ParkingLotLust

If youve been around for awhile, chances are you've seen my other thread(s) about it. I came across the board the other day, and now that I have a bit more experience, I figured I'd give it another shot.

The wall wart puts out ~6.9v, so it's pretty unregulated. The top chip is an RT9202 Single Synchronous Buck PWM DC-DC Controller, but I dont know what the bottom chip is - it says this on it:

9936
Δ31B9M

Anyways, as you can see in the picture, one one side of the chip, I have power, but on the other side, nothing. This could be why I have no power on the rest of the board.

If you have any ideas, need more info, or want to talk to me on MSN (so you can say try this and I can try it and tell you what happens in real-time), let me know. I would greatly appreciate any input!

EDIT Ignore the flux residue on CA14, I touched it up with a soldering iron (along with the rest of the points in the area) to make sure there were no cold joints.

Attached Images

P1000913.JPG (902.8 KB, 45 views)

Hero999

I don't know.

Did you measure the voltage at the other pint?

Perhaps if you trace out that section of the circuit it might give you a clue.

As a total guess it's likely to be an op-amp or some sort of voltage reference.

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ParkingLotLust

Could the chip be one of these? That would explain why pins 7 and 8 are connected together - if the FET was burnt out, that would cause it to not output, right? I'm not really familiar with FETs as I havent used them before.
http://web.archive.org/web/200601130...O8/CEM9936.pdf

ParkingLotLust

No suggestions, ideas, anything? Didnt think it would be that difficult for you guys to figure out

ericgibbs

hi,
That large inductor [enamalled copper wire] near the bottom of the picture, the soldering looks as dry as old boot...

Scrape off the enamel from the wire and resolder to the pcb.

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"Good enough is Perfect"

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ParkingLotLust

Didnt help. Next

ParkingLotLust

to the top!

ParkingLotLust

bump bump bump.

ParkingLotLust

Here is a high-res shot of the area with the mystery chip removed. Hopefully where the traces lead will help someone more intelligent than I recognize what kind of chip it is, what it does, or what kind of circuit Im looking at.

mneary

Look at Fairchildsemi or Vishay Si9936 dual MOSFET. 4R7 in series with each gate, wired as totem pole, perfect match.

ParkingLotLust

Would anyone be able to order one for me, and mail it up here? Mouser wants $15 just to ship two chips to Canada, and I know it sure as hell doesnt cost that much to drop a couple chips in an envelope.

Mouser # 781-SI9936BDY-T1-E3 (The Fairchild one is obsolete and Digi-key doesnt stock them).

mneary

You can probably use Si4834. I found 20 pcs on ebay for $4.50 plus $1.35 shipping. Send him an email to see if he ships to Canada. If he won't, send me a PM.

mneary

Don't know how I missed it. He ships to Canada for US$1.75. Lots better than $15.

ParkingLotLust

Awesome man, Ill pick that up and drop one in Hopefully that fixes it... If not, I know where you guys are

ParkingLotLust

Didnt fix it. I still have no voltage on the output side of the MOSFETs. However, I found that when I pressed down on the PWM IC with the tip of my multimeter probe, something on the board started buzzing! When I let go of the chip the buzzing went away. This time I pressed a bit harder and it stayed buzzing when I let go, so I unplugged the router right away!

Could the PWM chip be bad? Any way to tell without buying another one? While the experimenting is fun, I have no use for the other 19 mosfet chips, and if I have to order bulk PWM chips, I will have no use for them either.