Recently I had little problem with my RPi 4B board. I was working on some project and left my RPi connected to my lab bench power supply for half an hour. When I came back, I saw OCP (over current protection) on my power supply is activated and rPi is taking ca. 2A @ 1V. I turned off everything quickly and measured 5V line on my Pi. It turned out that there is short circuit on 5V rail. Other rails like 3.3V an 1.8V was ok. Propably, voltage that was set on my PS was to high (5.2V). First, I desoldered 5.1V zener diode here:
Unfortunatelly, nothing changed. Next i decided to desolder PMIC - > MXL7704
And v'oila, good news! There was any short circuit anymore! But bad news was I had to replace this PMIC and hopefully, only that. There was also quite high possibilty that some other ICs or passive components have been destroyed during PMIC fail but I was optimistic and decided to try fix my Pi4.
I checked exact part number and it was MxL7704-R4. Unfortunatelly I couldn't find any of these on Aliexpress, but i found MXL7704-R3 here:
https://pl.aliexpress.com/item/32991214 ... 5c0f9p1CI1
I ordered 2PCs and after 3 weeks i received my order. I checked markings and they were looking good so i decided to give a try. First, I have put some gel flux on IC and added some solder on pads, including big ground pad. Next, i used solder wick with gel flux and my soldering iron to clean pads on PCB. I cleaned everything with isopropyl alcohol and applied some more flux on pads, then I have put IC on pads with ESD tweezers. I used hot air, that was set to 390 degrees Celsius and soldered IC to board. You must have some soldering experience to do that. You can find videos on YouTube. I cleaned board again with alcohol and checked with mutimeter if there is no short circuit again. Everything was fine so I decided to power my Pi with my bench PS.
Board was sinking ca 700mA @ 5.0V, green led was blinking but system was not booting. I couldn't see anything on HDMI or serial console. SSH was also not working. I must admit that I expected that. I saw on some topic on rpi official forum that this IC must be correctly programmed over I2C to be compatible with RPI3B+ and RPI4B board. So i decided to do that. I was lucky because I had two other RPi'es laying around. I used RPi0 to read MXL7704 registers from good RPi4 over I2C. I couldn't find any test pins to connect to Pi4 PMIC I2C interface but I inspected PCB and saw two very small VIAS. I removed soldemask from them, soldered some very thin wires and second end of wire was soldered to goldpins. I used three signals, SDA, SCL and GND from Pi0.
i used i2ctools to read and write over i2c interface,
Kod: Zaznacz cały
sudo apt install -y i2c-toolsKod: Zaznacz cały
sudo i2cdetect -y 1Kod: Zaznacz cały
i2cdump -y 1 0x1B > good_pi.txtAgain, I have hit:
Kod: Zaznacz cały
sudo i2cdetect -y 1Kod: Zaznacz cały
i2cdump -y 1 0x1B > broken_pi.txtKod: Zaznacz cały
diff good_pi.txt bad_pi.txtKod: Zaznacz cały
sudo i2cset 0x1B register_addres register_value
Below you can find hexdump from good PI PMIC:
