I am sorry that I can not say much about opening the ESX. I also don't have the sampler anymore.

I roughly remember that you first have to remove all knobs by pulling on them. Then the ESX had some screws with allen heads on the front. You had to remove these as well. As a last part there were some screws on the back side of the unit.

If the ESX is open then the right PCB with the connection to the power supply and the big capacitors is the one you are interested in.

Place the PCB on some kind of isolating material if you plan to switch on the device for testing reasons.