Walter,
I suppose it is possible that U112 has beefy input protection (diode clamps to the supplies?) that might survive enough current to burn a small 62 ohm resistor. At maximum sensitivity the attenuators would be switched out of the circuit.
--John Gord
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On Sun, Jan 3, 2021 at 11:35 AM, walter shawlee wrote:
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.
