Liquid damage. Charging circuit short. Runs fine without battery.

[flatline]

There is a short somewhere in the charging circuit. Every cap that shows 1 or 2 ohm has been removed, but circuit still short, so caps soldered back on.

Measuring between positive and negative battery connector shows 1 to 2 ohms.

Mosfet PU4406 shows 2 ohm, it was removed, still show 2 ohm across circuit, so I soldered PU4406 back on.

Mosfet PU4403 was corroded, so it has been removed. and two caps above it were superheated and slightly welded to pads, they were removed.

Next I check PU4401

Pin1 33meg ohm
Pin2 27meg ohm
Pin3 89k ohm
Pin4 72k ohm
Pin5 1.5meg ohm
Pin6 42k ohm
Pin7 26k ohm
Pin8 52k ohm
Pin9 52k ohm
Pin10 1.5meg ohm
Pin11 1.5meg ohm
Pin12 9 ohm
Pin13 11 ohm
Pin14 0hm
Pin15 156k ohm
Pin16 110k ohm
Pin17 .5meg ohm
Pin18 135k ohm
Pin19 1.6 ohm
Pin20 .5meg ohm

I think pin 19 looks low, what should I do next?

[piernov]

Re: Liquid damage. Charging circuit short. Runs fine without battery.

That PCB doesn't look in good shape... What happened to it? If it's burnt just below PQ4201 label then that might be causing your short...
Easy way of finding a short is inject voltage and find the part that's heating up.

But if you excluded PU4406 and all the capacitors (PC4415-4418/4421/4423) then that leaves only PU4401 apparently. So I'd pull it first, and if the short is still there it's time to dig the burnt spot on the PCB I guess.

[flatline]

Re: Liquid damage. Charging circuit short. Runs fine without battery.

Injected voltage and that PCB got hot. Dug it out. Short is gone. Now time to replace Caps, a 470k resistor, and PU4403 (AON703).

I don't have any spare AON703. But I have a spare AON7401. Do you think AON7401 will be a good alternative? Would one 50V cap be ok instead of two 25v?

[mcplslg123]

Re: Liquid damage. Charging circuit short. Runs fine without battery.

Seems compatible as pulsed drain current is same though both differs a bit on other parameters but still should work. You should replace both caps and can get plenty from scrap boards.

[piernov]

Re: Liquid damage. Charging circuit short. Runs fine without battery.

Yes AON7401 for PU4403 should work fine.
For the capacitor you can go same or higher voltage, but in general you'd need to more or less match the capacitance and ESR (so the number of capacitors in parallel and their capacitance).
In this case they don't appear to be that critical so leaving one out shouldn't be an issue.

[flatline]

Re: Liquid damage. Charging circuit short. Runs fine without battery.

Looks like it's working good so far, it charges and runs off battery. Thanks for the idea to dig. I dug deeper then was necessary because I didn't know what I was looking for. It ended up being a huge ground plane right underneath the hot plane making contact. The coating between the to planes disintegrated and contact was made. So all that I needed to do was peel back the hot plane far enough to where there was no more contact. I Jumped a wire along the hot plane because there was hardly any left.

Thanks again.

[piernov]

Re: Liquid damage. Charging circuit short. Runs fine without battery.

Good job.
When the PCB burns, it becomes conductive so you also need to remove the burnt PCB material. Sometimes you even have to dig a crater through the board to remove all the burnt stuff. You just have to hope there are no signal lines in the internal layers and there are just power planes…