Author Topic: Old analog oscilloscope questions  (Read 5859 times)

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Online tggzzz

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Re: Old analog oscilloscope questions
« Reply #25 on: August 09, 2018, 03:27:28 pm »
I wouldn't worry too much about whether the scope is grounded or not. If it originally had a grounded plug which has had the ground cut off or been replaced with a 2-prong plug then it should be replaced with a grounded plug. If it was never grounded to begin with then don't worry about it, it's ok that it's floating. When people speak of "floating a scope" they often mean deliberately disconnecting the ground in order to use the scope to measure a high voltage non-isolated device, this is dangerous, not the fact alone that the scope is floating. So long as you are staying away from the mains side of whatever gear you're working on then there should be no problem. A good rule to follow is if you wouldn't touch something with your bare finger, don't connect a scope to it.

No, that is dangerous advice: it presumes an ideal world with complete knowledge and complete understanding.

If there is a mistake, or a misunderstanding, or a fault, then a floated scope can be lethal.

Example mistake: a probe slips.
Example misunderstanding: you misread a diagram, e.g. not recognising whether a protective mains earth connection is specified.
Example fault: mains has jumped to an unexpected place in the UUT - and that's why you are probing it in the first place.
And with a little imagination or knowledge, many many other examples can be found.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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Online tggzzz

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Re: Old analog oscilloscope questions
« Reply #26 on: August 09, 2018, 03:33:12 pm »
tggzzz used the wrong word - it is not probe "calibration" it is probe compensation.

Sigh. Quite right.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline james_s

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Re: Old analog oscilloscope questions
« Reply #27 on: August 09, 2018, 03:39:37 pm »


No, that is dangerous advice: it presumes an ideal world with complete knowledge and complete understanding.

If there is a mistake, or a misunderstanding, or a fault, then a floated scope can be lethal.

Example mistake: a probe slips.
Example misunderstanding: you misread a diagram, e.g. not recognising whether a protective mains earth connection is specified.
Example fault: mains has jumped to an unexpected place in the UUT - and that's why you are probing it in the first place.
And with a little imagination or knowledge, many many other examples can be found.

No, it presumes a very basic understanding that poking around in mains powered equipment is potentially dangerous. Lots of old scopes never had grounded power cords even when they were new, no need to modify the equipment to add one. If a person can't figure out not to poke around in dangerous stuff when they don't know what they're doing then they should not be using a scope. There is too much safety hysteria these days, can't protect everyone from themselves.

Your example fault of mains jumping to somewhere it shouldn't would be dangerous even with no test equipment involved. Loads of consumer equipment has no earth ground.
 

Offline james_s

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Re: Old analog oscilloscope questions
« Reply #28 on: August 09, 2018, 03:45:22 pm »
tggzzz used the wrong word - it is not probe "calibration" it is probe compensation. The probe automatically has a DC divide ratio of 10 (and 1 in the other switch position), the compensation trimmer in the probe is there to match the probe to the input capacitance of your particular scope. When compensation is correct, you will see a nice square wave with square leading corners, the tops of the squares should be nice and flat (not tilted). A bit of noise shouldn't stop you being able to do this.

That's true, however on quite a few scopes the terminal used to adjust the probe compensation is called "Calibrator" so I don't think the term is entirely inappropriate. If Tektronix thought it was reasonable to call it a calibrator that's good enough for me.
 

Online tggzzz

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Re: Old analog oscilloscope questions
« Reply #29 on: August 09, 2018, 04:50:13 pm »


No, that is dangerous advice: it presumes an ideal world with complete knowledge and complete understanding.

If there is a mistake, or a misunderstanding, or a fault, then a floated scope can be lethal.

Example mistake: a probe slips.
Example misunderstanding: you misread a diagram, e.g. not recognising whether a protective mains earth connection is specified.
Example fault: mains has jumped to an unexpected place in the UUT - and that's why you are probing it in the first place.
And with a little imagination or knowledge, many many other examples can be found.

No, it presumes a very basic understanding that poking around in mains powered equipment is potentially dangerous. Lots of old scopes never had grounded power cords even when they were new, no need to modify the equipment to add one. If a person can't figure out not to poke around in dangerous stuff when they don't know what they're doing then they should not be using a scope.

The OP is a beginner. Responsible responses should be tailored to that.

Quote
There is too much safety hysteria these days, can't protect everyone from themselves.

I've never over-protected my family; I backpacked round India with my 13yo, and before she could start to learn to drive a car, she was doing aerobatics that could have killed her within 10s, making forced landings, travelling at 90kt.

However, it is necessary to protect people from other people, and from other people's bad advice.

Quote
Your example fault of mains jumping to somewhere it shouldn't would be dangerous even with no test equipment involved. Loads of consumer equipment has no earth ground.

That's false, and a failure of your imagination.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline tedsorvinoTopic starter

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Re: Old analog oscilloscope questions
« Reply #30 on: August 09, 2018, 05:04:37 pm »
Thanks a lot Gyro. Your advice really really helps on what the next steps should be.
Thanks everybody for your kind effort and help.
 
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Offline Gyro

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Re: Old analog oscilloscope questions
« Reply #31 on: August 09, 2018, 05:49:33 pm »
That's true, however on quite a few scopes the terminal used to adjust the probe compensation is called "Calibrator" so I don't think the term is entirely inappropriate. If Tektronix thought it was reasonable to call it a calibrator that's good enough for me.

You're splitting hairs a bit too much there. Whatever you use as a source, the process is known as probe compensation and probe specs state a 'compensation range'. Many scopes call the terminal something like 'Probe Comp'.

The reason Tek (and a few others) call theirs a Calibrator is... (1) They normally implement it as a current loop for active current probe level calibration, and (2) It has specified accuracy for voltage and current. That's not general practice across all manufacturers.

Just because Tek implement it in a specific way doesn't justify an 'If it's good enough for Tek then its good enough for me' type response. More fundamental understanding is needed.


EDIT: ...and yes I am guilty too:

The lack of a probe calibration output is certainly a bit of a problem for compensating your probes.
:palm:
« Last Edit: August 09, 2018, 06:20:17 pm by Gyro »
Best Regards, Chris
 

Offline tedsorvinoTopic starter

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Re: Old analog oscilloscope questions
« Reply #32 on: August 19, 2018, 04:05:21 pm »
Finally, I found and purchased the manual (if anyone cares or needs it I can send the pdf) and I also bought a decent old Farnell signal generator. So I opened the scope, calibrated it and tested it.  Now it's really functional.
Some things (pots and trimmers) would be better replaced in the future (for more precise measurements), but after working out some solutions (changing the vertical subdivision to "calibration friendlier" values) and cleaning the whole thing with certain contact cleaners and compressed air sprays, I managed to revive it and make it work properly.
And in order to answer the question regarding the binding post ... after finding, purchasing and reading the manual. ...According to the manufacturer: "This output provides a positive going sq. wave of approximately 20V amplitude from a source impedance of 15kOhm. Its freq. is dependent on the setting of time/div and is suitable for probe compensation.
Mine is missing the jumper to the source impedance.
But in my eyes it's quite risky (things may fall apart or stop working) and troublesome to remove three old boards from an old oscilloscope. One day it may happen if it really needs to.
Thanks a lot everybody.
 


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