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MobilePro 790 Power Issue

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thenzero Page Icon Posted 2023-01-18 6:50 PM
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Got a new-to-me MP 790 off ebay recently. The case was in bad shape (cracked plastic, broken hinges) but it seemed functional apart from no power/charging light when on. The screen was in better shape than the one on my 780. I did a very involved case swap with my 780 (which I'll post up later). That seems to have gone very well, but the new system has a serious issue- if I remove the battery for more than a few seconds, it won't boot again until I do a hard reset, even after reinstalling the battery and hitting the reset button. It will fail to power up when I hit the power button or just hang on the boot screen (not the battery latch screen...battery latch is correctly closed). I can reliably reproduce this issue. Torch has suggested perhaps it's something to do with the backup battery- I put the one from my 780 in the "new" 790 because I thought it was in better shape. Supposedly they are the exact same battery- system reports that it is good.

Any thoughts? Troubleshooting steps I can try? Is this a known issue?

p.s. I tried again just now and after removing the battery for a while longer, it hard reset itself. So, that seems to indicate something wrong with the backup battery, right? Is it possible for it to show the battery as "Good" but it still need to be replaced? I have never seen this behavior on my 780, so I'm surprised if that's the cause but...

Edited by thenzero 2023-01-18 6:56 PM
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torch Page Icon Posted 2023-01-18 7:57 PM
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Can you switch it to the old battery and try replicating everything that happened before?
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thenzero Page Icon Posted 2023-01-18 8:15 PM
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Meaning use the backup battery that came with the 790? I could do that but it's very involved so I'd like to see if there are any other options...
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thenzero Page Icon Posted 2023-01-18 8:17 PM
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Let me ask this- if I had it open with the backup battery in my hand, what test could I do to verify the battery was "good" or not?
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torch Page Icon Posted 2023-01-18 9:02 PM
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Understood. I didn’t realize it was an involved process. Then yes you’re correct to wait to see if there’s any ideas to try before having to do that
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thenzero Page Icon Posted 2023-01-18 9:03 PM
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Sorry torch, don't want to sound like I'm dismissing your idea, I think you're absolutely right, just hoping someone will have seen it before.
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C:Amie Page Icon Posted 2023-01-18 9:07 PM
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Stick a multi-meter on the battery for 5 min and check the voltage is constant.

You also need to check any capacitors on the board. If the backup battery is running through a cap and the cap isn't holding charge to the DRAM...
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torch Page Icon Posted 2023-01-18 9:07 PM
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Nono. You didn’t.
If I was in your shoes, I’d want to try everything else first before having to go through that process too. We’re on the same page.
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thenzero Page Icon Posted 2023-01-18 9:28 PM
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Quote
C:Amie - 2023-01-18 9:07 PM

Stick a multi-meter on the battery for 5 min and check the voltage is constant.

You also need to check any capacitors on the board. If the backup battery is running through a cap and the cap isn't holding charge to the DRAM...


It's a steady 3.3V. (The one that came with it that I'm not using seems constant at 2.8V)

Any hints on how to check these caps? Visually they're fine. But I'm not sure how to test them past that.



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C:Amie Page Icon Posted 2023-01-18 9:38 PM
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Very difficult to do without the right tools. You'd have to work to what they are and then check to see that their output is stable - assuming they are in line from the DRAM battery of course
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thenzero Page Icon Posted 2023-01-18 9:46 PM
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Hm. OK. Well, I put my ESR meter on them (It's hard to tell where the test points are) and the reading is a teensy bit higher than it is on the 780 board but doesn't make a lot of sense unless they're either in parallel or also connected to something else with a capacitance because the reading is ~500uF (it was ~400uF on the 780 board).

They are on the exact reverse side of the board from the battery so I'm guessing they're connected but I don't have a schematic for this.

Edit: sorry, calling this thing an "ESR meter" is probably a stretch- it's a multimeter with a capacitance function

Edited by thenzero 2023-01-18 9:48 PM
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thenzero Page Icon Posted 2023-01-18 9:51 PM
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I verified that they are in parallel, so probably 500uF is a good reading.
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thenzero Page Icon Posted 2023-01-18 9:54 PM
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By the way- a quick test of the power LEDs themselves seems to indicate that they are bad...how the heck does that happen
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C:Amie Page Icon Posted 2023-01-18 10:05 PM
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Impact?
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thenzero Page Icon Posted 2023-01-18 10:09 PM
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So should I switch the batteries or nah?
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