tinynja98
Posts: 1
Joined: Thu Aug 29, 2019 5:59 pm

Pi 3 Model B USB not working

Thu Aug 29, 2019 6:20 pm

Hello, a friend of mine gave me his faulty raspberry pi for free, and now i want to try and repair it. I took the sd card of my known working personal raspberry pi, installed it, and sure enough the faulty raspberry pi booted right up, hdmi and ethernet were working perfectly and I could ssh into it. However, i noticed none of the usb ports were working, no power at all was given to any usb device i tried.

After some investigating, I found that dmesg gives me a long list of "usb 1-1-port2: over-current change" errors.

I then got my multimeter out and tested for power on the USB ports, but PP27 reads around 100mV when it is supposed to be 5V.

I did some more research and some threads suggested that maybe C97 was bad, so I tested the resistance between the two sides of the capacitor and it was reading close to 0.6Ohm, so I removed it, but then the same problem still persists.

Probing the two pads on the board where C97 used to be still gives me close to 0.4Ohm, and probing from PP27 to ground gives me a resistance of around 0.4Ohm also.

To my eyes there is clearly a short between PP27 and ground, but I don't know where it is... I thought about applying an external voltage on PP27 and ground to find if any component would heat more than the others and use some isopropyl alcohol to locate it (saw this on rossmann vids) but I'm afraid I'm gonna fry something on the board...

Could anyone help me on how I should diagnose this problem? All help is greatly appreciated!!!

User avatar
Burngate
Posts: 6002
Joined: Thu Sep 29, 2011 4:34 pm
Location: Berkshire UK Tralfamadore
Contact: Website

Re: Pi 3 Model B USB not working

Fri Aug 30, 2019 9:32 am

Several things come to mind, though how helpful you'll find them I don't know - probably not at all!

Finding short-circuits can be difficult, in part because the resistances involved are similar to the resistances between your meter probes and the copper traces.
You measured 0.6Ω across C97, and that dropped to 0.4Ω after you removed the capacitor? If that were true, it would mean the cap had a negative resistance (of -1.2Ω)! Far more likely is that you hadn't made perfect contact, first time round!

I also wouldn't bother applying an external voltage - the current involved is more likely to burn the copper traces, causing more damage. And if, by chance, it caused the short to vapourise, then the applied voltage could kill other parts of the Pi (they don't like it up 'em, to coin a phrase)

I suppose you've already looked inside the USB sockets to see if there's any foreign bodies, or bent pins?
The next thing I'd try is removing the sockets themselves, to see if that removes the short.

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