Re: Tek 465 o'scope - no display

"crispy" (cptnvortex@xxxxxxx) writes:

Before I did anything with the scope, I read the manual and set
everything the way the manual did to get what they call a "normal
display". To get this display of course, it needs to be hooked up to a
signal. I don't have one except for the wall outlet, and I didn't want
to use it at first, so I jumped to the probe compensation section so I
could use the calibrator. The only way I can get anything on the
display is with beamfinder. With beamfinder pressed, I can adjust
intensity, focus, vertical / horizontal, V/div, time scale, etc...
This is all true for both Chan 1 and 2. I was looking for a good
input, and thought that the wall outlet may be a bit much, so I looked
at the specs in the back of the manual and they say input AC voltage
limit is something way over 120V. Knowing that the impedence is also
very high, I decided that pulling out the scope plug enough to probe
the neutral prong would be safest, so I did that with nothing showing
up.

The reason to not do this isn't because the AC line is too high in
voltage.

The reason is that when you put the ground on one wire of the AC line,
and the probe on the other, you risk shorting the hot side of the line
to ground through the scope's ground line. Blow a fuse if you're lucky,
hurt something in your scope if you're not. I have a VTVM that I hooked
up to an AC/DC tv set without giving it any thought, and the ground lead
is not the original, having gotten real hot and melted the insulation
when it shorted the AC line to ground.

If you don't connect the ground line to the AC line, I'm not sure you'd
really be seeing a proper signal.

On the other hand, scopes are sensitive enough, and their input has
high enough impedance, that merely touching the probe will provide
a nice 60Hz (50Hz in some parts of the world), from stray pickup from
the AC line.

Michael
.

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