.... and no, it's not because it can't remember the words. ;-)

All the time my KEF PSW2000 subwoofer is powered up, there is a constant
background hum. (http://www.kef.com/history/2000/subwoofers/sub2.asp)

Pretty sure it's just a mains frequency hum coming from its power supply (it
doesn't change whether any input is connected or not).

Does this sound like a fairly basic/simple repair job? Is it likely to be
just the smoothing capacitors in the power supply or might it be more
complicated?

I know which end of a soldering iron to hold and recognise the dangers of
big capacitors. Could I fix it myself?

Tim

Tim Downie wrote:
... and no, it's not because it can't remember the words. ;-)

All the time my KEF PSW2000 subwoofer is powered up, there is a constant
background hum. (http://www.kef.com/history/2000/subwoofers/sub2.asp)

Pretty sure it's just a mains frequency hum coming from its power supply (it
doesn't change whether any input is connected or not).

Does this sound like a fairly basic/simple repair job? Is it likely to be
just the smoothing capacitors in the power supply or might it be more
complicated?

I know which end of a soldering iron to hold and recognise the dangers of
big capacitors. Could I fix it myself?

Tim


Is it a gritty him or a fairly 100zh hum or a really 50hz hum..

I am surprised..how long has it been doing it, and how long have you had it?

You MIGHT have an earth loop.. unplug it and see whether the audio
input earth is connected to the mains earth.(use a continuity tester).if
so try a quick test with the mains earth disconnected...

The Natural Philosopher wrote:
Tim Downie wrote:
... and no, it's not because it can't remember the words. ;-)

All the time my KEF PSW2000 subwoofer is powered up, there is a
constant background hum.
(http://www.kef.com/history/2000/subwoofers/sub2.asp) Pretty sure it's
just a mains frequency hum coming from its power
supply (it doesn't change whether any input is connected or not).

Does this sound like a fairly basic/simple repair job? Is it likely
to be just the smoothing capacitors in the power supply or might it
be more complicated?

I know which end of a soldering iron to hold and recognise the
dangers of big capacitors. Could I fix it myself?

Tim


Is it a gritty him or a fairly 100zh hum or a really 50hz hum..

I'm pretty sure it's 50 hz. I thought somesome somewhere would have
recorded mains hum on the net for comparison but I haven't found it yet.


I am surprised..how long has it been doing it, and how long have you
had it?

Had the sub for 5-6 years now. Hum has slowly been getting worse.


You MIGHT have an earth loop.. unplug it and see whether the audio
input earth is connected to the mains earth.(use a continuity
tester).if so try a quick test with the mains earth disconnected...

Don't think so. Doesn't matter where in the house I plug it in and nothing
has changed with regards to its wiring in donkey's years. I'll get the
multimeter out after I've walked the dogs.

Tim

HI Tim

On Tue, 6 Mar 2007 08:35:55 -0000, "Tim Downie"
wrote:

The Natural Philosopher wrote:
Tim Downie wrote:
... and no, it's not because it can't remember the words. ;-)

All the time my KEF PSW2000 subwoofer is powered up, there is a
constant background hum.
(http://www.kef.com/history/2000/subwoofers/sub2.asp) Pretty sure it's
just a mains frequency hum coming from its power
supply (it doesn't change whether any input is connected or not).

Does this sound like a fairly basic/simple repair job? Is it likely
to be just the smoothing capacitors in the power supply or might it
be more complicated?

I know which end of a soldering iron to hold and recognise the
dangers of big capacitors. Could I fix it myself?

Tim


Is it a gritty him or a fairly 100zh hum or a really 50hz hum..

I'm pretty sure it's 50 hz. I thought somesome somewhere would have
recorded mains hum on the net for comparison but I haven't found it yet.

There's (allegedly) a sample of 50hz hum in Wikipedia
en.wikipedia.org/wiki/Mains_hum





I am surprised..how long has it been doing it, and how long have you
had it?

Had the sub for 5-6 years now. Hum has slowly been getting worse.

Now that sounds like one or more large electrolytic caps slowly drying
out.....? As they dry out their capacitance decreases, and their
effectiveness at 'smoothing' decreases.....

Should be fairly obvious which the culprits are (assuming it's a
transformer-type 'linear' power supply). If you can identify the
culprits, then you should be able to find replacements from the usual
suspects....

To confim the diagnosis you could try adding a couple of
(suitably-rated) caps across the suspected duff caps on short plying
leads - taking due care to safely discharge the stored energy, which,
athough it's _probably_ not lethal g might well make you jump, or
even weld your screwdriver to something ! BTDT !

Hope this helps

Adrian

In article ,
Tim Downie wrote:
I am surprised..how long has it been doing it, and how long have you
had it?

Had the sub for 5-6 years now. Hum has slowly been getting worse.

First call would be the main smoothing caps.

--
*24 hours in a day ... 24 beers in a case ... coincidence? *

Dave Plowman London SW
To e-mail, change noise into sound.

Tim Downie wrote:
The Natural Philosopher wrote:
Tim Downie wrote:
... and no, it's not because it can't remember the words. ;-)

All the time my KEF PSW2000 subwoofer is powered up, there is a
constant background hum.
(http://www.kef.com/history/2000/subwoofers/sub2.asp) Pretty sure it's
just a mains frequency hum coming from its power
supply (it doesn't change whether any input is connected or not).

Does this sound like a fairly basic/simple repair job? Is it likely
to be just the smoothing capacitors in the power supply or might it
be more complicated?

I know which end of a soldering iron to hold and recognise the
dangers of big capacitors. Could I fix it myself?

Tim


Is it a gritty him or a fairly 100zh hum or a really 50hz hum..

I'm pretty sure it's 50 hz. I thought somesome somewhere would have
recorded mains hum on the net for comparison but I haven't found it yet.


Its probably 100hz.

I am surprised..how long has it been doing it, and how long have you
had it?

Had the sub for 5-6 years now. Hum has slowly been getting worse.

Ok. Its possible that the main capacitors are going high impedance-ish

It's also possible that as components are ageing its power supply
rejection is worsening.



You MIGHT have an earth loop.. unplug it and see whether the audio
input earth is connected to the mains earth.(use a continuity
tester).if so try a quick test with the mains earth disconnected...

Don't think so. Doesn't matter where in the house I plug it in and nothing
has changed with regards to its wiring in donkey's years. I'll get the
multimeter out after I've walked the dogs.


Ok, thats ruled out..

Check first by switching it off while running..there should be a few
seconds of power ..if the hum vanishes as soon as the PSU is off the
mains, but it still works, its definitely self generated.



Tim

The Natural Philosopher wrote:

Check first by switching it off while running..there should be a few
seconds of power ..if the hum vanishes as soon as the PSU is off the
mains, but it still works, its definitely self generated.

Yep. Hum stops as soon mains cut.

Tim

The Natural Philosopher wrote:

Check first by switching it off while running..there should be a few
seconds of power ..if the hum vanishes as soon as the PSU is off the
mains, but it still works, its definitely self generated.

Yep. Hum stops as soon mains cut.

Tim

Tim Downie wrote:

I am surprised..how long has it been doing it, and how long have you
had it?

Had the sub for 5-6 years now. Hum has slowly been getting worse.

Ah, that does change the probably cause a bit... I would have to go
along with dodgy smoothing cap as well (the time frame would also
possibly coincide with a period when a large number of duff
electrolytics were made, many of which started going tits up after a
couple of years.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
Tim Downie wrote:

I am surprised..how long has it been doing it, and how long have you
had it?

Had the sub for 5-6 years now. Hum has slowly been getting worse.

Ah, that does change the probably cause a bit... I would have to go
along with dodgy smoothing cap as well (the time frame would also
possibly coincide with a period when a large number of duff
electrolytics were made, many of which started going tits up after a
couple of years.

Next question, where's a good place to buy replacements?

Before you say "RS" or "MAPLIN", neither of them seem to stock capacitors
with the same spec.

The present ones were made by Licon. 12000 microfarads, 63V. On the other
side they say LSM, 85C(M), VENT.

Terminals seem to be two pins that pass through pcb. (Don't know the spacing
yet). Overall package about 34mm diameter & 46mm high.

How crucial is it to replace like with like? 12,000 seems to be an odd
value. Most catalogues that I've looked at so far list 10,000 & 15,000
microfarad capacitors but not 12,000. Is there any harm in "over-specing"
slightly?

Tim