Five years ago my father and I found in a village of the province of Lleida (Catalonia) our first Alphatronic P2. Soon, problems regarding the power supply started to make an act of presence. The first day, the filter was blown. I can't remember exactly how much time passed but in a three months term the power supply was dead. In our case, the 5v module failed. As some of its components were popular only in West Germany we couldn't find direct replacement or information was missing so my father decided to innovate "a bit" and bypassed the +/-5V unit.
The bypass adapter, the connector of the ATX can be seen over the left border of the floppy drive surface. The 5V unit is removed and contacts placed on thhe screws, which are used in the design to send power between both boards.
Here connection to the P2 power supply can be seen with detail. Ground is common.
A pair, random pictures showing the connector of the ATX power supply with the adapter connected.
We use an old ATX power supply (those with a switch), but if there isn't any available a modern one (those with push button) can also be used. Prior to that we used one of those and if you do, remember that, in order to start it you need to connect the contact of the green wire in the main connector of the PSU with any ground contact on the came connector.
This does not fix your power supply, but allows the computer to turn on. I consider it an emergency repair and should not be considered a proper solution. It is very helpful, however.