[Sander deWaal]

Prune said:


A lot of time you can float ground partially, without fully
disconnecting it, say using chains of diodes or high power zeners, so if
the potential you're floating to exceeds what you expect, it can pass
and hopefully blow the fuse.


Yep, that's a good one.
Just like those people who ground their guitar amps via a quarter-watt
resistor to the case, to avoid hum.

Ever thought about what happens when that string of zeners (or that
resistor) burns out *and forms an open circuit*, hmmm?

Are you one of those who think that the ground lift switch on an amp
actually lifts the safety ground from the case, instead of the signal
ground?

Phil has a rather rude way of saying it, but he's damn right in his
conclusions: "solutions" like yours are dangerous and can have lethal
consequences.

You *don't* want to be the "engineer" who "fixed the hum" , attending
the funeral of a friend because of it, trust me.

--
"All amps sound alike, but some sound more alike than others".

[Sander deWaal]

"zack" said:


were do you go to get the codes for ng's
LOL meens lough etc.


http://www.mugsy.org/asa_faq/glossary/usenet.shtml

0.43 seconds in Google.

--
"All amps sound alike, but some sound more alike than others".

[Phil Allison]

"Sander deWaal"

were do you go to get the codes for ng's
LOL meens lough etc.


http://www.mugsy.org/asa_faq/glossary/usenet.shtml

0.43 seconds in Google.


** Fancy - long as that .....




........ Phil

[[email protected]]

On Thu, 20 Jul 2006 00:28:18 GMT, Prune wrote:

A lot of time you can float ground partially, without fully
disconnecting it, say using chains of diodes or high power zeners, so if
the potential you're floating to exceeds what you expect, it can pass
and hopefully blow the fuse.


That's not good advice.... you blew it with the word "hopefully"...

You can float the ground on things when you are a 55 year old tech like me and
haven't gotten a shock in 30 years... otherwise, leave the grounds intact.

I used to use a battery powered scope to avoid shorting to ground various
communication systems in the field, but they tend to run on battery's, and AC
was never a problem.

The only battery system I stayed away from was in the old railway switching yard
(long gone). They had a battery backup system for the car retarders, 575 V of
200 AH battery's in series! Now THATS dangerous!

As for the original poster, he should have observed that his filament wiring was
already grounded, and therefore used the ground connection to his advantage.
People who don't know this much shouldn't be using electric test equipment
un-supervised.

Normally, you DO NOT use a scope to measure AC mains leads, unless you have a
special attachment to allow it, usually a signal transformer. At work, we rent a
power analyzer when we want to see the AC.

[Prune]

Sander deWaal wrote in
:
Are you one of those who think that the ground lift switch on an amp
actually lifts the safety ground from the case, instead of the signal
ground?

My mistake; I was thinking indeed of signal ground. I always have safety
ground directly earthed.

[[email protected]]

On Fri, 21 Jul 2006 09:43:15 +1000, "Phil Allison"
wrote:




Normally, you DO NOT use a scope to measure AC mains leads, unless you
have a
special attachment to allow it, usually a signal transformer. At work, we
rent a
power analyzer when we want to see the AC.


** That seems over cautious to me - a standard 10:1 probe is well able to
safely cope with 120 / 240 volts AC.

I'm just going with the idea that you don't need a scope to check your AC, you
can use a meter. And if you really want to check the AC, a power analyzer can
tell you wonderful things. You just leave it on for the weekend and it logs all
kinds of events. The last one we rented was worth about $15,000... logs
frequency, harmonics, voltage fluctuation, everything.

Anyway, the problem with a scope is where to put the ground lead! LOL! Of
course, we know it goes on the fat pin...

( I also own a 100:1 probe rated at 1500 volts max - guess what THAT is
for, Bob.)

Gee, Phil, it might come in handy on vacuous equipment... I also have a
current-to-voltage probe, but have never tried it... it's good down to 1ma I
think.

Also, when working with triac and SCR circuits that "chop" the AC supply,
one cannot use a transformer to safety isolate the waveform from the scope
as it may well have a large DC component.

Yes, a transformer isn't a good idea with a switcher anyway, with all those
possible harmonics, you want to look directly.

Transformers are such snobs, don't like DC one bit - they get all snooty
and refuse to pass it and just sit there, fuming at you !


I've seen lots of gear that had a cap in series with the transformer... not sure
that's a good idea either.


....... Phil

[Phil Allison]

Normally, you DO NOT use a scope to measure AC mains leads, unless you
have a special attachment to allow it, usually a signal transformer. At
work, we
rent a power analyzer when we want to see the AC.


** That seems over cautious to me - a standard 10:1 probe is well able to
safely cope with 120 / 240 volts AC.

I'm just going with the idea that you don't need a scope to check your AC,
you
can use a meter.


** Nonsense - you *need* a scope to VIEW the AC wave .


( I also own a 100:1 probe rated at 1500 volts max - guess what THAT
is
for, Bob.)

Gee, Phil, it might come in handy on vacuous equipment...


** Yep - the plate waveforms on tube guitar amps.



Also, when working with triac and SCR circuits that "chop" the AC supply,
one cannot use a transformer to safety isolate the waveform from the scope
as it may well have a large DC component.

Yes, a transformer isn't a good idea with a switcher anyway,


** Wrong topic - Bob.

The subject is light dimmers and the like.



I've seen lots of gear that had a cap in series with the transformer.


** AC coupling is NO GOOD.

You need to see that DC component on the scope.





.......... Phil

[[email protected]]

On Sat, 22 Jul 2006 11:36:25 +1000, "Phil Allison"
wrote:




Normally, you DO NOT use a scope to measure AC mains leads, unless you
have a special attachment to allow it, usually a signal transformer. At
work, we
rent a power analyzer when we want to see the AC.


** That seems over cautious to me - a standard 10:1 probe is well able to
safely cope with 120 / 240 volts AC.

I'm just going with the idea that you don't need a scope to check your AC,
you
can use a meter.


** Nonsense - you *need* a scope to VIEW the AC wave .


I use a meter to check the AC, and very rarely see the need to scope it... most
power supplies don't care a lot about the AC except for amplitude. Newer stuff
works from 100 to 250v at from 50 to 60hz. and that covers it all!

Anyway, you can't tell much by looking at a wave, compared to actual harmonic
content analysis. Hard to guess distortion by eyeballing a trace. If the AC
trace is poor enough to see, it's really bad! Sometimes the AC here gets it's
peak clipped, due to the industry in the area.


( I also own a 100:1 probe rated at 1500 volts max - guess what THAT
is
for, Bob.)

Gee, Phil, it might come in handy on vacuous equipment...


** Yep - the plate waveforms on tube guitar amps.


I guess an Ampeg V22 can max out your probe!


Also, when working with triac and SCR circuits that "chop" the AC supply,
one cannot use a transformer to safety isolate the waveform from the scope
as it may well have a large DC component.

Yes, a transformer isn't a good idea with a switcher anyway,


** Wrong topic - Bob.

The subject is light dimmers and the like.

Which we call switchers here...



I've seen lots of gear that had a cap in series with the transformer.


** AC coupling is NO GOOD.

You need to see that DC component on the scope.



Yes, DC offset can be important and is readily visible on a scope.



......... Phil

[Phil Allison]

Normally, you DO NOT use a scope to measure AC mains leads, unless you
have a special attachment to allow it, usually a signal transformer.
At
work, we rent a power analyzer when we want to see the AC.


** That seems over cautious to me - a standard 10:1 probe is well able
to
safely cope with 120 / 240 volts AC.

I'm just going with the idea that you don't need a scope to check your
AC,
you can use a meter.


** Nonsense - you *need* a scope to VIEW the AC wave .


I use a meter to check the AC,


** Yawn - that is *utterly* beside the point.

When servicing or designing SCR/ triac *dimmers* it is ESSENTIAL to be able
to see the incoming AC wave and the output wave of the dimmer - this MUST
be done with a *DC* coupled scope to avoid phase shift and correctly
observe AC waves with possible MISSING half cycles.

Perfectly safe to do with any grounded scope & using a 10:1 probe.


Also, when working with triac and SCR circuits that "chop" the AC supply,
one cannot use a transformer to safety isolate the waveform from the
scope
as it may well have a large DC component.

Yes, a transformer isn't a good idea with a switcher anyway,


** Wrong topic - Bob.

The subject is light dimmers and the like.

Which we call switchers here...


** ********.

They are known as "dimmers" world wide while " switcher " is shorthand for
"switching PSU".





........ Phil