Switch: 0.05A current draw - BQ Shorts / SYS Rails ? - diagnostic help needed

I have a switch which does not power on - only has a 0.05A on the voltage meter.
I checked for shorts and found a short on the BQ.

I removed the BQ but the short still remain - so my conclusion was that the BQ probably is not the problem.

Looking on the MAX77620H I have several shortages, but now I do not know how I can go on with the diagnose or how this might can be fixed. So any help is appreciated.


Shortage still exists without BQ

max - shorted_org

Many shortages round the MAX77620H (red dots are the shorts)
Measuring the SYS-Rails as I have seen in another thread and the yellow one is shorted:

Other measurements:

red - 3.848 kOhms
blue 13.82 kOhms
yellow - shorted
purple 0.890 kOhms

Anything I can do to to find out the problem ?

When you say yellow is shorted, just how low is it? I have a good boards where it reads around 50 ohms.

Yellow is about 47 kOhms

I’m fairly new this this, but I think the rails coming out of the MAX77620H look good.

Thanks for the help - now I am a little bit unsure what I should do next.

As the “yellow sys-rail” seems to be ok and not shorted, but I still have shorts round the MAX77620A and also on the MAX77621 (and on the BQ)

Not sure if this is a good indication which chip might be faulty or where these shorts are coming from ?

Yellow isn’t your SYS rail, yellow is your CPU-0 rail, and I imagine it’s actually 47ohm and not 47K (which would be normal on a regular rev Switch board)

Though given that this is apparently a Mariko board I would actually expect somewhere in the region of 13 to 30 ohms on this rail (which is normal)

Though you’ll have to confirm, in one post you state it’s a MAX77620H and the other it’s a MAX77620A which tells me which hardware rev your board is.

SYS rail can be found at the 2R2 coil near the BQ IC.

The cap which is showing as shorted near the BQ IC doesn’t seem right, I’ll get back to you with info on this

This is one rev 1 Switch - so it’s the MAX77620A I was talking.
And yes - CPU-0 Rail is 47 ohm…

The cap which is showing as shorted near the BQ IC doesn’t seem right, I’ll get back to you with info on this

This one is still shorted - even without the BQ IC installed.

So the cap near BQ IC is on the SYS rail.

What is your resistance to ground on this shorted rail?

Many things reside on thi rail, the two Max CPU/GPU bucks, the PMIC, 8316 IC, backlight IC and a whole host of other misc ICs who VIN is SYS.

Generally speaking, in my experience, it’s been the fault of the ICs on this rail which are closest to the edge of the board in cases of liquid, so backlight IC, CPU buck etc.

You could solder a wire to the 2R2 coil and another to ground, then set a bench PSU to 1V with an initial current limit of 1A and see which IC gets hot, if nothing increase the current limit, if still nothing and your confident nothing else is shorted to one another you can increase your voltage to 4V, but be warned, if something else is shorted to one another you may expose this 4V where it wouldn’t be welcome :slight_smile:

All that being said, this will ultimately depend on the shorts resistance, it might be it too high for this method.

Thanks for the additional infos - much appreciated and will help me a lot.

What is your resistance to ground on this shorted rail?

Well, tried to measure (black probe on ground, red on BQ IC).
On one side of the IC I can measure 0.4 ohms on the other 0.1 ohms - if done correctly.

So not sure what this means to be honest…

The last time I had a short on SYS, I found a shorted cap at the MAX44621 left from the SoC.

Like Severence adviced I would inject voltage at the SYS line and check both sides with freeze spray/IPA/thermal cam for a hot spot.

Thanks for the answer - unfortunalty I do not have a thermal cam / freeze spray - so I don’t think I can go this way (I am still in the learning mode :wink: )

Is it correct that once I have pulled off the faulty part the short should be away ? So wouldn’t it be an option to pull off one part after the other to see which was making the short ?
If so - are there any experience in which order I should start ?
Or any other idea for identifying the faulty part ?

I have done some measurements from the pics and it seems that all red marked ICs are shorted, but I guess this is normal due to the short on the Sys-Rail … ?

You have a dead short on this rail, so as a result, when “Injecting” voltage the faulty component should present itself quite easily in terms of heat, so just place your finger round the locations Calvin has highlighted above as well as on the ICs next to them. If your still having trouble then pour a little IPA at these locations and see how quick it evaporates, thermal cam/freeze spray is not needed in this case.

Right, once you’ve removed the offending component the short will clear, though it has to be said, if the culprit is the main PMIC you stand a 50% chance the console will ever work as often what happens is the SYS rail kills some lower voltage lines at the SoC.

Once you have identified the culprit for the short, and if it isn’t a passive then chances are the IC is BGA at which point you’d have to order up a replacement, and in regards to the PMIC you’d have to pull one off a donor and reball with stencils and paste as the IC afaik is not avaliable to buy preballed… aside from some shady chinese seller in and of unknown condition. pretty sure all the other ICs on this rail can be bought new pre-balled.

Right, PMIC and others voltage in is SYS

Ok - now it is getting clearer how to go on the best way - thanks for the explanation.
Sorry for asking maybe some additional stupid questions, but the procedure is still not fully clear for me.

Here is my understanding what I need to do (please correct if wrong)

  1. Soldering a wire from 2R2 to ground.

I would put a wire on the side of the 2R2 any maybe the other end on one anchor of the USB-C port - if this will work ?

  1. set a bench PSU to 1V with an initial current limit of 1A

That’s not clear for me how to do this. First for my understading the power will be injevted normally via the USB-C connector ?
“set a bench PSU to 1V with an initial current limit of 1A” - PSU is a power supply right ? Is a special hardware needed here or how can this be done ?

No such thing as a stupid question :+1:

No, not to ground

You’d solder a wire (make it a red wire to avoid confusion) to the 2R2 coil, doesn’t matter which side of it you choose to solder to.

Then you solder a wire (make it black to avoid confusion) to ground, you can use the USB through legs pad as you suggested.

Then, you’d connect the positive (red) from you power supply (PSU) to the red lead soldered to the 2R2 coil, and the negative (black) from your PSU to the black lead at the USB through ground.

no, the USB/battery is not involved here and neither should be connected

Connecting either will simply result in no current draw as short circuit protect at several ICs will be enaged, and not only that but even if current could flow it would not be benificial as it would be (in the case of USB) 5V or greater. As it is, the ICs on the board are protected as is.

Bench/Lab power supplies are adjustable, meaning you can set the voltage level and current limit, if you do not have a bench power supply then I would suggest you look into getting one as they’re incredibly useful.

I suppose in a pinch you could use a AA battery (<1.2V) but you’d have to faff about with and place a current limiting resistors in series with it, issue here is you’d have to parralel up quite a few as your board is going to be drawing a serious amount of current and 1/4W resistor is likely not going to cut it.

Ah - was not aware of that - but I am happy to have a closer look into that
I have seen that these bench power supplies could be really expensive, but I have seen the following one:

SKU: CE-A001-246(30V 10A )

3010D 30V 10A DC Power Supply is suitable for mobile phone repair or equipment within 30V voltage and 10A current!

605D Operating Conditions Specifications:
Output Voltage: 0~30 V
Frequency: 50Hz~60Hz
Working Temperature: -10~40 degree
Output Current: 0~10A
Storage Conditions: -10~40 degree
Relative Humidity: less than 80%
Size(LWH): 312118 CM
Weight: 7.303 KG

SKU:CE-A001-247( 0-15v 0-2A )


Adjustable DC power supply


voltage: 0-15v Continuously adjustable

Electric current: 0-2A Continuously adjustable

Test voltage range: 0-99.9V

RF signal detection: test for GSM、CKMA、PHS phone signal

This should be finde for such tasks, right ?

I’m not familar with those PSU but the second one would likely be not that useful as it only has a max current of 2A.

The first one with the 10A limit might be good, but one thing you’ve got to watch out for with these cheap PSU is, on some of them you have to short your positive and negative leads together to first set your current limit… this is a really dated way of doing it and a well designed PSU no longer requires having to do this, so if you can verify that the first PSU doesn’t require having to do this then I think it would fit your purpose just fine.

If you can’t verify, I’ll dig up a link for the PSU I use if you want?

That would be great as I am not not too familar with these…

So this is the version I have,

I’d opt for the 30V 5A version. You can likely find it cheaper, this was just the first one in the list.

It also goes by a few different names from the venerable distibutors such as (i think) farnell/element 14 and a few others but you’ll be able to tell as the case is identical, might just go under the Tenma or other brand.

PSU is good quality, got a bit of a bad wrap when it first came out but the issues have long since been resolved and I’ve had no issues with mine.

Great thanks - had some research for that as shipping to Germany could be quite expensive with VAT and shipping etc. but I have found another one which I have ordered now.

Will take some time until delivered and I am ready for the next step - quite sure I’ll come back to this thread :wink: