This seems to be a common problem so I thought I would report on my repair.
Summary:
Apparently fixed by replacing the U500 hybrid with a known good one.
Details:
This is my main scope. In 2015 it received new caps, repaired SMD A5, and new cal. memory. It worked flawless up until a few months ago. At that point it started failing the power on self test with ¡°Test 05 Fail 40¡± messages.
I set the scope aside dreading trying to find what I assumed would be another damaged trace on A5. After a while, I pulled it off the shelf and started testing it. Of course, it passed all self-tests.
So I put it back in service and waited for it to fail again. Eventually, it did.
This time I carefully measured the power supplies and checked that the DAC was basically functioning. Everything was okay. Before trying to troubleshoot the A5 sample-and-hold circuits I thought I would try an easy experiment. I swapped the U500 hybrid between a 2430A and this scope. Surprisingly, both scopes now work and pass self tests.
I don¡¯t have a good explanation as to why this worked (or indeed if it has ¡ª only time will tell). Perhaps the hybrid is marginal and the 2430A uses it under slightly different conditions. Perhaps an issue with contacts ¡ª although I had already tried cleaning the contacts and reseating it.
