Odd 2247A problem, looking for insight


 

I have a nice looking Tek 2247A scope here that has been sitting on the shelf for a while, due to a bad Channel i input. the signal is a horribly attenuated and differentiated replica of the actual input. Everything else is perfect. In a moment of covid-induced boredom, I felt I had to get to the bottom of this. I had a hunch that this was the input resistor being broken or damaged (there is always a flying resistor from the input BNC to the vertical input on 2200 series scopes). It was the only thing I could think of that could give this result.

It takes some effort to de-can the scope, and remove the complex shield over the vertical inputs, but sure enough, when opened, the barely visible input resistor (62 ohm carbon film) was BURNED. I replaced it, and ta-da, everything is working. What the heck?

Here's what I just can't figure out, this scope has no 50 ohm input function, so no low impedance path exists to help burn this resistor due to over-driving, SO HOW DID it get burned and leave the scope undamaged?

I am totally mystified by this, and keen to hear any opinions. I just can't see how this fault happened without vaporizing the vertical input channel.

all the best for the new year,
walter (walter2 -at- sphere.bc.ca)
sphere research corp.

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