I had one on the bench for ages that didn't work. Every key on the front panel was dead (the internal membrane was always high resistance). I found some switches that were close to the originals to bring the old girl back to life.

Almost like new.

 

Beautiful! That is a clean unit indeed!
As you mentioned, Advantest (as well as Anritsu) Service Manuals are very rare.

 

dear @nsaspook
i got the same instrument with exactly the same error in keys.
I searched half of the internet, mouser digikey rs components and others but i did'nt found switches to replace.
do u have a hint for me which kind of switches u used to repair this beatuful voltage/current source and where to buy them?
AndreasR
a nearly broken man

 

dear @nsaspook
i got the same instrument with exactly the same error in keys.
I searched half of the internet, mouser digikey rs components and others but i did'nt found switches to replace.
do u have a hint for me which kind of switches u used to repair this beatuful voltage/current source and where to buy them?
AndreasR
a nearly broken man

Tactile Switches 6mmX6mmX5.5mm
Not an exact fit but can be made to work as in the above example:
https://www.mouser.com/ProductDetail/Alps-Alpine/SKEGAEA010?qs=m0BA540hBPcuEkLfJ1JfyA==

These are in stock: https://www.mouser.com/datasheet/2/15/alps_s_a0002904755_1-2258550.pdf

You need to be really careful when removing the caps from the original switches.