Vintage 2 Prong - Are They Isolated Mains Supply? - Page 2 - diyAudio
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Old 17th July 2013, 07:10 PM   #11
Champ is offline Champ  Canada
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Quote:
Originally Posted by Mooly View Post
No problem

The ground of the scope would still connects to ground in the amp but can you see now that there are two ground paths both connected together via the mains leads. In practice that means that you could still measure with one or other disconnected (not that I'm saying you should disconnect the amp ground wire). And just as in audio, you could get a ground loop that affects sensitive (really low amplitude) measurement.
So we do not use the probe ground?
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Old 17th July 2013, 11:09 PM   #12
Enzo is offline Enzo  United States
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There always need to be a shared reference between the unit under test and the test instrument. Usually that is circuit common, which in turn is usually ground.

If your scope has a grounded mains cord, and your UUT is also so equipped, then in many cases the mains cord can serve as the ground connection. But you do run the risk of ground loops that way - may or may not happen.

In my view, I use that crude method for crude measurements. If I am just looking for overall signal level or presence of power supply or whatever, good enough. If I am looking for low level sensitive scoping, I will ground my probe to the circuit common, assuming that is at earth potential.

I say it that way because I work on a lot of switching power supplies, and it is common to find in the primary side switching circuits that the control IC and such work at a relatively low voltage, but the reference at their common is to -170vDC. (In other words a 12v circuit has -170 as ground and -160 as +12)

If you rely on the wiring from one chassis, back through the mains cord, through the mains outlets, and back up the other mains cord to the other chassis, it is a lot of opportunity for noise to enter the system.


In your opening post you refer to 2-wire mains cords and what we refer to as "hot chassis" gear. The way to service that is to have an isolation transformer on your bench. The UUT plugs into that so it is no longer directly connected to the mains. So that way the chassis can still be "hot", but it is no longer referenced to the mains. That affords you the safety you need, and you can clip your grounded scope to the chassis.
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Old 18th July 2013, 08:11 PM   #13
Champ is offline Champ  Canada
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Quote:
Originally Posted by Enzo View Post
There always need to be a shared reference between the unit under test and the test instrument. Usually that is circuit common, which in turn is usually ground.

If your scope has a grounded mains cord, and your UUT is also so equipped, then in many cases the mains cord can serve as the ground connection. But you do run the risk of ground loops that way - may or may not happen.

In my view, I use that crude method for crude measurements. If I am just looking for overall signal level or presence of power supply or whatever, good enough. If I am looking for low level sensitive scoping, I will ground my probe to the circuit common, assuming that is at earth potential.

I say it that way because I work on a lot of switching power supplies, and it is common to find in the primary side switching circuits that the control IC and such work at a relatively low voltage, but the reference at their common is to -170vDC. (In other words a 12v circuit has -170 as ground and -160 as +12)

If you rely on the wiring from one chassis, back through the mains cord, through the mains outlets, and back up the other mains cord to the other chassis, it is a lot of opportunity for noise to enter the system.


In your opening post you refer to 2-wire mains cords and what we refer to as "hot chassis" gear. The way to service that is to have an isolation transformer on your bench. The UUT plugs into that so it is no longer directly connected to the mains. So that way the chassis can still be "hot", but it is no longer referenced to the mains. That affords you the safety you need, and you can clip your grounded scope to the chassis.
Interesting points and advice.

How is a hot chassis component grounded at all, considering there is no direct link to "Earth Ground". Is the neutral ground?
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Old 18th July 2013, 09:13 PM   #14
Enzo is offline Enzo  United States
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We tend to be sloppy in our informal use of terms.

Ground refers to the earth connection, but we tend to call all circuit commons ground, because usually one is connected to the other. Clearly if you make a battery powered headphone amp for a guitar and plug into it, or the ubiquitous wireless unit, there is no "ground" connection, but within the system, there is a common point that serves the same purpose.

Neutral in your wall is supposed to be wired to ground in your service panel in the basement or wherever. Nowdays even the plug on a table lamp is polarized, but for many years they were not. That is why a zillion Fender and other brand amps had a ground switch. It connected a cap to chassis from one side of the mains or the other, you select. IN practice we'd plug the amp in, and flip the ground switch to the position that gave the lest shocks when touching other gear. And hopefully that was the position that hummed least. You could more or less do the same thing by flipping the plug over in the wall socket.

In the old days, mom and dad were used to flipping plugs. If the radio gave you shocks, turn over the plug. In an old hot chassis radio or the amp in a record player, once we got the plug in the wall the best way... Typically the preamp circuit was "grounded" to the chassis. The power amp and power supply common was not. The3y were linked by a high value resistor and a cap.
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Old 19th July 2013, 12:13 AM   #15
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The first thing to do is get a GFCI (Ground Fault Circuit Interrupter) receptacle or box.

Don't get confused about making connections to good old Mother Earth (she is not part of this situation) and making a connection to the in house Safety Ground (EGC).
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Last edited by Speedskater; 19th July 2013 at 12:16 AM.
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Old 19th July 2013, 12:19 AM   #16
Champ is offline Champ  Canada
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Originally Posted by Speedskater View Post
The first thing to do is get a GFCI (Ground Fault Circuit Interrupter) receptacle or box.

Don't get confused about making connections to good old Mother Earth (she is not part of this situation) and making a connection to the in house Safety Ground (EGC).
Yep, using one. Hope it works when I need it Hopefully never that is
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