I know this is an older thread, but I may be able to assist in fixing a common failure on these boards since they apparently are no longer available. I am not an authorized Yamaha repair person, but I have repaired 3 of these boards. These boards are are found in the P-80 & P-90 keyboards. It may be in others, but these are the units on which I have worked. As I mentioned I realize this is an old thread, but I suspect it is looked up often given that this seems to be a common problem.
To begin, referencing the initial photo in the initial post. One should only find voltage on the two connectors outlined in green. The bottom is just a jumper to the power switch, the middle is the connector that carries the power to the other circuit boards in the unit. The top connector, outlined in red, is actually the audio coming from the main board to the output connectors on this jack panel. (lets hope there is no power there!)
I believe most everyone already knows, the most common problem is with the power input jack breaking and usually becoming intermittent before just breaking all together. The simple fix is to just re-flow the solder where the jack connects to the circuit board. Note the two solder pads to reflow in the picture to hopefully reconnect the jack to the circuit board. Make sure no solder crosses any traces! (see pic)
Reheating and flowing the solder on these two points was the simple fix on two of the units I did. Once I flowed the solder, I used a meter to confirm continuity between the center pin and the solder pad, and also confirmed continuity from the outer ring and its solder pad. Then it's a good idea to confirm there is no short between the two. If there is continuity between those two pads, there are other problems because there is a short.
On the third unit I repaired, It was a little more puzzling. Someone had already worked on it and already tried to re-flow the solder by adding a bunch of solder and It needed to be cleaned up. However once cleaned up, it still wasn't working. After doing a little more digging, I finally realized that the solder pads of the jack had broken so much that simply reflowing the solder didn't make a good contact with the trace.
I needed to add a jumper to jump the solder pads of the jack to the next component on the respective trace. Once I added the jumper, all came together! (see second pic)
A quick way to check for this problem is to simply use a meter and check continuity between the pads of power jack to the next component on the same trace. If you have continuity you should be good. If there is no continuity, then you probably need to add a jumper.
Hopefully this can help someone down the line.
