This is a switch-mode PSU, so the mains is rectified to around 400 volts DC inside.

Extra care is required to avoid touching this dangerous part of the PCB.

This can be remote controlled by GPIB, RS232, USB or Ethernet.

The user manual is here: CPX400S+SA+SP_Instruction_Manual-Iss9.pdf

I have requested the service manual.

Fault:

Output trips at over 20 watts.

Error on display.

Suspected cause:

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Examination:

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Action:

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Result:

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Status:

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Examination:

Remove three screws at the back:

Pop the plastic rivets at the sides:

They would not pop out so I ran a knife between the metal side and bottom.

You can see the display and controller boards (left), the power board (nearest), network board (furthest right) and GPIB board (middle right):

A better view from this side:

Be very careful, don't poke around willy nilly!

It is densely packed. The case forms a tube that the fan blows air through.

Networking boards:

Front panel display and control boards:

I can't see what that microcontroller is.

I suspect the 4-way cable is a serial comms link to the networking board. Maybe I2C?