I had a Sharpy that had a low voltage supply problem (fans, motors, and screen were dead when plugged in). The voltage was correct for the PSU that splits the mains to high and low voltages. And the cable that goes from the PSU to the the bus that splits power for the low voltage was good. However the bus only metered at 0v instead of the 28v coming from the cables plugged into the PSU. My question is how is that possible and what would cause that? It was theorized that an negating voltage was getting sent back through one of the lines but I'd still expect the correct voltage to be recieved by all the lines that were good. Anyways, after unplugging and plugging all the cables one by one, I found I only had problem when the cable that supplied power to the pan/tilt PCB was plugged in. I still got 0v on the bus when that PCB was completely disconnected from all motors and encoders. Changing out that PCB ended up doing the trick.