Electronics > Beginners
Old analog oscilloscope questions
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tggzzz:

--- Quote from: drussell on August 08, 2018, 12:34:57 pm ---Obviously if you're floating a measuring instrument to do something, you need to know exactly what you're doing and not touch the chassis of your instrument or have any stray accidental connections touching the chassis anywhere, etc.  Certainly could be a shock hazard and not something that a beginner should be doing for any reason.

--- End quote ---

Like the person in this incident?... From https://groups.io/g/TekScopes/topic/7632418#10795 with my emphasis:

What makes it so dangerous to float the scope is that it is very easy for you to come in accidental contact with the floating scope chassis and receive a very bad shock, possibly lethal. One of my first customer contacts as a Sales Engineer for Tektronix was to call on the Sylvania Lighting Center in Danvers, MA and investigate a rumor about an engineer working there that was killed while using a Tek scope. I found it it was true. During lunch, one of the engineers was working alone in the lab on a lighting experiment that was using some 3 phase, 220 volt power. He needed to make some measurements between points none of which were at earth ground. So, he floated the scope . . . He even has the scope sitting on a scope cart with a sheet of insulation material between the bottom of the scope and the metal tray it normally sits in so the scope cart would not be "hot" with the scope. He also had a "tunnel" of plexiglas on both sides and over the top of the scope in a crude attempt to prevent anyone from accidentally touching the hot scope. The back was not covered with plexiglass in order to allow the fan to do its job and the front was not covered so the engineer could access the scope controls. This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway.

Anyone who teaches you that floating a scope by defeating the power cord ground lead is a very poor teacher, indeed. They simply do not know enough about making SAFE measurements to be in a position to teach electronics.
Gyro:

--- Quote from: drussell on August 08, 2018, 04:08:51 pm ---He has the schematic.  It shows that it is floating.  He could probably earth it using the banana jack on the back (assuming it is reasonably well connected to the chassis, not just connected to the transformer shield like it shows in the schematic) if he chooses (my Stark has no such provision, BTW...) but it came from the factory this way, it hasn't been modified.

--- End quote ---

The fact that it did come from the factory with a 3-core mains lead (and still has it) rather spoils your assertion.  ;)

As both I and tggzzz pointed out, the schematic quite clearly shows mains ground connected to chassis.
tedsorvino:
I think it's about time to leave behind the disagreement. All of you help in your way. And I'm REALLY THANKFUL .
So PLEASE help me with the rest of my INITIAL questions, since the GROUND issue is SORTED.
Gyro:

--- Quote from: tedsorvino on August 08, 2018, 05:00:53 pm ---Thanks everybody. I replaced the mains plug to a modern one. There was a ground connected but it was floated at the end. Here are some some pictures.

https://ibb.co/fUYsmK
https://ibb.co/ijoQ6K

What about my other questions regarding the binding post and the probe compensation - calibration. Any help.

--- End quote ---

The lack of a probe calibration output is certainly a bit of a problem for compensating your probes. What you need is a nice clean squarewave of around 1kHz. You could maybe make a basic oscillator using a 555 timer or logic gate - as long as there isn't too much overshoot. My feeling is that this would probably be 'good enough'.

[Edit: Sorry, yes you're right, I was just getting around to trying to give you some practical advice.]

P.S. I've lost track - which binding post? ... If it's the gate output, then yes, it's the same as the Tek manual. It can be used for things like frequency counters (to monitor the exact timebase frequency - not the signal frequency directly), to measure the exact sweep time, possibly detecting that the scope has triggered. With a some delay logic and the Z-Mod input, you could make a 'cursor' that accurately times between two points on the trace, etc. Not a lot of use to you to be honest.
tggzzz:

--- Quote from: tedsorvino on August 08, 2018, 05:53:58 pm ---I think it's about time to leave behind the disagreement. All of you help in your way. And I'm REALLY THANKFUL .
So PLEASE help me with the rest of my INITIAL questions, since the GROUND issue is SORTED.

--- End quote ---

We are being insistent since somebody else might come along in the future, read this thread, and somehow come away with the (dangerous) misapprehension that it is OK to float a scope. We're glad you've got the message! :)

Probe calibration. Some people think a scope's CAL out enables the timebase (ns/div) and vertical deflection V/div) to be calibrated. It doesn't, and usually has a roughly 1kHz frequency and roughly a volt amplitude. What is does have is reasonably fast edges (<1us) and a flat top with no ringing. That enables a probe's low-frequency compensation to be tweaked so that the displayed signal has a flat top as shown in the usual pictures.

So, in order to "calibrate" a probe (i.e. tweak its frequency response), all you need is such a flat-top square wave from some source or other. If you tell us what you have, we could say whether it could be used.
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