Well, they have a schematic and block diagram to help them find errors, but I have to say that Kudelski was very picky about what they used to build Nagra.
We are not talking about an Uher Report or a Revox, Telefunken Studer...
Normally I always change electrolytic capacitors, but Nagra has electrolytic capacitors that were really only found in military applications.
I know also no Nagra which quit the service because of broken Elkos. If you have a component tester, unsolder the suspected electrolytic capacitors, test them and replace them if they are defective, but I