Electronics > Beginners
Old analog oscilloscope questions
tedsorvino:
Thank you very much for your time.
These pictures will help you understand what I mean by saying "constructed not grounded - floated". Drussell is closer to what's going on.
https://ibb.co/k8i3bK
https://ibb.co/hVcwwK
Any ideas how to ground this thing in order to be safe?
I attach the proper schematic
Gyro:
You might be best just replacing the mains cable if you're confident enough - that one looks pretty manky. According to the schematic, there ought to be an easily identifiable grounding point where it terminates inside. Posting a photo would help identify it. If you do open it, don't touch any components associated with the CRT or the EHT lead, they might still have a high voltage charge.
Also have a look inside that plug, you wouldn't be the first person to discover a ground wire either cut short or tucked away in a 2-pin plug! [Edit: In fact it's a round section cable on a piece of equipment manufactured in the UK, I'd lay odds that there's a ground wire in there!]
P.S. That ground banana socket looks more like a convenient ground reference for the Z-mod input than a mains earthing point.
drussell:
--- Quote from: tggzzz on August 08, 2018, 07:03:34 am ---If it is not connected and a scope probe's shield is connected to a dangerous voltage (e.g. mains), then then entire scope's case will be at that dangerous voltage.
--- End quote ---
True, but if it were earthed it would probably just blow up your probe or scope. :)
You shouldn't be attaching your input/probe ground to anything other than a grounded point on your device under test or bad things are bound to happen.
--- Quote ---It is also possible, depending on why the scope was "floated", that there is internal damage to the mains transformer.
--- End quote ---
No, it was designed to be floating, just like most older scopes I've seen, especially the old vacuum tube based ones intended for the TV repair industry.
--- Quote ---It appears to have a metal case that is connected to the probe shields, which is normal for a scope like this. Such instruments must be earthed - for other people's safety[1] and to prevent other equipment being damaged.
--- End quote ---
Hyperbole much? :)
You're more likely to damage your scope or your DUT if your scope is grounded. Don't get me wrong, I would normally run with it grounded too, but it actually makes equipment damage more likely, not less. It reduces the chance of you shocking yourself if you do something silly but not the chance of equipment damage. If you're probing a device where the spot that you put your measurement ground is at a voltage that will give you a shock, you should be running your DUT on an isolation transformer.
--- Quote ---What do you mean "constructed floated"? Do you mean that:
* the original documentation states it is designed to be operated when floated. But you don't have that information
* you have examined the interior circuit, and it is clear that it has never had a protective mains earth and never needed one. But you are a beginner, and so could not tell
--- End quote ---
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.
He could change the input cord to permanently ground the chassis but that really doesn't make sticking your finger or probe ground to anything but ground any safer and removes the ability of sticking the scope ground on, say, a 12v power rail, to reference to that for measurements. Having your scope chassis at 12v isn't dangerous, you just need to know what you're doing. :)
If you want to be safest, use a GFCI upstream of the scope and ground the scope frame. Then your GFCI will trip (hopefully) preventing any damage to anything should your probe ground go astray. :)
tedsorvino:
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.
tggzzz:
--- Quote from: drussell on August 08, 2018, 04:08:51 pm ---
--- Quote from: tggzzz on August 08, 2018, 07:03:34 am ---If it is not connected and a scope probe's shield is connected to a dangerous voltage (e.g. mains), then then entire scope's case will be at that dangerous voltage.
--- End quote ---
True, but if it were earthed it would probably just blow up your probe or scope. :)
--- End quote ---
That would be a better outcome.
--- Quote ---You shouldn't be attaching your input/probe ground to anything other than a grounded point on your device under test or bad things are bound to happen.
--- Quote ---It is also possible, depending on why the scope was "floated", that there is internal damage to the mains transformer.
--- End quote ---
No, it was designed to be floating, just like most older scopes I've seen, especially the old vacuum tube based ones intended for the TV repair industry.
--- End quote ---
The OP's scope wasn't. This thread isn't about your scope.
--- Quote ---
--- Quote ---It appears to have a metal case that is connected to the probe shields, which is normal for a scope like this. Such instruments must be earthed - for other people's safety[1] and to prevent other equipment being damaged.
--- End quote ---
Hyperbole much? :)
You're more likely to damage your scope or your DUT if your scope is grounded. Don't get me wrong, I would normally run with it grounded too, but it actually makes equipment damage more likely, not less. It reduces the chance of you shocking yourself if you do something silly but not the chance of equipment damage. If you're probing a device where the spot that you put your measurement ground is at a voltage that will give you a shock, you should be running your DUT on an isolation transformer.
--- End quote ---
No hyperbole.
I don't think you are replying to what I wrote, viz: "prevent other equipment being damaged".
--- Quote ---
--- Quote ---What do you mean "constructed floated"? Do you mean that:
* the original documentation states it is designed to be operated when floated. But you don't have that information
* you have examined the interior circuit, and it is clear that it has never had a protective mains earth and never needed one. But you are a beginner, and so could not tell
--- End quote ---
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 ---
You are hallucinating. His schematic shows the mains input should be connected to the protective mains earth.
--- Quote ---He could change the input cord to permanently ground the chassis but that really doesn't make sticking your finger or probe ground to anything but ground any safer and removes the ability of sticking the scope ground on, say, a 12v power rail, to reference to that for measurements. Having your scope chassis at 12v isn't dangerous, you just need to know what you're doing. :)
If you want to be safest, use a GFCI upstream of the scope and ground the scope frame. Then your GFCI will trip (hopefully) preventing any damage to anything should your probe ground go astray. :)
--- End quote ---
There are many ways in which a GCFI can fail to protect you. A couple of examples, but it is easy to think of others:
https://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=4503
https://midas-uk.co.uk/its-alright-guv-its-got-an-rcd-on-it/ (Nice URL :) )
You appear to be forgetting that the OP is a beginner and doesn't know what he is doing. In that context your statements are dangerous.
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