Well, to make the long story short – it’s dead. I was setting up my router with OpenWrt to share the internet, and connected the wrong power supply unit that provided 7.5 instead of 5 volts. The router is ok – there’s a good stepdown converter onboard, while the usb host is directly powered by what comes from power supply. So, that’s 2.5 volts overvoltage.
So, the modem got totally screwed: Blink and it’s dead. So I took the screwdriver and disassembled it.
By the way, the modem usually acts a bit weird. If it’s ledt disconnected for a long time, once you connect it to your laptop, the laptop just gets turned off via usb short-circuit protection. My guess was, that that happened because of the big inrush current into some capacitors out there.
And the guess was correct. The 5V usb supply are connected to two 6V 470uF caps connected in parallel (almost 1000uF!), then it goes into an unknown noname IC, and after that – into an LC filter consisting of 2х470uF 6V caps + 2 tantalum caps (value unknown, all caps are connected in parallel). AFAIK that is done because GSM modules do consume a lot of current while sending out bursts of data.
Whe I plugged the modem in the usb the multimeter showed 0 volts at the output of that unknown stepdown.
“It’s dead, Luke”
Let’s hope that the main SoC is still alive. Since no datasheets were found on the stepdown, I decided to guess the output voltage. My guess was 3.3, but I wanted to be sure. The chipset is Qualcom, with ARM core inside, so that could be anything from 1.2 volts, AFAIK. So I hacked another project, I called “The dropper“, to guess the correct voltage. At 3.3 volts the modem came back to life.
So I glued a 3.3 linear voltage regulator to the PCB:
Connected that with 0.3mm coated wires, so that it would fit in the original casing and no mod would be visible.
It worked for 15 seconds before the voltage regulator was shut down by overheat protection. Looks like the current was too big for it to handle.
So I said “goodbye” to preserving the original look, drilled the case, attached a powerful 3.3 volt stepdown, and assembled it. You know, who cares about design when the whole thing is going to work in the attic for the time being?. The result looks somewhat like that:
I removed one of the 470uF caps at the input, so that it will not bug me any more while I’m debugging the software stack on the laptop.
Once assembled, the modem connected to the network, completed a ping test. So, I guess it can be called a success.