Hello,

I am trying to repair a Logitech LS21 multimedia speaker system (Model Number S-00042).

There is a small switched-mode PSU which I have opened up inside the main sub-woofer unit. The soldered inline mains fuse was blown.

There was some blackening around a few surface mount resistors. There is a short across the Drain-Source of the MOSFET transistor (WFF2N60).

Should I just replace the transistor and fuse or there likely to be some other cause?

On 2013-04-03 22:30:12 +0200, M.Joshi said:


Hello,

I am trying to repair a Logitech LS21 multimedia speaker system (Model
Number S-00042).

There is a small switched-mode PSU which I have opened up inside the
main sub-woofer unit. The soldered inline mains fuse was blown.

There was some blackening around a few surface mount resistors. There
is a short across the Drain-Source of the MOSFET transistor (WFF2N60).

Should I just replace the transistor and fuse or there likely to be some
other cause?

hi,
often when the main mosfet transistor is blown, alsoone or more
diode(s) of the bridge get shorted too you must verify that.
and also when the mosfet gets shorted it's often because the main
capacitor has heated and has too much esr - you should measure it and
change it if needed.
hope this helps.

--
----------
Kripton

the ESR Repository @ http://kripton2035.free.fr/esr-repository.html
the Geiger Repository @ http://kripton2035.free.fr/geiger-repositor.html

Thanks for your reply Kripton. I do not own an ESR meter. Will the capacitance range on my multimeter give me an indication that the capacitor has been damaged?

On 2013-04-05 01:42:23 +0200, M.Joshi said:


Kripton;3041140 Wrote:
On 2013-04-03 22:30:12 +0200, M.Joshi
said:
-

Hello,

I am trying to repair a Logitech LS21 multimedia speaker system (Model
Number S-00042).

There is a small switched-mode PSU which I have opened up inside the
main sub-woofer unit. The soldered inline mains fuse was blown.

There was some blackening around a few surface mount resistors. There
is a short across the Drain-Source of the MOSFET transistor (WFF2N60).

Should I just replace the transistor and fuse or there likely to be
some
other cause?-

hi,
often when the main mosfet transistor is blown, alsoone or more
diode(s) of the bridge get shorted too you must verify that.
and also when the mosfet gets shorted it's often because the main
capacitor has heated and has too much esr - you should measure it and
change it if needed.
hope this helps.

--
----------
Kripton

the ESR Repository @ 'ESR-Repository'
(http://kripton2035.free.fr/esr-repository.html)
the Geiger Repository @ 'Geiger-repository'
(http://kripton2035.free.fr/geiger-repositor.html)

Thanks for your reply Kripton. I do not own an ESR meter. Will the
capacitance range on my multimeter give me an indication that the
capacitor has been damaged?

often when the esr is too high, the capacitance changes also but not always.
so you can measure the capacitance and if it is really out of spec
change the capacitor
in my signature below, on my web site there are easy to build esr meter
- build one it is always handy to have an esr meter when you fix
electronic stuff.

--
----------
Kripton

the ESR Repository @ http://kripton2035.free.fr/esr-repository.html
the Geiger Repository @ http://kripton2035.free.fr/geiger-repositor.html

"M.Joshi" wrote in message
...

Hello,

I am trying to repair a Logitech LS21 multimedia speaker system (Model
Number S-00042).

There is a small switched-mode PSU which I have opened up inside the
main sub-woofer unit. The soldered inline mains fuse was blown.

There was some blackening around a few surface mount resistors. There
is a short across the Drain-Source of the MOSFET transistor (WFF2N60).

Should I just replace the transistor and fuse or there likely to be some
other cause?




--
M.Joshi


It's almost impossible to say. SMPS repair is a black art. I do loads and
loads of them. Some are very well behaved repair-wise, and replacing a blown
MOSFET is all that is required. Or you can get them at the other end of the
spectrum where there has been so much collateral damage to the MOSFET
failing, that they defy reliable repair. And then there is every condition
between those two points ....

If a MOSFET has blown, there may well have been a cause, such as an
electrolytic with a poor ESR, but also, you can have a faulty control IC
that will do in the MOSFET. If the MOSFET failed for no particular reason,
it may well take out any series resistor between its gate and the control
IC, and / or any little diodes in the gate circuit. It also might take out
the control IC. Check also that the MOSFET source resistor has not gone
open, if the device failed D-S short.

Make sure that you determine which component let go of its magic smoke to
put the soot on those sm resistors, and make sure that you scrub all of that
soot off, using a solvent like IPA, applied with a toothbrush.

Finally, remember that switchers are EXTREMELY DANGEROUS. I would never
advocate attempting to repair one without it being plugged into a variac, in
turn plugged into a proper bench isolating transformer.

Arfa

Quarta-feira, 3 de Abril de 2013 21:30:12 UTC+1, M. Joshi escreveu:
Hello,



I am trying to repair a Logitech LS21 multimedia speaker system (Model

Number S-00042).



There is a small switched-mode PSU which I have opened up inside the

main sub-woofer unit. The soldered inline mains fuse was blown.



There was some blackening around a few surface mount resistors. There

is a short across the Drain-Source of the MOSFET transistor (WFF2N60).



Should I just replace the transistor and fuse or there likely to be some

other cause?









--

M.Joshi

I had the same problem. After reading a lot on line about this specific problem in this model, I gave up, because there isn't any pcb spares for this and it is just impossible for me to know what is the resistance value of the burnt ones, because they are burnt!!! Well, I have loads of AC DC transformers lying around the house... I guess I kind of collect them... and as the problem is in the internal current transformer, I just replaced the internal burnt one, for an external that I just had Lying around. Cost... NOTHING!!! works perfect.
I tried a 12V- 1A out...
Stop wasting time looking for the resistances...
I lost hours, and i am sure my fix will work for years to come!
Hope this helps you out!
Ana

Thanks for replying - I didn't realise this was a known issue with this model. I'll take a look at the burnt out PSU components again and if they aren't identifiable, I will replace the PSU PCB with a 12V SMPS module.