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Old 17th July 2013, 05:22 PM   #1
Champ is offline Champ  Canada
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Default Vintage 2 Prong - Are They Isolated Mains Supply?

I will be getting my first oscilloscope any day now. I also do not want to blow myself up

My research indicates that probing around an amp with an Oscilloscope could be dangerous if correct grounding procedures are not observed. I am interested in
troubleshooting vintage gear. Since most of this stuff uses mains 2 prongs with no dedicated ground, does this mean they are isolated from the mains earth.

I do understand that I should stay away for the power supply of the amp, but am I ok to ground to various circuit board ground points in the signal path while probing?

Newbie question to be sure.

Thanks for any help.
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Old 17th July 2013, 06:18 PM   #2
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That's a very good question.

Vintage gear (particularly old radios and TV's) can in reality mean stuff that is non isolated and so the dangers are real. OK, studying a service manual or circuit diagram if available would help. If there is a mains transformer powering the unit then that's a good sign that its isolated but don't count on it. It could be something called an autotransformer which doesn't provide isolation.You can always measure with a meter (resistance) to see if there is continuity between either prong and chassis. You can also measure voltage (AC and DC) between the scope ground and what you think is ground in the unit. There should be no voltage but in practice leakage and "floating" means that some voltage may be present. Lets say you did measure say 100 (or maybe a lot more) volts AC or DC. In that case connecting a resistor of say 47K 2watt between the suspect points should kill the voltage difference. If it doesn't and the voltage is maintained then the point you thought was isolated isn't.

And if your talking audio amps with normal inputs then these are pretty sure to be safe to be grounded, after all its what would happen when it was all connected up anyway.
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Old 17th July 2013, 06:27 PM   #3
Champ is offline Champ  Canada
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Thanks Mooly.

Here is the attached HK 930 receiver I will be troubleshooting. If you have a sec can you have a look and let me know your thoughts on this layout are with regards to the grounding.

Last edited by Champ; 17th July 2013 at 06:34 PM.
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Old 17th July 2013, 06:32 PM   #4
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Where
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Old 17th July 2013, 06:36 PM   #5
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Too big to upload I see. Try here.

Harman Kardon 930 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
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Old 17th July 2013, 06:39 PM   #6
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Just seeing the picture and description is enough for that one. Fully isolated. The chassis metal work will be "audio ground".
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Old 17th July 2013, 06:41 PM   #7
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So that means I can ground to either the chassis or ground on a PCB being tested without fatal error?
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Old 17th July 2013, 06:50 PM   #8
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Yes. Just don't connect the scope to anything on the primary side of the mains transformer.

All the points marked on the circuit with a ground symbol can be connected to the scope ground lead.
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Old 17th July 2013, 06:55 PM   #9
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Then this begs the question... What about a component that is 3 pronged and connected to earth..
How does one handle this? Where does the ground probe go?

I know this is very basic, but I am having trouble finding this info on line. Lots of info on how to use a scope, but not much practical stuff out there.
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Old 17th July 2013, 07:06 PM   #10
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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.
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