Next I did two things:
- visual inspection of the 7A13 but nothing was obviously burnt (but I met many nice looking components)
- searched on TekScopes Yahoo group for similar failures
Back to the book then! The user and service manual is a work of art itself, so it was a pleasant experience to go through it. Looking at parts list and their position I finally spotted a polarised capacitor (C518) hidden under wires and behind the module structure:
|C518 is the burnt blob in the centre.|
I really doubt it originally looked "burnt brown", with a shade of orange towards its pins. So, this is the starting point to fix this instrument.
I will not go much further, since I don't really need this module and it already had a broken relay when I acquired it (debugging and broken relay posts from 10 years ago). Needless to say, this piece of history is the result of 1960's and 1970's engineering and I am glad to have had the chance to see and touch it. Oh, is has a digital display too!