AM radio noise

  • Thread starter Thread starter Randy
  • Start date Start date
Vanguard °°° said:
He must've been communicating to you separately via e-mail. I read both
of Randy's posts in this newsgroup and neither mentioned the use of an
ungrounded outlet.
Click to expand...

I did not communicate separately... my outlet is grounded.
I'll have to check this on my next system build to see if a load is
really needed or if just shorting the PS-ON signal pin to ground will
work. If it doesn't then I'll know that I need to generate a load (but
would that be 1A across all taps or just 1A on one of the taps)?


The Antec PS tester does let you use a voltmeter but does provide the
green LED "idiot light" to show the PSU is okay. It has a 25W 5.4-ohm
load to create a load (but they don't mention if they load all outputs
or maybe, say, just +5V), but it's a fixed load so you cannot increase
the load (current draw) to check for degradation in regulation (which
would require an oscilloscope to see the change in the ripple, anyway).


Again, I didn't see Randy mention using an ungrounded wall outlet. I
did see him mention about trying to use a surge protector that also
included some line conditioning (although it looks like simply a passive
setup using a capacitor) but then that wouldn't protect the line from
getting noise from the output side of the surge protector. Putting both
the computer and radio on the line conditioning surge protector wouldn't
protect the radio since both devices are on the output side of the
filter. Maybe if he had plugged the computer into the wall outlet
instead of the surge protector and then plugged the only radio into the
surge protector he might've seen some relief if the line conditioning in
the surge protector was of any value (but it sounds like the problem
might be with RF instead of induced EMF).
Click to expand...

first, thanks to everyone who has responded... it has definitely made
me think about a few things.

To set the record straight I just finished many tests.

My outlet is grounded. With the computer off but the ps switch in back
set to "on" I get am radio interference (bad) in all that circuit's ac
outlets in the room. I also get interference on a battery operated
radio in the same room... maybe a smidgen better but still
unlistenable to. I had plugged the computer into the new surge
protector and it didn't help. I also plugged the clock/radio into the
surge protector and it didn't help. My case is metal. The power supply
is 300w adt-300 (came with my DTK WT-PT074W ATX Tooless case).

I disconnected everything and shorted the power supply... the fans
came on and it still wrecked the AM radios. I turned off the power
switch in back of the case, left it connected to the motherboard but
tried disconnecting each drive in turn... still AM radio problems. I
then left the drives connected and disconnected the motherboard...
there was a quick (maybe 1 second) AM radio disturbance and then it
faded away to become pretty good. Plugging the motherboard back in
made it bad again. I disconnected all the cards from the motherboard
but it was still bad if the power supply was plugged into it.

Now, could it be the motherboard? I think not because this is the
second motherboard I had in this case and both were AM radio
destroyers. First a ECS-K7S5A and now an Asus-A7N8X. I'm thinking that
maybe the motherboard connection clicks something special on the power
supply that shorting it does not.

Anyway there it is in a nutshell. I think I might try a better more
powerful (430w) ps. With a dvd, dvdrw, 2 hd, floppy (useless), canopus
analog video converter, radeon 9600pro, modem and three case fans I
might be pushing it with the original 300w cheapo ps. But... I've
never had a single problem with it other than the AM noise.

Randy
 
I did not communicate separately... my outlet is grounded.


first, thanks to everyone who has responded... it has definitely made
me think about a few things.

To set the record straight I just finished many tests.

My outlet is grounded. With the computer off but the ps switch in back
set to "on" I get am radio interference (bad) in all that circuit's ac
outlets in the room. I also get interference on a battery operated
radio in the same room... maybe a smidgen better but still
unlistenable to. I had plugged the computer into the new surge
protector and it didn't help. I also plugged the clock/radio into the
surge protector and it didn't help. My case is metal. The power supply
is 300w adt-300 (came with my DTK WT-PT074W ATX Tooless case).

I disconnected everything and shorted the power supply... the fans
came on and it still wrecked the AM radios. I turned off the power
switch in back of the case, left it connected to the motherboard but
tried disconnecting each drive in turn... still AM radio problems. I
then left the drives connected and disconnected the motherboard...
there was a quick (maybe 1 second) AM radio disturbance and then it
faded away to become pretty good. Plugging the motherboard back in
made it bad again. I disconnected all the cards from the motherboard
but it was still bad if the power supply was plugged into it.

Now, could it be the motherboard? I think not because this is the
second motherboard I had in this case and both were AM radio
destroyers. First a ECS-K7S5A and now an Asus-A7N8X. I'm thinking that
maybe the motherboard connection clicks something special on the power
supply that shorting it does not.

Anyway there it is in a nutshell. I think I might try a better more
powerful (430w) ps. With a dvd, dvdrw, 2 hd, floppy (useless), canopus
analog video converter, radeon 9600pro, modem and three case fans I
might be pushing it with the original 300w cheapo ps. But... I've
never had a single problem with it other than the AM noise.

Randy
Click to expand...

No, plugging it back into the motherboard just allowed the +5V turned on
(even with the unit powered off). As long as AC power is supplied to
the power supply unit (because the rear switch is on), the +5V power is
active in an ATX-style power supply. Someone else already re-mentioned
this to me since I had missed or forgot it. When you shorted the
power supply (presumably meaning PS-ON signal to ground), this would
have enabled the +5V tap to go active again, and when it did you got the
AM radio noise again. You've had 2 motherboards in this machine but the
same power supply and the problem continued. With no power to the
motherboard or drives, the problem lingered. Sure looks like a power
supply problem to me.

How old is the power supply (or the case setup that included it)? Is it
still under warranty? Maybe you could get a warranty replacement power
supply. It's possible that this power supply was defective from the
start and that a replacement of the same model would eliminate the
problem. However, the cost of shipping it to them might be too
expensive and could be better used in purchasing a new one.

I didn't find any computer cases at http://www.dtk-computer.com.tw/, and
none of the cases listed at
http://www.dtk.at/products/products_cases.htm were for your case model
(and none of the cases that they list show a model of the power supply
that matches yours). So I can't lookup what power supply was included
with the case. The model number you mentioned might've been a part
number from the reseller from whom you got the case.
 
Vanguard °°° said:
No, plugging it back into the motherboard just allowed the +5V turned on
(even with the unit powered off). As long as AC power is supplied to
the power supply unit (because the rear switch is on), the +5V power is
active in an ATX-style power supply. Someone else already re-mentioned
this to me since I had missed or forgot it. When you shorted the
power supply (presumably meaning PS-ON signal to ground), this would
have enabled the +5V tap to go active again, and when it did you got the
AM radio noise again. You've had 2 motherboards in this machine but the
same power supply and the problem continued. With no power to the
motherboard or drives, the problem lingered. Sure looks like a power
supply problem to me.
Click to expand...


And it was! After listening here and searching on review sites I went
out and bought an Antec Truepower 430w. No more problems (even when
the system is up and running) and it's quieter than the old sucko 300w
psu. Maybe the old one got damaged in one of our 3-4 power blackouts
over the last 3 years or maybe it always sucked. Either way the new
one seems great... though I'm sure it costs more to run :-(

Thanks everyone... I've probably built more than 100 machines for
people but never really worried about the psu. On some I still won't,
but on my own I'll want a good one.

Randy
 
larrymoencurly said:
Is a ground needed for the input line filter to be effective even if
the filter doesn't connect to ground, as is the case with many Antecs,
including my 300W SmartPower? My Antec causes little AM radio noise,
but I've never plugged it into a 2-wire outlet.
Click to expand...

The input line filter in the PS connects to "chassis ground", i.e. the
metal PS enclosure. This is connected to the computer case, which is
"grounded" (earthed) by the green wire in the line cord, which is, in
turn connected to the "U" shaped ground in the outlet.

The outlet ground, in a correctly wired situation, is connected to the
wiring conduit or to a ground wire if "Romax", (no conduit), is used.
At the power inlet to the building, these ground wires are connected
to the "building ground" and all the return wires, (neutrals), are
connected to the same point. Older buildings may not comply with
this requirement, which is now almost universal, at least in the US.

Wired correctly, this ground prevents the two power wires from acting
like an antenna and radiating noise fed into it by the power supply.
I have seen "3 prong" outlets where the ground was not even connected
or did not exist in the old wiring.

This is an unsafe situation as well as contributing to other problems.

Every RF interference situation is different, depending on the length
and position of the supply wiring, the shielding provided by the source
and the receiver, and maybe the position of the moon and stars! ;-)

After doing all the usual things, including providing a proper ground,
a lot of "cut and try" is usually called for. Of course, bad equipment
may produce interference which is very difficult to eliminate.

Virg Wall
 
FWIW, I had the same problem. I upgraded to another, larger HD and
noticed the problem was gone. I'm assuming it was the old HD causing
the noise. Sometimes you just luck out.
 
The input line filter in the PS connects to "chassis ground", i.e.
the metal PS enclosure.
Click to expand...

How can my PSU's filter connect to ground when it has no any low-value
Y-rated disk capacitors from line-ground, just X2 capacitors across
the lines? It looks a lot like the filter in this 380W Antec
TruePower (lower left):

http://terasan.okiraku-pc.net/dengen/no48/true/open.jpg

and has a .47 uF yellow boxy capacitor across the AC receptacle, a
choke in series with one of the AC lines, followed by a .22 uF
capacitor across the lines, then a choke in series with each AC line
(both wrapped around the same core), and finally another .22 uF
capacitor across the lines.

Another whose filter that doesn't connect to ground is on page 12 of
this example 90W ATX PSU (C1, C2, LF01):

http://us.st.com/stonline/books/pdf/docs/6412.pdf
 
larrymoencurly said:
How can my PSU's filter connect to ground when it has no any low-value
Y-rated disk capacitors from line-ground, just X2 capacitors across
the lines? It looks a lot like the filter in this 380W Antec
TruePower (lower left):

http://terasan.okiraku-pc.net/dengen/no48/true/open.jpg

and has a .47 uF yellow boxy capacitor across the AC receptacle, a
choke in series with one of the AC lines, followed by a .22 uF
capacitor across the lines, then a choke in series with each AC line
(both wrapped around the same core), and finally another .22 uF
capacitor across the lines.
Click to expand...

It's hard to tell from the picture. See:

http://www.pavouk.comp.cz/hw/Alim_PC_ATX_200W_fr.pdf (page 1)

for the kind of circuit I was refering to. Note the two 47 mmfd capacitors
with the mid point connected to the ground (shield). In an actual PS,
some heat sinks are connected to the case ground, and some are floating.
Note the center point of the two main filter capacators connected to
ground thru a varister.

The actual enclosure is bolted to the computer case so it is always at the
line ground wire potential.
Another whose filter that doesn't connect to ground is on page 12 of
this example 90W ATX PSU (C1, C2, LF01):

http://us.st.com/stonline/books/pdf/docs/6412.pdf
Click to expand...

There doesn't seem to be any ground (shield) here. You're correct, the
filter capacitors are between the hot line and neutral. There are three
different ground symbols in the diagram. It's hard to figure which is which.
I assume the "three pronged" ones donate case ground.

RFI prevention is an emperical kind of thing. Each situation is unique.
I have seen several examples of the building wiring acting as a good
antenna, especially when not properly grounded.

Virg Wall
 
In order for that ground wire to be part of a filtering
system, that wire must be short. Take a portable shortwave
receiver. Connect directly to earth ground - inches. Signal
is substantially reduced if not totally cut off. Connect same
shortwave receiver to same ground with 50 feet of copper
wire. Now that ground connection become a perfectly good
receiving antenna.

Grounding the filter through 50+ feet of safety ground wire
does not earth the filter. Instead that 50 foot wire would
become a transmitting antenna for any RF noise that leaked
out.

Many filters require that ground wire because of leakage
currents from AC electric. A line filter, like other low pass
filters, does not require an earth ground to be functional.
That filter simply acts as a dam - to stop higher frequency
signals from passing out of power supply into any AC wires.
Any signal that gets through that filter is then connected to
a good transmitting antenna - those AC wires. Purpose of
filter and of that 'lump' ferrite bead on monitor cable are to
stop (block, impede) RF signals from escaping the computer.
Connection to safety ground is necessary in some designs to
bleed off leakage currents. But a 50 foot connection to earth
ground would only act as a transmitting antenna.
 
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