[GreenKeys] [External] Re: UPDATE- shockingly basic question - Teletype Model 33 ASR

[Jones, Douglas W]

From:  Nick England [navy.radio at gmail.com] -- Thursday, February 11, 2021 11:29 AM

> *Across* the resistor???? Do you mean scope probe to one side and ground clip to the other?
> Careful! If part of the loop is grounded, and you hook up the scope ground to the wrong spot, you can short out the power supply. Sparks sparking! Fuses fusing! Transistors blowing quickly to protect the fuse!

My scope is a Tektronics 321, older than the Model 33, it turns out.  Great little scope, and when I need to use it in a context like this, I run it from an isolated 12V power supply so the whole scope floats relative to ground.  That way, I can avoid blowing anything up.  Of course, all the metal parts of the scope then float at an unknown and potentially unsafe level, so I put the scope on a wooden board and avoid touching the metal parts.

The one-hand rule is important when doing this kind of thing.  Keep one hand behind your back when working with exposed conductors at potentially harmful voltages.

A scope with differential input would be far better to use for this kind of thing.

         Doug Jones
         jones at cs.uiowa.edu