Raspberry Pi B+ I2C cannot work

you do have pull up resistors right......

Because he's using the raspberry pi with built-in pullup resistors, I very much doubt that he desoldered them. (Nowadays they are hard-to-find as there is no schematic... [update: If you know what you're looking for, you can find them. R23, R24 on the pi3]).

The built in pullup resistors are part of the BMC chip set and are used for inputs on the GPIO etc. I am not confident that when the pins are used for dedicated I2C that the pullups are enabled as they will be the wrong value probably and in the past I have had to add my own to make it work with some ADC/DAC chips I have tried. + I2C standard calls for open collector operation and the pull ups often will be placed at the end of the line, or several high value ones along the line (No preferred!).

Either way it may be worth putting a couple of 10K pullups to 3V3 to see if it helps. I have yet to have I2C not work fo me when this is done.

The pullups in the chip are usually in the 50kOhm range. Those are insufficient to use as I2C pullup resistors. Most processors configure the pins as "open drain" when using them as I2C, so often not even the internal pullups are active. On both of these things, I don't know the exact facts on the broadcom.

However what I do know for sure is that the raspberry pi PCB has a pullup to 3.3V on both (*) I2C lines. They are R23 and R24 on the raspberry pi 2 and 3, they are the unmarked resistors between the "GPIO" label and the SD card slot on the pi zero, they are documented as being 1.8k on the raspberry pi 1, and they measure as just under (i.e. they could very well be exactly) 1.8k on my pi 3.

(*) Actually, I suspect "all four".

The pullups in the chip are usually in the 50kOhm range. Those are insufficient to use as I2C pullup resistors. Most processors configure the pins as "open drain" when using them as I2C, so often not even the internal pullups are active. On both of these things, I don't know the exact facts on the broadcom.

However what I do know for sure is that the raspberry pi PCB has a pullup to 3.3V on both (*) I2C lines. They are R23 and R24 on the raspberry pi 2 and 3, they are the unmarked resistors between the "GPIO" label and the SD card slot on the pi zero, they are documented as being 1.8k on the raspberry pi 1, and they measure as just under (i.e. they could very well be exactly) 1.8k on my pi 3.

(*) Actually, I suspect "all four".

Well that is good news then, but in the past I have still had to add pull ups to get things working so next time im doing that I will dig further