0.41a, no screen, possible fix

so I’ve had a couple of motherboards that will only slow charge and have no display. From following Steve’s videos I was under the same impression that these motherboards couldn’t be recovered but today I managed to recover 1 (well I have fast charge back and the display works).

Normally you will get 5v but only 0.41/0.42 amps and it stays steady at that ampage and doesn’t go to 0 amps before switching to fast charge (1.3 to 1.8 amps depending on amount of charge in the battery).

This was a water damaged board (milk actually and what a stink when I opened it up!), so I haven’t put it back together until I check for corrosion and I need to replace a capacitor I lost when changing the chip. I did put it back together enough to confirm the display now works and the unit is charging.

The problem seems to be the 77621AEWI chip located below the main chip, next to the speaker connection. it’s a little black chip surrounded by capacitors. I was going to add some pictures but don’t have the option.

I used a doner chip from another knackered board and had to reball using flux and a soldering iron but it’s not too difficult to do, especially if you can see the reballing is good using a scope.

I also noticed that when I removed the 77621AEWI chip I got an open loop on both sides of 1 of the capacitors next to the charge chip BQ24193 (one of the larger ones to the left hand side of the BQ chip). Once I added the doner chip this short disappeared and for the record this capacitor wasn’t shorting with the knackered 77621 chip on the board. This kinda indicates that there is a link between the main charge chip and the 77621 chip.

I thought I’d open this topic as it might help others get around the 0.4 amp no display problem. I have another board here somewhere with the same problem and when I get a chance I’ll do the same fix and see if I get the same results.

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I unfortunately can not report the same success from a donor board. I will see about ordering a new chip and trying with that.

Thanks for this info. I’ll have to try this out sometime.

Please Google “Auto RCM”
How to detect it , how to inject payload to disable it for the unit to boot “normally”.
hth

I think I got lucky with this original switch as I’ve tried this fix again and it doesn’t fix the problem.

What I have found is that if you take a good Nand board and plug it on to a switch that has the 0.4a no display problem you can get the switch screen to come on and the battery indicator to flash up for a second or 2. (proves Backlight and LCD are working as least).

This will be because the nand isn’t paired with the switch so will not boot all the way. This could mean that if you could get the files off the suspect Nand board and flash a new Nand board then most likely the switch could be recovered.

I put a Switch into RCM mode for the first time today so I’m a long way from figuring out how to flash a new/spare Nand but maybe someone else has some ideas.