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AC polarity help. Can't get a DMM reading?

hopjohn

hopjohn

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Ok so I'm trying to mark the non polarized plug on my TU-717 for lowest voltage by measuring AC on chassis to outlet ground. I've removed all the connections except the power, put my DMM on the 500 setting with AC (V~ with wavy line, right?), put the red probe on a chasis screw, the black probe on the center screw to wall outlet and /or inside the ground at the wall outlet (scary!) and NO reading. Also tried reversing the plug, and putting DMM setting at 200 V~ (only other one I have ) no luck there either. I've isolated the transformer with rubber grommets already so I don't know if that would make a difference, but I'm stumped. Oh , yes and I did have the tuner powered up.
 
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To find the non polarized plug,i just hold the black probe in my hand and the red to any in/out RCA plugs negative.
 
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Yes the cord is marked with a white stripe, but is that side to go the wide or narrow spade receptacle at the wall outlet. Also, I've heard that isn't always a reliable method of getting the lower of the two voltages.
 
Well, there might be an easier way.

If your receiver is designed and wired properly, the power switch will be on the hot leg. So, find the wire that goes into the chassis and to the power switch. Either simply follow it back to the plug and mark that side, or else set the DMM to ohms or continuity check, and use that to find the matching side of the plug.

Just as an fyi, in case you're not sure, and this applies both to plugs and receptacles, wide is neutral and narrow is hot.
 
Fisherdude said:
Well, there might be an easier way.

If your receiver is designed and wired properly, the power switch will be on the hot leg. So, find the wire that goes into the chassis and to the power switch. Either simply follow it back to the plug and mark that side, or else set the DMM to ohms or continuity check, and use that to find the matching side of the plug.

Just as an fyi, in case you're not sure, and this applies both to plugs and receptacles, wide is neutral and narrow is hot.
Click to expand...

I still dunno why I get no reading, but in any case I guess I can do this and assume it's wired properly.

Any possibility my DMM isn't capable of reading these voltages. What types of readings are typical for a tuner?
 
Well, I'm not exactly sure what readings you're expecting. If you set your DMM to AC Volts, and put one lead on the chassis, which is at ground potential (usually), and the other lead on the center screw of the wall outlet, (assuming it's not painted), which should be at ground potential, your DMM should read close to zero volts. Or, if you go from chassis to the wide (neutral) socket, zero volts, or chassis to the third (ground) in the socket, zero volts. When you say you're getting "no reading" do you mean zero volts, or literally the meter is showing nothing?

If you're reversing the plug and then expecting to see a different reading from chassis to earth ground at the outlet, well, you might see a different reading and you might not. Depending on whether or not there are floating grounds inside the circuit, and/or if there are safety caps, there might be a difference in the reading, or there might not.

Bottom line, you want the hot lead from the wall going through the power switch, because that's the only safe way to hook something to the mains, and I assume that's the way the Sansui engineers designed the receiver to work. Your best course of action is to simply identify which pin on the Sansui's plug goes back to the power switch, put a Sharpie dot on that side, and always make sure to plug that side into the narrow slot on the wall outlet.
 
The white stripe of the original power cord is the neutral side, thus should go to the wide spade of your new plug.

Scott T.
 
Clay...

Because the gear is technically floating from ground, the AC voltage on the chassis could be anywhere from a few volts to 100+ volts. It's a very high-impedance source and thus very unlikely to hurt anyone, but the root cause of the voltage can be flux leakage from the transformer (and the chassis acts as another 'winding' of sorts), or perhaps the manufacturer intentionally tied the chassis to one of the AC cord conductors with a high-value (2.2M ohms or more) resistor, as was commonly done on a lot of gear. Either way, sensitive circuits (like an FM front end) can benefit from having the chassis at the lowest possible voltage potential.

Why the OP is not reading anything, who knows? Maybe his three-prong outlets are really floating too.
 
Thanks, Glenn. I really appreciate the explanation. I knew that it's quite common to see a few volts on the chassis with respect to ground due to the reasons you mentioned, but I had no idea that it could get that high. I knew about the line resistors, but the transformer "connecting itself" to the chassis...that's another new one.

Thanks again!!
 
EchoWars said:
Why the OP is not reading anything, who knows? Maybe his three-prong outlets are really floating too.
Click to expand...

There is a test to make. Measure from the smaller AC lug on the plug to the ground on the plug. You should read 120V plus or minus something. Then from the larger lug to ground. This should be 0V.
 
Echo and others,

Just to clarify when I say, "I'm not getting a reading", I mean a 000 reading (meter on the 500 setting) with no change from the time I turn on the meter, and no change with respect to the position of the plug.

I have one of the wall outlet testers you speak of, and the outlet in question tests normal.

I will use a cheater plug next time I want to do a similar test to make it easier to reach earth ground.

I looked for resistors between the power and the chasis and found none on the TU-717. There are ceramic capacitors tied to the chasis from the negative side of each of the RCAs, a ceramic capacitor between the pins on the power switch , and a ceramic capacitor between the neutral side of the ac cord and the rear of the user replaceable fuse.

BTW I would vote for this to be a sticky, in DIY or some place where it can be more easily found.

http://www.audiokarma.org/forums/showthread.php?t=7802

There's also the below link which reads basically the same, but has some additional info which may be useful.

http://www.gcaudio.com/resources/howtos/acpolarity.html

hopjohn
 
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Perhaps you should reduce the meter scale. It could be there is little or nothing there. On a XXX scale I'd presume anything less than 0.5 volt would show as 0.
 
I had tried the 200 setting (00.0), with no changes there either. I guess zero and zero are just the results.
 
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