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Old 9th March 2011, 03:21 AM   #1
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Default Help-AC input connected directly to case!

I hope this is the right place for this - it does concern a linestage I'm building. I put a Lite Audio A-15 in an old Marantz receiver case, and I decided to use as much of the Marantz as possible, including the AC wiring uo to the transformer, fuse container and on-off switch (which has a small value cap across it). It works pretty well and sounds anywhere from OK to great depending on the equipment I pair it with (too much gain though). I was having a bit of trouble with noise, and finally determined that I need to connect the case to earth ground (it only had a two-prong plug). Sooooo....I started poking around the AC inlet to see how I could manage it, and found to my utter surprise that one wire of the AC wall cord is connected directly to the case!!!!!! (It's also connected to the ground of the unswitched power outlet on the back of the unit - small red arrow in pic). My question is – why is this thing not a toaster, with me as the toast?

I'm thinking of undoing the connection to the case to see if that's the cause of some of my noise (hum & buzz - it's not audible over the music or even audible at all at lower listening levels, but I still want to get rid of it). Is that a good idea?
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Last edited by schubert; 9th March 2011 at 03:22 AM. Reason: typo
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Old 9th March 2011, 03:53 AM   #2
Bonsai is offline Bonsai  Taiwan
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Something sounds wrong here. I'd be damn careful . . .

Do you have a last will and testament?
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Old 9th March 2011, 10:58 AM   #3
DF96 is offline DF96  England
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Proceed with caution! However, when something seems totally wrong it sometimes turns out that it is not wrong but merely confusing. Check, double-check and then check again.

Can you find a circuit diagram for the original receiver? This might show the power arrangements.
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Old 9th March 2011, 11:05 AM   #4
SY is offline SY  United States
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It's hard to tell from the picture which wire it is. If it's the "third" wire, that's normal and safe. With everything unplugged, measure the resistance to the chassis of each of the power cord pins. If the third pin (the round one in the US) is grounded to the chassis, that's correct. If either the hot or neutral pin (the flat ones in the US) are connected to the case, someone has been fiddling in there and given you a time bomb.
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Old 9th March 2011, 11:35 AM   #5
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Quote:
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It's hard to tell from the picture which wire it is. If it's the "third" wire, that's normal and safe. With everything unplugged, measure the resistance to the chassis of each of the power cord pins. If the third pin (the round one in the US) is grounded to the chassis, that's correct. If either the hot or neutral pin (the flat ones in the US) are connected to the case, someone has been fiddling in there and given you a time bomb.
It's not the third wire because there isn't one - it's a two prong, unpolarized plug. I thought maybe there was a resistor in there (the connection is a piece of bare wire covered by a loose plastic sleeve) but measurements show no resistance between the black AC input wire and the case. It certainly appears to be part of the original design. Obviously the whole setup is OK because I'm still here and there's no measurable voltage between the case and earth ground. About 3V leaks into the power supply when the unit is turned off because of the cap across the on-off switch; I can't help but think that cap has something to do with this and the case would probably be live without it when the unit is switched off.
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Old 9th March 2011, 11:50 AM   #6
SY is offline SY  United States
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OK, that must be a really old unit, made before consumer safety standards were implemented to prevent once-common electrocution. Get that plug out of there! Replace it with a three prong cord and plug, with the safety ground attached to the chassis and the hot/neutral leads isolated from the chassis.
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Old 9th March 2011, 01:22 PM   #7
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If I had to guess I'd say it's from the 70's. I'll replace the inlet with an IEC three prong and connect the third wire to the case - that's usually what I do, but I wanted to re-use as much of the Marantz as possible (makes for less casework in the back!). Needless to say I'll be inspecting things a lot closer next time I do something like this!

I'm still curious as to why this arrangement works and why a manufacturer would wire a receiver like this, if anyone could shed some light on the subject.

Last edited by schubert; 9th March 2011 at 01:27 PM. Reason: addition
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Old 9th March 2011, 01:31 PM   #8
SY is offline SY  United States
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It works as long as neutral in the mains socket is really neutral and hasn't had its connection to ground deteriorate, and that this is also true of all other pieces of equipment that are connected. Unfortunately, this isn't always the case, and the injuries and deaths which resulted (see "Stone the Crows" for a colorful example) forced manufacturers to adopt the current safer standards.
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Old 9th March 2011, 03:17 PM   #9
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I had no idea what "Stone the Crows" is, but I looked it up and - point taken. I may end up selling this anyway, so that makes the grounded plug mandatory.
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Old 9th March 2011, 07:25 PM   #10
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....... and found to my utter surprise that one wire of the AC wall cord is connected directly to the case!!!!!! (It's also connected to the ground of the unswitched power outlet on the back of the unit - small red arrow in pic).
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......... Obviously the whole setup is OK because I'm still here and there's no measurable voltage between the case and earth ground. .......
I'm confused.....

You have an unpolarized AC power cord but a grounded (ie 3-terminal) unswitched power outlet?

If you measure the voltage between earth ground (ie ground at the wall outlet) and the case there is 0 volts AC? And if you reverse the unpolarized plug in the outlet, there is ??? volts AC?

BTW, definitely a good idea to get these issues resolved before selling.
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