Re: WRT54G fried - plugged AC into a DC device. D'oh. Any suggestions?



On Wed, 23 May 2007 18:58:17 GMT, slade969 <slade969@xxxxxxxxxxxxx>
wrote:

Hi,

I recently bought a Linksys WRT54G v.1.0 from someone on ebay 'as-is'. The
price was right and I was in the mood for a little project. It came with the
old 5v2a power supply and when plugged in, only the 'power' and 'diag'
lights came on. They were blinking. The unit didn't respond to pings.

<snip>
OK now is a good time to add some info... I also have a WRT54G v.1.0
that I got not working. THe OEM wall-wort was the problem.
Here is the schematic of its output ckt...

+---+----|>|----+-----+-----L1-----+----+------->+5V
| | | | | |
( +--10R--1n--+ 330uF 220uF +-1K-+--(1)KA431 Vref=2.5V
( 25V 10V |
( 105° 105° 1K
| | | |
+---------------------+------------+---------+--(2)KA431

The 330uF, 25V input cap is adjacent to the rectifier diode & had
overheated. It had a "rounded top".

The wall wort would put out 5V unloaded but dropped to almost nothing
with any load.

Unit worked just fine with a good +5V wall wort.
(also flashed to dd-wrt)


About this time, I noticed that something was loose in the power supply. It
rattled around inside when shaken. So I tested the power supply w/ a
multimeter which reported that it still did put out 5v.

Some research indicated that these units can handle more voltage than what
they're rated for. I noticed that the power adapter for my DSL modem was
12v. It fit into the receptacle on the WRT54G, so I plugged it in and was
greeted with a POP sound. I unplugged it immediately. There was a burnt
circuitry smell, but no smoke and no visible damage to the PCB or any chips
- even under magnification.

Here is a partial schematic I had made (ascii art)

+------>7705 chip
|
Jack1(+)--L2---F1---+-----+-----+-----+-----|>|--->SMPS
(-) | | | | 0.188V
| +-----+ | | |
| 1000uF| _|_ --- --- ---
L1 16V --- /_\ --- --- ---
| 105° --- | | | |
_|_ _|_ _|_ _|_ _|_ _|_
- - - - - -
ID= DS1 CK1 CA3 C3 D1

Device Zss smt smt smt Zss
ID 33 chip chip chip 33

"F1" is the yellow thing directly behind the pwr input jack.
using the pic Jeff posted...
<http://www.linksysinfo.org/images/Content/Autopsy/WRT54G-mblg.jpg>

"DS1" is mostly hidden in that pic - you can see part of it between F1 &
L1, the inductor closest to the pcb edge.

Well, upon closer inspection of the power adapter, I noticed it's output is
12v AC, and the router needs DC. Of course the router doesn't respond to any
power supply now and the lights no longer come on at all. Clearly a
boneheaded move on my part, but what's done is done.

My question: don't most electronic devices like this have some sort of fuse
protection on the board, to protect the circuitry from boneheads like
myself? Does anyone know if there's a varistor or something on the board
that I could just replace to bring it back from the dead?


The markings on "F1" are...
30V
JF34

Its not easily replaceable (soldered in).

Find a friend that knows some electronics & he/she should be able to
find the problem & maybe repair it.

Now for some more info on powering this unit. I had traced out the
input +5V lead & found that it also goes to some pins of the "radio
card". BUT I could not detect any connection to these pins (on the radio
card).

What I did discover is that the +5V also goes to a TI 7705AC chip at the
front edge of the main pcb & under the radio card. Its a "Supply Voltage
Supervisor" chip monitoring the health of the +5V line.

The 7705 chip pdf shows...
Vcc: 3.5-18 V
Vsense: Recommended Max= 10V

So it seems the resistors in the "Supply Voltage Supervisor" ckt would
be the limiting factor in higher voltage (DC) input to unit.

I have run this unit on a 6.5V wall-wort for a few days with no
problems.

kc - learned everything I know about wireless from this NG - thanks guys
& girls!

PS - Don't try to run a D-Link DI-524 above 5V. I was testing the
voltage/current curves & at 6.5V the PA amp shorted. I was able to
remove it & bridge the _tiny_ gap (what a pita!!) & the unit still works
fine, but _somewhat_ reduced power :)
.