles - the role of USB host, and the role of USB device. You want it to be a USB host to drive your keyvoard.
Now notice that the u-USB plug has FIVE pins. These pins include the pin named ID:
If you need your tablet to act as host, the ID pin must be grounded. In all regular cables the ID pin is floating. The ID connection is buried under the plastic overmold, and it is usually a challenge to change it.
First solder a type A plug back on and confirm that the keyboard is still good.
Then plug the adaptor and keyboard into a micro socket break-out board and trace all the connections from the socket break-out to the keyboard cable.
Then plug the micro plug into the socket and copy those connections.
PS. perhaps you need to do something with the OTG pin?