Is my raspberry unrepairable?

Hi Mauricio,

This is an interesting finding.

It seems both the 1.2 and 1.8V rails are quite ok in your case. However, the 3.3V rail being so low is not good. There are many chips on the board using the 3.3V power rail. If the CPU is getting really hot when powering the board, then it is probably gone. What about the ethernet and USB hub chip? This the one with the SMSC logo onto it. Is this chip getting hot too?

Other ideas would include to desolder L1, which is the inductor used by the 3.3V regulator. By doing so, you will disconnect the 3.3V rail. If afterwards, the 5V rail is quite normal (the voltmeter readings should show a value close to 5V), then this means the fault is only in the 3.3V rail and it can be in one or more chips using it.

To definitely know which chips are faulty, you can power the board from USB (having the L1 inductor disconnected, thus the board's 3.3V switching regulator won't interfere) and then connect an external 3.3V power supply (you can connect it at the point where you should have 3.3V) capable of delivering a few amps (at least). Then, all the faulty chips should get very hot.

With the 3.3V at 0.4V the SMSC chip (the lan chip) will not be powered. It is powered only from the 3.3V rail. So with the power rail at 0.4V, it doesn't have the power to become hot.

The 3.3V is used by:

* the lan chip. (main power).

* the SD card (main power)

* the CPU (as IO voltage).

If you have a lab power supply, setting the voltage to 3.3V and the current to say 40mA, remove the SD card  and power the 3.3V rails through the GPIO pins. If it goes into current limit that might be the LAN chip which might not even be broken. Increase the current to about 100mA. It should stay below that. All this of course afteryou've disconnected the regulator from the 3.3V by removing L1. as bbolo1 suggests.

Ok so, I tried to power it through my home plug and the 5v (red ones) are now at 4.85v, and all the others stayed the same voltages. All chips gets hot( they feel pretty much the same temperature). I didn't tried to desolder anything since the new voltage for the 5v changed. Should I still desolder anything or plans changed?

@Roger Wolff I don't have a lab power supply, for now I can use a battery right? I can perhaps buy a low mA battery. Are lab power supply really usefull?

Yes, lab power supplies are really useful!

(note that you mentioned the word "really" and I answered with the word "really" in my response. Those are different meanings of a word spelled and written the same. Think about it. :-) )

A lab powersupply has a configurable current limit. This means that you can set that limit at say 30% above what you expect your circuit to use, and then not blow stuff up if there is a mistake in a circuit.

Example: I'm working on a motor-control-PCB. During startup, the CPU apparently tells the powerstage: connect V+ to motor pin A, while at the same time: connect GND to motor pin A. Of course that should never happen, but apparently it happens during startup. So far the "current limit" led of the powersupply blinks an instant and nothing blows up. If I connect a good LIPO battery as the powersupply I'm pretty sure that the FET switches would blow up.

If you say that your chips get hot, that means that something is seriously wrong. A hint that nothing more can be done.

If you still measure 0.4V on the 3.3V line, then it is very difficult for the LAN chip to become hot: It doesn't have any power. I question the observation....

If the 3.3V line is "shorted" to GND (about 0.4V) by the regulator, then I can imagine the CPU getting hot. It MIGHT have a diode from its 1.2V supply to the 3.3V IO voltage. Normally not conductive, but now suddenly a current can flow.

Many people confuse mAh ratings and mA ratings of batteries. I have a 3Ah LIPO battery rated at 120A. It can deliver 3A for an hour, or say 0.3A for 10 hours. But it can also deliver 120A for about 1/40th of an hour (90 seconds). Take a 10x smaller version of that battery and it says "300mAh", and you'd think: find that won't harm my 'pi. But it can deliver 12A, quite a lot end easily enough to damage a 'pi.

Roger Wolff wrote:

 

A lab powersupply has a configurable current limit. This means that you can set that limit at say 30% above what you expect your circuit to use, and then not blow stuff up if there is a mistake in a circuit.

Yes, that is an extremely useful feature.  Another fun thing you can do with a lab supply is to clear a thin conductive bridge that's shorting together two signals because of a PC board defect.  You set your lab supply voltage to 0.5V or so and set the current way up to 10A or so.  Then you connect the leads to the two signals.  If the bridge is thin enough, the 10A will blow the bridge like a fuse and the 0.5V makes sure you don't apply too much voltage when the short clears.  This only works for some shorts, but when it works it rescues a board.

Absolutely. Did exactly that a week or two ago. Although my lab powersupply does not go any further than 3A, but it worked. :-)

Thanks for the info . On another notes, I took my meat thermometer to calculate the temperature of the chip and it reached up to 28°C (around 80°F).

Yea I'm definitively that kind of person xd i'll read more about batteries. Do you suggest me to power up them with an external battery, or can I desolder anything, or change the diodes?

28°C is kind of normal. The normal operation of the RPi CPU can get higher than this value.

The idea here is to use a 3.3V power supply capable of delivering at least a few amps. Then, by disconnecting the L1 (to disconnect the 3.3V switching regulator output) and using an external more powerful 3.3V power supply, you should be able to tell which chips are definitely fried. The faulty chips should get really hot (more than 60-80°C).

If the main CPU is fried, you might cut the PCB area with the switching power supply and use it for future projects whenever you need 3.3, 1.8 or 1.2V

Hi!

ok so at first, I tried to desolder it, and damn it was hard to, so I was scared to touch the others things around it. I ended up by breaking L1 carefully and then I was able to put some rosin core on it to make it easier for me to conduct things up. Can you tell me where am I supposed to join things? like the + and -  of the 3.3v battery.

I'm going to attache some photos. Would you be able to tell me if I did a good or if I did not x.x

Thanks!

The picture got rotated..

Before going any further, just make sure you have a regulated 3.3V supply (have a powerful 3.3V LDO on your battery so it will have enough amps to supply).

Now, put the "-" to any "GND" point on the board and the +3.3V to the upper part of the capacitor that filters the 3.3V rail (marked in orange).

Then, power up the board using a USB power adapter. The switching regulators will supply the 1.2V and the 1.8V rails.

Afterwards, connect your 3.3V power supply and check which chips are getting really hot.