Should outside of RCA jack be electrically hot?

[Michael Volow]

Recently noticed hum at high volume when receiver switched to phono input
(with phono connected). Phono works well otherwise. Phono ground intact.
However, when I unplugged my new HarmonKardon CD recorder from the mains the
hum went away. Same thing when I disconnected the patch cords from the CD
recorder to the receiver. Grounding the H-K to the receiver did not help.

When it measured the outside (shield) of any of the RCA jacks on the rear of
the H-K against ground in the outlet strip, it measured 60-70 VAC!!! when
the unit was plugged into the strip. The voltage of the outsides of RCA
jacks on other components vs the same ground was almost 0 VAC.

I have concluded that the abnormal voltage at the outside of the H-K's RCA
jacks is the source of the hum, and that the unit is defective. (It records
OK.) It is picked up in the phono input because it's such a low level
input.

Do others agree with this conclusion? Is my reasoning sound? I plan on
returning the H-K.

[unitron]

"Michael Volow" > wrote in message >...
> Recently noticed hum at high volume when receiver switched to phono input
> (with phono connected). Phono works well otherwise. Phono ground intact.
> However, when I unplugged my new HarmonKardon CD recorder from the mains the
> hum went away. Same thing when I disconnected the patch cords from the CD
> recorder to the receiver. Grounding the H-K to the receiver did not help.
>
> When it measured the outside (shield) of any of the RCA jacks on the rear of
> the H-K against ground in the outlet strip, it measured 60-70 VAC!!! when
> the unit was plugged into the strip. The voltage of the outsides of RCA
> jacks on other components vs the same ground was almost 0 VAC.
>
> I have concluded that the abnormal voltage at the outside of the H-K's RCA
> jacks is the source of the hum, and that the unit is defective. (It records
> OK.) It is picked up in the phono input because it's such a low level
> input.
>
> Do others agree with this conclusion? Is my reasoning sound? I plan on
> returning the H-K.


As much as it pains me to offer help to a "dookie" :-), (okay, it
actually makes me insufferably smug) when you say "phono ground
intact" do you mean that there's a green wire running from the
turntable to a grounding lug on the receiver? Are you measuring that
60-70 Volts AC with a high input impedence meter, like a digital?
Does the HK have a 2 prong plug on the wire that goes into the wall
socket or a 3 prong? If 2 prong, is one blade bigger than the other
so that it can only go in one way? Are you in an older building, say
pre-1965?
It does sound as though there might be something wrong with the HK
(and if you bought it locally take it back there along with your
meter, plug it in there, see if you get the same measurement, do the
same with another identical unit from their stock), but it's such an
unlikely fault I'm tempted to look for other causes, hence all the
questions.

[Michael Volow]

> As much as it pains me to offer help to a "dookie" :-), (okay, it
> actually makes me insufferably smug) when you say "phono ground
> intact" do you mean that there's a green wire running from the
> turntable to a grounding lug on the receiver?

Yes it's there and intact (although gray on my unit).

> Are you measuring that
> 60-70 Volts AC with a high input impedence meter, like a digital?

Yes, DVM.

> Does the HK have a 2 prong plug on the wire that goes into the wall
> socket or a 3 prong? If 2 prong, is one blade bigger than the other
> so that it can only go in one way? Are you in an older building, say
> pre-1965?

Yes 2-prong polarized plug. House built 1978. Also another CD player plugged
into the same outlet strip measures only 0.3VAC between the ground pin in
the outlet and the outside of this unit's RCA jack.

> It does sound as though there might be something wrong with the HK
> (and if you bought it locally take it back there along with your
> meter, plug it in there, see if you get the same measurement, do the
> same with another identical unit from their stock), but it's such an
> unlikely fault I'm tempted to look for other causes, hence all the
> questions.

Perhaps I should check to see if for some reason the H-K's plug
polarization is wrong. Thanks for your suggestions.

Mike V.

> > Recently noticed hum at high volume when receiver switched to phono
input
> > (with phono connected). Phono works well otherwise. Phono ground intact.
> > However, when I unplugged my new HarmonKardon CD recorder from the mains
the
> > hum went away. Same thing when I disconnected the patch cords from the
CD
> > recorder to the receiver. Grounding the H-K to the receiver did not
help.
> >
> > When it measured the outside (shield) of any of the RCA jacks on the
rear of
> > the H-K against ground in the outlet strip, it measured 60-70 VAC!!!
when
> > the unit was plugged into the strip. The voltage of the outsides of RCA
> > jacks on other components vs the same ground was almost 0 VAC.
> >
> > I have concluded that the abnormal voltage at the outside of the H-K's
RCA
> > jacks is the source of the hum, and that the unit is defective. (It
records
> > OK.) It is picked up in the phono input because it's such a low level
> > input.
> >
> > Do others agree with this conclusion? Is my reasoning sound? I plan on
> > returning the H-K.

[unitron]

"Michael Volow" > wrote in message >...
> > As much as it pains me to offer help to a "dookie" :-), (okay, it
> > actually makes me insufferably smug) when you say "phono ground
> > intact" do you mean that there's a green wire running from the
> > turntable to a grounding lug on the receiver?
>
> Yes it's there and intact (although gray on my unit).
>
> > Are you measuring that
> > 60-70 Volts AC with a high input impedence meter, like a digital?
>
> Yes, DVM.
>
> > Does the HK have a 2 prong plug on the wire that goes into the wall
> > socket or a 3 prong? If 2 prong, is one blade bigger than the other
> > so that it can only go in one way? Are you in an older building, say
> > pre-1965?
>
> Yes 2-prong polarized plug. House built 1978. Also another CD player plugged
> into the same outlet strip measures only 0.3VAC between the ground pin in
> the outlet and the outside of this unit's RCA jack.
>
> > It does sound as though there might be something wrong with the HK
> > (and if you bought it locally take it back there along with your
> > meter, plug it in there, see if you get the same measurement, do the
> > same with another identical unit from their stock), but it's such an
> > unlikely fault I'm tempted to look for other causes, hence all the
> > questions.
>
> Perhaps I should check to see if for some reason the H-K's plug
> polarization is wrong. Thanks for your suggestions.
>
> Mike V.
>
<snip>

Okay, the house is new enough to have been properly wired, but you
can take your meter and check to be sure that all your outlets are 120
Volt AC between the smaller "slot" (the "hot" wire) and the larger one
(which is connected to the building's ground rod but carries just as
much current as the hot wire) *and* the same reading between the
smaller slot and the "U" shaped hole (which also connects to the
ground rod, but doesn't usually carry current) and no reading between
the larger slot and the U-shaped hole (since both are supposed to be
at ground potential there should be no electrical difference between
them), or at least a very low reading, say under a Volt. It's
unlikely, but not impossible that an outlet could have been wired
incorrectly, with the large slot being the "hot" and the small slot
being the grounded one.
That 60 to 70 Volts might drop considerably if measured with a much
lower impedence meter, like a ten buck moving needle type, because
there isn't the ability to deliver much current there and the voltage
sags under load, but I'm pretty sure that there's still something
wrong with the HK that's not going to get any better on its own,
because it shouldn't be doing that even if the cord had been attached
backwards internally or the wall socket was wired backwards and we
seem to have eliminated outside causes.
Rather than spend any more time on what might be wrong with it to
cause the problem, just take it back where you got it and take your
meter along to make sure that the (at no additional cost to you and
with their most sincere apologies for your inconvenience) replacement
doesn't have the same problem.
Please note that my reply "simplifies" some things, not
(unnecessarily) for your benefit, but that of others who may read this
thread.

[CJT]

Michael Volow wrote:

>> As much as it pains me to offer help to a "dookie" :-), (okay, it
>>actually makes me insufferably smug) when you say "phono ground
>>intact" do you mean that there's a green wire running from the
>>turntable to a grounding lug on the receiver?
>
>
> Yes it's there and intact (although gray on my unit).
>
>
>> Are you measuring that
>>60-70 Volts AC with a high input impedence meter, like a digital?
>
>
> Yes, DVM.
>
>
>>Does the HK have a 2 prong plug on the wire that goes into the wall
>>socket or a 3 prong? If 2 prong, is one blade bigger than the other
>>so that it can only go in one way? Are you in an older building, say
>>pre-1965?
>
>
> Yes 2-prong polarized plug. House built 1978. Also another CD player plugged
> into the same outlet strip measures only 0.3VAC between the ground pin in
> the outlet and the outside of this unit's RCA jack.
>
>
>> It does sound as though there might be something wrong with the HK
>>(and if you bought it locally take it back there along with your
>>meter, plug it in there, see if you get the same measurement, do the
>>same with another identical unit from their stock), but it's such an
>>unlikely fault I'm tempted to look for other causes, hence all the
>>questions.
>
>
> Perhaps I should check to see if for some reason the H-K's plug
> polarization is wrong. Thanks for your suggestions.
>
> Mike V.
>
>
>>>Recently noticed hum at high volume when receiver switched to phono
>
> input
>
>>>(with phono connected). Phono works well otherwise. Phono ground intact.
>>>However, when I unplugged my new HarmonKardon CD recorder from the mains
>
> the
>
>>>hum went away. Same thing when I disconnected the patch cords from the
>
> CD
>
>>>recorder to the receiver. Grounding the H-K to the receiver did not
>
> help.
>
>>>When it measured the outside (shield) of any of the RCA jacks on the
>
> rear of
>
>>>the H-K against ground in the outlet strip, it measured 60-70 VAC!!!
>
> when
>
>>>the unit was plugged into the strip. The voltage of the outsides of RCA
>>>jacks on other components vs the same ground was almost 0 VAC.
>>>
>>>I have concluded that the abnormal voltage at the outside of the H-K's
>
> RCA
>
>>>jacks is the source of the hum, and that the unit is defective. (It
>
> records
>
>>>OK.) It is picked up in the phono input because it's such a low level
>>>input.
>>>
>>>Do others agree with this conclusion? Is my reasoning sound? I plan on
>>>returning the H-K.
>
>
>
>
>

It probably has an RF bypass capacitor from signal ground to each
side of the 120V power, which for a high impedance measurement
would look like a voltage divider (hence yielding 120V/2=60V +/-).

I think I'd strap signal / chassis ground to ground on one of
the components that has a three-wire cord and not worry about it.
However, be aware of the possibilities of a ground loop, and use
a fairly beefy piece of wire (for reasons in the next paragraph --
you want a fuse to blow rather than the strap to melt if there's a
fault).

Also be aware that those bypass capacitors have sometimes been
called "death capacitors" in the guitar world, because if the one
to the hot lead fails as a short, it can cause lethal voltages to
be present on the chassis where an unwitting guitarist can contact
them. Guitarists have died that way.

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