Hazardous thing happened at work

[fezder]

now one hazardous thing happened at home, managed to create s/c with 'scope. it was stupid thing, but i was thinking that psu i was using was floating! luckily, only thing that took damage was one wire and pc fan.....oh well, mistakes happen

 

[Nigel Goodwin]

now one hazardous thing happened at home, managed to create s/c with 'scope. it was stupid thing, but i was thinking that psu i was using was floating! luckily, only thing that took damage was one wire and pc fan.....oh well, mistakes happen

That's why the majority of service engineers refuse to use an earthed scope - it's just too dangerous.

 

[fezder]

what's funny about this, is the fact earth is for safety purposes! IF i'd checked the psu whether or not it's grounded device, this hasn't happened. Also, i've used before only floating psu's (wall-warts)

and other thing that saved psu was because it has current limiting feature, only bad about it is that it cannot be changed, it's 25A IIRC.

 

[Nigel Goodwin]

what's funny about this, is the fact earth is for safety purposes!.

Not 'exactly' - earthing gives 'some' protection under certain exact circumstances - in just as many cases it makes it much more dangerous.

The reason for earthing is that the cases where it can be useful is mostly for the general public, and those who understand the reasons for it are generally safer without it. But, as always, it depends entirely on the exact circumstances and what's involved.

For servicing purposes it's almost always far safer to work in a completely earth free environment.

But the first thing I ever do with a scope is remove the earth lead (and leave it dangling out the back of the plug so it's obvious it's been done). Never had a modern scope with a moulded plug, in which case I'd have to remove the moulded plug and fit a normal one.

Funnily enough, whilst typing this my stomach is about 6 inched from a plug feeding a scope - just felt round it, yep - earth wire dangling out

But as I always say in these threads, if you don't understand the reason for earthing, or why it's a BAD idea in many circumstances - then leave it earthed.

 

[tvtech]

As a tvtech that has worked on both Live and Isolated chassis I will agree with Nigel 100%

Earth off for all Live chassis and also for SMPS if you are working on the Primary side.

None of the Scopes I have used in the TV game have ever used earth. Never shocked myself either (with scope) or blown one up or damaged one.

 

[fezder]

nigel, yeah you are right, i forgot ''almost'' word from sentence....

 

[4pyros]

I always used and isolation transformer for my scope, and I always left the ground lift switch's enabled.

 

[Misterbenn]

now one hazardous thing happened at home, managed to create s/c with 'scope. it was stupid thing, but i was thinking that psu i was using was floating! luckily, only thing that took damage was one wire and pc fan.....oh well, mistakes happen

I did exactly the same thing at school when I was 16. I was working off a 'safe' supply, cant quite remember the detials it was either 24Vac or -/+24Vdc. anyway it was usually used for the lab soldering irons on a ~20A circuit, but in this case I'd been allowed to use it for my electronics project.

Obviously the project wasn't fused, I was too young to think of such things. So when I connected the oscilloscope ground to the negative rail, something I'd done lots of times when using bench supplies, I'd actually shorted out the -24V rail of my project. The small ~2A rated supply harness burst into flame, and as I stood in shock the wires (eventually) melted and broke the current path leaving a nice scorch mark on the bench.
BUT I learnt a valuable lesson!

These days the majority of the scopes I work with have a battery pack so when in doubt I just unplug the mains entirely!

 

[throbscottle]

I nearly killed myself when I was 19, all by myself in my damp basement flat. Method of death could have been electrocution, starvation or burning (but really all by being stupid) - you choose. I had a '70's amp I'd been given that had some weird fault - one channel would go off after a while (thermal fault on a 741, I found). Anyway, this thing had a a wooden case, earthed metal chassis. I had to keep flipping it over to probe around, looking for the fault. Because of the exposed live terminals I was turning it off at the switch every time I flipped it, however the switch was in an awkward place to reach, and it quickly became a real PITA. So I thought I would just handle it really carefully, and flip it over without turning off the mains.

One tiny moment of inattentiveness - my thumb was near the amp's live mains connection, next moment it was on that connection, and of course I was holding the earthed chassis with both hands. Most of the current thankfully went through that same hand, but enough went up my arm and down my other arm to paralyse my arms and hands and part of my body. Unable to move, no-one to call out to, my thumb smoking where the live terminal was burning into it (the palms of my hands I discovered afterwards had also been starting to burn)...

Tune in next week for the next thrilling installment!

Yeah ok I won't do that.

I had to go to a Very Calm Place inside to work out which muscles I could actually use - ok the ones between my shoulder blades seemed to work. Pulled on those, the amp dropped out of my hands. Phew! Needless to say, I've always been very careful of live circuits since then.

 

[tvtech]

Hi Throb

Earthing is a much understood thing. Glad you survived

Regards,
tv

 

[tvtech]

By any chance one of these...John Linsley Hood built these with beautiful real wooden cabinets:

https://www.google.co.za/search?q=j...Fjohn-linsley-hood%2F75-de-luxe.shtml;800;269

Regards,
tv

 

[Nigel Goodwin]

Earthing is a much understood thing. Glad you survived

Presumably you meant 'misunderstood'

 

[Nigel Goodwin]

By any chance one of these...John Linsley Hood built these with beautiful real wooden cabinets:

https://www.google.co.za/search?q=john linsley hood&biw=1280&bih=671&tbm=isch&tbo=u&source=univ&sa=X&ei=GyysVNN4xNnuBrTAgKgO&sqi=2&ved=0CCIQsAQ#facrc=_&imgdii=_&imgrc=v0wPkZ_28-rdwM%3A;ndQ2LDNSiQ4QpM;http%3A%2F%2Fwww.hifiengine.com%2Fimages%2Fmodel%2Flinsley_hood_75_de_luxe_amplifier.jpg;http%3A%2F%2Fwww.hifiengine.com%2Fmanual_library%2Fjohn-linsley-hood%2F75-de-luxe.shtml;800;269

Those pictures are of professional kits, based on JLH circuits - no idea what he was like making cabinets, but he was certainly VERY active and influential on audio design.

I even bought one of his books a VERY long time ago, but it had a nasty accident with acid spill from a caving lamp - and had a tendency to fall apart if you touched it

 

[throbscottle]

Same style, but some cheap mass produced brand of its era - can't remember what.

 

[tvtech]

Presumably you meant 'misunderstood'

Hi Nigel

I had a brain fart. You are correct. Spelling of mine and writing what I mean seems to be getting worse

To be honest...JLH was the only one I could think of from yonks back with nice wooden cabinets on amps..

Regards,
tv

 

[Nigel Goodwin]

I had a brain fart. You are correct. Spelling of mine and writing what I mean seems to be getting worse

It's not really 'spelling' - it's just using entirely the wrong word, I do it myself as well

To make matters worse, if you read it back yourself your brain puts the correct word in for you, so you don't spot the mistake.

 

[tvtech]

At least you are here and listening/watching. Thanks for that Nigel

You went from CRT to Digital flat screen...did anyone help you understanding...or did you have to work it out for yourself?

Regards,
tv

 

[Nigel Goodwin]

At least you are here and listening/watching. Thanks for that Nigel

You went from CRT to Digital flat screen...did anyone help you understanding...or did you have to work it out for yourself?

Work it out for yourself

But now it's pretty well down to board changing, but the biggest problems (on decent makes) are LCD panel failures.

The crappy makes have normal PSU problems, often S/C rectifiers - due to their stupid idea of putting them in parallel, without any balancing resistors.

 

[tvtech]

I should probably start a new thread with this...not yet though.

Can I count on your help if I run into problems trying to fix non CRT...like Digital. I am clueless with that. And once it starts here...and word gets around...I will be inundated.

I only have to get lucky with one set....and then word gets around. Trying to plan and not be silly. I am trying to see what I am in for.

Not saying it will ever happen....but you never know.

Call it "looking ahead". And of course, I am a quick learner. I won't waste your time.

I just need guidance from you maybe with all and everything Digital that could come up.

I am bad with Digital...even my Meter is Analogue like me

All the best,
tv

 

[BGAmodz]

I remember one time repairing a SMPS welder , i changed two IGBTs , powered up the unit and a very bright light came out when those have blown .

Thank god my eyes are healthy , now i use welding glasses when i work also i use anti-short-circuit protection.