repairing a d-link wireless access point power supply

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I have a couple of d-link wirless accesspoints on whose power supplies have
failed - they should deliver 5volts at 2 amps.
The power supply takes the form of a slim-line plug pack.
Inside the pack is a little switch mode power supply.
I'm looking for some tips on what to look for in repairing this item.  there
is nothing obviously wrong on a visial inspection.  Fuse is intact, no heat
stress on the board, no visually distressed components. Just nothing on the
output.
On the board there is a pwm chip - CM3842 - an 8 pin device.  There is 8
volts on its Vcc pin (pin 7) - nothing appreciable on any other pins.
On there board there is also a 3 legged semiconductor - probably a FET -
Number is ST W55403 (then Morocco P4nK602 FP0) it's an all plastic device -
ferrite sleeve on its middle leg. It has 320 DC volts on its middle leg -
nothing on either of the other 2 legs.
Windings of the transformer show continuity.
I'd like to get this working again - I've been quoted $55 for a replacement.
Pic of the board can be seen at:
http://members.ozemail.com.au/~pebarhug/images/d-linkpsu.jpg
TIA
Peter



Re: repairing a d-link wireless access point power supply



"Peter"
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 **  Wrested with a couple of those just lately.


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** Takes 14 volts to initiate the kick start process.

First check the circa 100 kohm resistor that feeds the PWM  IC with  DC -
it may have gone high value.

Then if that is OK, connect a scope to the tranny secondary winding and
watch for signs of kick starting.



.......   Phil




Re: repairing a d-link wireless access point power supply



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Pin 7 is fed from 320 Volts via a 300K (on value)nd it is held at 8 volts by
a zener...no intelligible markins on the zener though. I see in the specs
the chip needs 14.5 to start and 8.5 min operating voltage.
I'll try to get a working unit and compare



Re: repairing a d-link wireless access point power supply



"Peter"
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** An external zener needs to be between 20 and 32 volts.

There is a 34 volt zener ( or equivalent ) inside the IC from Vcc to ground.

Standard 3842s will not reliably start with a 300 kohms feed resistor  -
only the low current versions will.

There should also be a small electro from Vcc to ground -  check it has not
gone high ESR.

You DO have a Bob Parker ESR meter

DON'T  YOU   ????





.......  Phil



Re: repairing a d-link wireless access point power supply



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The problem was a faulty 0.01 ceramic cap from pin 2 to pin 4 of the pwm
chip.
After replacing that all worked

Thanks

Peter
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Re: repairing a d-link wireless access point power supply



"Peter"

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**  So you lied about there being two such units  ?

 Just goes to show that all NG questions are trolls.







.......    Phil






Re: repairing a d-link wireless access point power supply



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No I did not lie about anything. On the other unit while testing the FEt my
probe slipped thus shorting it  The middle leg had 319 volts on it.  On
reconnecting to the power - sparks, smoke, flash, the pwm chip
disintegrated.
Peter



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