06 February 2017

Tektronix 7A13 module on fire (almost)

I spotted it in a drawer, so I tried to plug a 7A13 input module into my Tek oscilloscope chassis. In less than 10 seconds on the CRT a bright spot appeared and then all lights went faint. I shouldn't admit that I replugged the unit and tried a second time, and the experiment lasted just two seconds since all lights were faint since I pulled the switch (on this device you pull to power up and push to switch off). No smoke/smell was released.

Next I did two things:
  1. visual inspection of the 7A13 but nothing was obviously burnt (but I met many nice looking components)
  2. searched on TekScopes Yahoo group for similar failures
The search revealed that tantalum capacitors do fail abruptly, and they were used to decouple power lines inside the instrument.

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!

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