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?
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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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.
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.
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.
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.
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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.
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.
Can you perhaps tell us which model Marantz this started life as, and then we'd have a sporting chance of finding out how it was originally wired. Even in the 70s I think there were few products supplied with one side of the mains intentionally connected to chassis. This is sounding slightly incredible.
No, the unswitched outlet is also two-prong, (presumably) one being hot and the other neutral. I took the one that was connected to the case to be the neutral because it was shorted to the other (switched) outlet and because I, so far, have lived to tell the story. The AC voltage from the case to the metal trim on my kitchen counter is 0V and while I haven't played around with reversing the plug and measuring, I haven't always plugged it in the same way while using it either, and I haven't been zapped yet. I do intend to replace this mess with a proper IEC plug. I won't have the outlets any more, but I never use them anyway.
Upon reflection I wonder if I've been saved from becoming fried bacon by the fact that the current has an easier path to ground through the transformer - either by means of the on switch or (if the switch is off) the small cap across the switch.
Upon reflection I wonder if I've been saved from becoming fried bacon by the fact that the current has an easier path to ground through the transformer - either by means of the on switch or (if the switch is off) the small cap across the switch.
This sounds even more incredible; the 2 prong plug is UNPOLARIZED and one wire was connected directly to chassis? I'd have to agree with Huntingdon, this is just too incredible. I'd say this can't be true, there must be some error in your decription or what you are assuming is the stock Marantz wiring.
Believe me, I'm quite mystified myself!!! 
It certainly looks like stock wiring - the connections all look the same and mass-produced. There's a small protrusion in the case that appears to have been put there specifically for this connection. There's no doubt about the connection or the fact that the case isn't live, and I just can't figure it out.
It certainly looks like stock wiring - the connections all look the same and mass-produced. There's a small protrusion in the case that appears to have been put there specifically for this connection. There's no doubt about the connection or the fact that the case isn't live, and I just can't figure it out.
Well, this unit spent its childhood as a Marantz SR-325 stereo receiver from 1984-85. I cursorily looked around for a schematic/service manual but couldn't find a free one.
One thing I considered was that there was a 1M or so resistor in the sleeve, but I measured the resistance between the input wire and the case and got the familiar "beep" from my DMM indicating a short. Now I'm wondering if I was accidentally shorting my meter some other way, 
for when I measured the AC volts between the poles of the unswitched outlet I got 120V, but only 19V between the hot pole and the case, and 1-2V from the neutral pole to the case, depending on whether it's switched on. Sooo....there must be a resistor in there. Now I'll take it apart and find out for sure. 
for when I measured the AC volts between the poles of the unswitched outlet I got 120V, but only 19V between the hot pole and the case, and 1-2V from the neutral pole to the case, depending on whether it's switched on. Sooo....there must be a resistor in there. Now I'll take it apart and find out for sure.
You need a decent ground reference for troubleshooting....either a proper earth ground at your workbench (direct connection to good ground rod) or (better) a connection to the ground of your AC house supply.
'Cheater' plugs assume that the wall box is grounded and connect to the faceplate screw...so that is one place to look.
With 'modern' plumbing, it's not a sure thing that you can get an earth ground from a plumbing connection - too many ways that you can get a plastic 'break' in the line, IMO.
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