battletux
Posts: 39
Joined: Mon May 21, 2012 7:33 pm

model b boot issue. odd voltage readings.

Thu Apr 02, 2015 7:42 pm

So I have a 512mb model b pi that seems to have stopped working while I was away at work last week.
This was a replacement modmypi sent me after a SD card adaptor from them broke the SD card holder on a previous pi and this pi is about a year or so.

I tried to fire up the pi to run osmc and plug in the i2c DAC to the HiFi however it won't boot. I either have no lights or a very dim power light. I have tried different PSU's and USB cables that I know work but there is no change in the behavior of the pi.

I hooked up the multimeter to it to check the voltage over the test points and this is where it is a little odd.

With no SD card inserted I get a reading of 4.2 to 4.4v with any PSU. Drop a SD card in and the voltage goes to 7.4v then down to 6.3v then to around 3.8/3.3v before the power light goes out and the voltage reading goes to pretty much zero. I also note that, unsurprisingly the CPU/ram package is VERY hot to touch during this and that the capacitor by the power port sometimes gets quite hot too, but inconsistently.

Any ideas what can be the cause of this? There have been no power issues over the week that I'm aware of and I can see Jo sign of damage to the pi's PCB or any of the components on it.
http://about.me/andy_mc

battletux
Posts: 39
Joined: Mon May 21, 2012 7:33 pm

Re: model b boot issue. odd voltage readings.

Thu Apr 02, 2015 7:46 pm

Just to add I have also tried powering it from a USB TTL console cable. The voltage with the SD card in reads 4v and the power light is unsurprisingly dim. The CPU/ram package also feels too hot to touch as well but not as much as it does when connected to the mains.
http://about.me/andy_mc

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Burngate
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Re: model b boot issue. odd voltage readings.

Fri Apr 03, 2015 11:15 am

With no SD card inserted I get a reading of 4.2 to 4.4v with any PSU.
That's a bit low.
Drop a SD card in and the voltage goes to 7.4v ...
Way too much, and if your reading is correct then your PSU is faulty.
There's nothing on the Pi that can cause this. Just putting in an SD card can only increase the current drawn by the Pi, and if that causes the PSU to drastically over-volt, then its design is terrible - if not unbelievable.
... then to around 3.8/3.3v before the power light goes out and the voltage reading goes to pretty much zero.
It's tripping the poly-fuse.

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FTrevorGowen
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Re: model b boot issue. odd voltage readings.

Fri Apr 03, 2015 1:25 pm

Burngate wrote: ...
Drop a SD card in and the voltage goes to 7.4v ...
Way too much, and if your reading is correct then your PSU is faulty.
There's nothing on the Pi that can cause this. Just putting in an SD card can only increase the current drawn by the Pi, and if that causes the PSU to drastically over-volt, then its design is terrible - if not unbelievable.
...
An extreme variation of the behaviour of this A1399 clone perhaps? :o :
http://www.cpmspectrepi.webspace.virgin ... 1399_Clone
Trev.
Still running Raspbian Jessie or Stretch on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W, 2xP3A+, P3B+, P3B, B+, A+ and a B2) but Buster on the P4B's. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm

battletux
Posts: 39
Joined: Mon May 21, 2012 7:33 pm

Re: model b boot issue. odd voltage readings.

Fri Apr 03, 2015 9:36 pm

I have tried the 2amp charger from both my ee 4g mifi and my oneplus one. As well as using a USB TTL cable. All show issues with the voltage as in my first 2 posts. These psu's have never had any issues before now.
http://about.me/andy_mc

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