I have a 468 as well but have not used it in a very very long time.
Unless a near impossible part to find part has burned out it should be repairable.
Since this is a processor controlled scope, you need to make sure the processor part of the scope is working to properly troubleshoot. The ROMs could, over time, lose their programming and need to be replaced. A couple of bits here or there could stop the scope from working. This is what people may refer to as "ROM rot". ROM code is available and the ROMs were originally MK36000s, but can be programmed in a 27C64, since finding and programming MK36xxxs ROMs can be a challenge for most people. I seem to remember that you may need to make some wiring change to use the 27C64 chips or use an adapter, if it fits, in place of the original ROMs.
If you have an EPROM reader/programmer that can read the old 36xxx ROMs, the checksums and part numbers should be:
If the checksums don't match, the ROM is probably bad. If you are going to replace one, it is best to replace them both.
Hope this helps.