Ground point causes a bang

I've just started working on a monitor PSU - its completely dead. I though I'd connect up the scope and have a look around. After connecting the scope ground to the circuit ground (see attached pic - circled in red) I switched on the monitor.

Good job I didnt have a large breakfast - there was an almighty bang. After checking the circuit the track to the 220V fuse was melted.

What gives? Why would connecting the scope ground to this point cause this? I didn't even connect the probe anywhere yet.

**broken link removed**

If it helps any its a Tektronix 2213A scope.

Sometimes especially with TV's and the like it pays to run these of a 230 / 230 volts 500 VA or 1 kVA isolating transformer to avoid having a hot chassis shorting out to earth via the scope leads.

I've done this by hooking up a waveform generator to a simple mains totallising counter. It tripped my RCB in the house leaving me in the workshop in the pitch black.

any way around this - other than having a isolating transformer

Check this thread https://www.electro-tech-online.com/threads/oscilloscope-and-grounding.15623/

gregmcc said:

any way around this - other than having a isolating transformer

Click to expand...

hi Gregg,
Unplug the monitor from the mains, use your ohm-meter to check which of the twin mains lead wires goes to the chassis,
it should be the NEUTRAL wire.
If the house wiring to the mains plug outlet has been incorrectly wired ie: reversed, it may look as though the Black/Blue wire in the mains cable is going to the chassis.....BUT

[set the monitor power switch to ON so it dosn't give an o/c reading]

It sounds as though the LINE/PHASE wire was connected to the monitor chassis when it blew the fuse.

Once you have the NEUTRAL connected to the chassis, plug the monitor in, then using a AC voltmeter,
measure between the chassis and a good earth point, the voltmeter should read close to zero.

DONT touch the chassis until you have confirmed its close to earth potential.

If you can get a neon mains tester screw driver its a good tool to have in your box,
use it always before you touch a monitor or older TV chassis.

Eric

Thanks for the link and other tips. I'll have a read through and apply.

GOOD DOD! You basically connected your scope ground to the power line. A ground symbol does not indicate “earth ground”. There is a “chasse ground” that is OK to connect to. There can be GND1, GND2, GND3 etc. They are only a reference point just like VCC, VEE and +5V are points that do not have to be referenced to ground. The ‘ground’ you used is the “power supply” ground. The full wave bridge 601 has the power line connected to it’s two AC leads (~). The power supply ground is one diode away form the line voltage.

I design power supplies. If you work on supplies that are connected to the power line go back and read the thread from IRL3103. The ground leads on my scope probes have burn marks from doing the same thing and I know better.

Keep safe!!!

ericgibbs said:

hi Gregg,
Unplug the monitor from the mains, use your ohm-meter to check which of the twin mains lead wires goes to the chassis,
it should be the NEUTRAL wire.
If the house wiring to the mains plug outlet has been incorrectly wired ie: reversed, it may look as though the Black/Blue wire in the mains cable is going to the chassis.....BUT

[set the monitor power switch to ON so it dosn't give an o/c reading]

It sounds as though the LINE/PHASE wire was connected to the monitor chassis when it blew the fuse.

Once you have the NEUTRAL connected to the chassis, plug the monitor in, then using a AC voltmeter,
measure between the chassis and a good earth point, the voltmeter should read close to zero.

Click to expand...

Sorry Eric, but that is complete nonsense!. Nothing has been connected like that for many decades, everything uses bridge rectifiers which leave the chassis at half mains potential regardless of the mains polarity. Back in the 60's (and very early 70's) TV's used to use half wave rectifiers and if you connected the mains the wrong way round it meant the chassis was connected directly to live - but this practice ended 30+ years ago.

It's a long running discussion, but this is the exact reason that I (and most service engineers) consider earthed scopes too dangerous to use - it's far safer to have an unearthed scope - although in either case, safety is a question of understanding what you're doing.

Nigel Goodwin said:

Sorry Eric, but that is complete nonsense!. Nothing has been connected like that for many decades, everything uses bridge rectifiers which leave the chassis at half mains potential regardless of the mains polarity. Back in the 60's (and very early 70's) TV's used to use half wave rectifiers and if you connected the mains the wrong way round it meant the chassis was connected directly to live - but this practice ended 30+ years ago.

It's a long running discussion, but this is the exact reason that I (and most service engineers) consider earthed scopes too dangerous to use - it's far safer to have an unearthed scope - although in either case, safety is a question of understanding what you're doing.

Click to expand...

or use diff-amps
but yer tek scopes with a battery pack saved my bacon a number of times

Or run at least one or both items via 230/230 volts isolating transformers which will give you maximum safety as i suggested in an earlier post.

Apply the one hand rule during testing.

Nigel Goodwin said:

Sorry Eric, but that is complete nonsense!. Nothing has been connected like that for many decades, everything uses bridge rectifiers which leave the chassis at half mains potential regardless of the mains polarity. Back in the 60's (and very early 70's) TV's used to use half wave rectifiers and if you connected the mains the wrong way round it meant the chassis was connected directly to live - but this practice ended 30+ years ago.

It's a long running discussion, but this is the exact reason that I (and most service engineers) consider earthed scopes too dangerous to use - it's far safer to have an unearthed scope - although in either case, safety is a question of understanding what you're doing.

Click to expand...

hi Nigel,
You are right, that info was dug from my 'archive' memory banks!

All I can say in my defence is:
I have found in my many trips to South Africa, [Gregg's country] that a lot of the domestic electronic/electrical [computer] equipment in circulation in SA are items that were current in the UK over 10/20 years ago.

It seems that the developing countries are used as dumping grounds for old stock.

I tried buying a TV a couple of years ago in SA, asked the sales guy where the Scart connector was,
he didn't know what I was talking about!
All I could get was mono/video RCA/Phono connectors.

No offence intended to SA, its a great country.

Recall the other day a member in Jo'burg with the blown heater fuse, NO fused plugs, just the old round 15Amp, remember those Nigel, in the UK, 30 years ago.

The best solution would a mains isolation transformer.

Regards
Eric

I've done a trace through the ground points and you are all correct, the GND on the scope is connected to the mains GND (as it should be), the point I 'thought' was GND on the circuit (as ronsimpsom says, is marked gnd but was not really gnd) was really the mains neutral!
No wonder there was a bang!!

Thanks for all the replies. ericgibbs - I think if you still ask around here for scart the sales guys will look at you funny. Its never really taken off here.

BTW The mains ground is also connected to the chassis ground on the monitor.

ericgibbs said:

I tried buying a TV a couple of years ago in SA, asked the sales guy where the Scart connector was,
he didn't know what I was talking about!

Click to expand...

Why would he?, SCART is a European standard (actually French), and isn't used outside Europe, so you wouldn't expect a South African to have heard of it (or an American).

I've repaired a number of sets from SA, the stereo sound is quite amusing! - with a built-in FM radio for the second channel!

Nigel Goodwin said:

Why would he?, SCART is a European standard (actually French), and isn't used outside Europe, so you wouldn't expect a South African to have heard of it (or an American).

I've repaired a number of sets from SA, the stereo sound is quite amusing! - with a built-in FM radio for the second channel!

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hi Nigel,
It seems we often differ in our opinions, however, I do try to keep an open mind and where possible do some homework
to confirm or not my understanding of a point of difference.

The European PAL I standard, DVB and many other euro standards are already used widely throughout South Africa.
So why do you would think the European Scart standard wouldn't also be adopted?

In light of your reply, I did some homework to bring my self upto speed.

Looking at the SA market sales and forecasts, shows that the number of electronic devices, having a SCART connector,
being offered/sold and supplied to South Africa is increasing and also are the associated peripherals, cables etc.

I would predict, using the information that I have read thru, that SA will have the same common Scart usage as the UK market
within the next 2 to 3 years.

As I said in an earlier post, the developing countries lag behind Europe, by about 10 Years,
where else would businesses unload their old stock.

I cant imagine any manufacturer missing the opportunity to 'addict' another society in order to sell his product.

Perhaps in two years time, in SA, I could get a TV to DVD with a Scart connector

As a side issue, as we in Europe and the Americas have a fully developed electronics component infrastructure,
we should remember that many countries do not have that luxury..

To us, a 741 or 16F84 are old hat, but in some places thats the level of components they have to manage to work with.

I appreciate that you did not raise this point, I just felt it should be noted, as quite often OP's are criticised for using 'old' components.

Eric

ericgibbs said:

As I said in an earlier post, the developing countries lag behind Europe, by about 10 Years,
where else would businesses unload their old stock.

Click to expand...

I don't think they do?, sets for SA are more likely specifically made for that market, and not old sets from elsewhere - certainly the sets I've seen from SA are SA specific. What you will probably notice, is that the sets will most likely have optional provision for a SCART, just as UK sets have optional provision for the SCART to be left out and phono sockets fitted instead. It's common practice to make a chassis, and a cabinet, easily changeable to different markets.

BTW, the SCART socket is 27 years old, it became mandatory in France back in 1980.

Nigel - I agree there. I've seen a few TV's from Sony/Samsung with the model no. information found on their local websites (co.za) and no info to be found on the international web sites. The sets must be therefore be made only for SA