Worship the GROUND that I walk on

[TrentO]

I just got off the phone with my Italian buddy Frank, talking about the crappy (from abuse) Tektronix 2445A Oscilloscope he recently gave me-- Ch.1 and Ch.2 appear to be very flaky or non-functional at all, based on ambient temperature. Anyhow, since he and I used to work for the same employer, where this scope came from, I guessed that someone measured a signal (probably RS-449) without ensuring that the ground potential was the same, thus cooking the O-scope channel(s)-- this was a big problem at my previous Japan-based employer, as they would frequently rip the grounding lugs off of US made equipment that spent time there (and even shave off the polarizing aspect off of edison-type plugs.) This practice would sometimes cause interesting problems when re-connecting this equipment upon return to the 'States. I have seen differences of 30 or more DC volts between the chassis' of devices in the same lab/datacenter, where this hand-me-down oscilloscope was once being used. I now have this Tektronix paper-weight which probably isn't worth getting fixed-- time to buy that Rigol.... Anyhow, this lead to an interesting conversation about grounding-- and a question from Frank about why grounding straps are (in his experience) always flat braided, plated cables. He believed that it has something to do with the "frequency" or some electrical characteristic that determined it construction.  His examples where-- the flat braid grounding straps between body panels in automotive applications, and straps in old TV-sets between the chassis and case. My assertion was that such cables only existed in applications where movement and/or vibration may be a problem, (and possibly corrosion,) and nothing to do with the electrical characteristics of the cable/connection per se. Can someone more authoritative than either me or Frank settle this argument? Frank was an EE by profession and I consider him extremely knowledgeable on many subjects, but in this instance I think the years of hard living in the '70's is coming through.

Also, any guidance on how to fix this Tektronix using RadioShack parts, would be most appreciated.

TIA,

-Trent

[Mastro Gippo]

I don't know about your problem, but my father works with CO2 lasers at home and he removed the ground pin from the AC plug because every time he was measuring something in the HV circuit he would trip the protection circuit of the whole house.. And i had to restore it out in the cold!

[GeekGirl]

I just got off the phone with my Italian buddy Frank, talking about the crappy (from abuse) Tektronix 2445A Oscilloscope he recently gave me

Also, any guidance on how to fix this Tektronix using RadioShack parts, would be most appreciated.

TIA,

-Trent

Trent,

First thing you need is a service manual http://www.bitsavers.org/pdf/tektronix/scope/070-3829-00_2445svc_Jan83.pdf.

I am downloading it now and will look over it and direct you to things to check

Regards,

Kat.

[GeekGirl]

I don't know about your problem, but my father works with CO2 lasers at home and he removed the ground pin from the AC plug because every time he was measuring something in the HV circuit he would trip the protection circuit of the whole house.. And i had to restore it out in the cold!

Oh dear, I hate it when people "play" with things 

You should NEVER disconnect the ground, it is there for a reason (if you disconnect the scope ground, then measure a high voltage, if you touch something metalic on the scope (bnc case etc) and something that is grounded, you become the ground link, which can give varying results from a slight tingle to death, depending on the voltage, the current it causes to flow through your body and the path it takes.

There are ways to do HV measurements without removing a ground (isolation transformers, isolated differential probes etc) sure these cost money to make or buy, but how much is your continued health worth ?

OK rant over, I am just a safety conscious person (after seeing a few horrible HV deaths !!!)

Regards,

Kat.

[GeekGirl]

BTW Mastro,

If your dad wants to join up (or ask question through you) a fair few of us would be able to advise him how he could do measurements safely (we need to know the set up etc)

Regards,

Kat.

[Mastro Gippo]

He's not playing with things, it's his job... Well, second job. I'm pretty confident that he knows what he's doing. One time, through, he got a big shock repairing our automatic gate, that made quite a damage to his arm while travelling towards ground. I'm really scared by mains voltage since when one of his coworker died while repairing an hair dryer, that was open and vibrating on the table and was falling off, and by instinct he tried to catch it. He left a wife and a children. I will never forget. 

[Dude]

Ground straps and bars are used because of the greater surface area, to reduce the impedance.
Search for things  like ground strap "skin effect".

[TrentO]

Oh dear, I hate it when people "play" with things 

The included is a public-service message from the AoEEWBG (the Association of Electrical-Engineer Wanna-Be Guys.)

Important rules to follow when in the company of AoEEWBG guys, especially for EE-savy women--

Rule #1-- No mention of guys 'playing' with things, regardless of actual intent.

Rule #2-- No communication of relative size by pinching your thumb and forefinger together, (i.e.: "yeah, he was THEEES close to getting it to work" and definitely never -- "his circuit was only this |------| big.'

Rule #3-- No comparisons of the tools that you use, versus those of AoEEWBG members-- "You really should get a STOUT, ROBUST, BEEFY multimeter, like the Fluke 289 that I use-- the one that you have is worst than the Hello-Kitty one I had in middle-school."

WARNING: Failure to follow the above rules may result in any present AoEEWBG members giving up the EE disciplines and taking up duck hunting instead.

Thank you, from the AoEEWBG, awaiting response from the AFL-CIO for membership.

[Neilm]

I don't know about your problem, but my father works with CO2 lasers at home and he removed the ground pin from the AC plug because every time he was measuring something in the HV circuit he would trip the protection circuit of the whole house.. And i had to restore it out in the cold!

Oh dear, I hate it when people "play" with things 

You should NEVER disconnect the ground, it is there for a reason (if you disconnect the scope ground, then measure a high voltage, if you touch something metalic on the scope (bnc case etc) and something that is grounded, you become the ground link, which can give varying results from a slight tingle to death, depending on the voltage, the current it causes to flow through your body and the path it takes.

There are ways to do HV measurements without removing a ground (isolation transformers, isolated differential probes etc) sure these cost money to make or buy, but how much is your continued health worth ?

OK rant over, I am just a safety conscious person (after seeing a few horrible HV deaths !!!)

Regards,

Kat.

I quite agree. Mind you, I did see a supposedly CATIII 600V multimeter explode when subjected to a typical mains surge on 600V so maybe I'm just paranoid.