Motherboard problem between battery and AC adapter on a Compaq's CQ40 laptop


Here's my problem:

A Compaq's CQ40-627LA was working fine until one day it started to shut off as if the CPU were extremely hot. I cleaned the fan and the cooler, removed the old thermal paste and added some new one but the laptop kept shutting off after being on just for a couple of minutes. While I was trying to find what could be wrong I noticed that when I tried to turn on the laptop with the battery alone, the charging indicator LED blinked (telling me that there was no charge in the battery), and the laptop wouldn't start. Also noticed that when both, the adapter and the battery were plugged in, the laptop would go on for a minute or two and then off (just the way the problem started). And finally, noticed that when I tried to turn on the laptop just with the adapter, it wouldn't do anything, as if the adapter were dead. So I changed the adapter for a "known to be working" one, but the exact same thing happened: just the battery gave me the blinking LED, just the adapter gave me a dead laptop, and both of them gave me the charging indicator on (as if everything is ok) and two minutes of "not dead" laptop. Obviously I thought that the problem was the battery and got one that should be working (though I haven't verified that) because it is from an almost new laptop that had a MoBo problem. Anyway, with this "new" battery the laptop's always dead. No signs of charging or life at all.

I have already opened the laptop and verified the voltage at the internal DC jack connector, which seems to be right. The voltage that is missing is the one that should be present at the battery contact. So, my question basically is where (or what) should I be testing in order to find what's causing my problem?

Any comment would be extremely appreciated.

Thanks in advance.
milnerfloresAsked:
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jmaggio322Commented:
detach the battery and unplug the power hold down the power button for 60 seconds then reconnect everything. If this does not work you may need to update the bios.
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Grant1842Commented:
Well I all ways check the ac adapters.
Please see my article here on EE.
http://www.experts-exchange.com/Hardware/Laptops_Notebooks/A_3413-Check-Laptop-Power-Adapter-Voltage.html

Next see my article on Battery's here on EE
http://www.experts-exchange.com/Hardware/Laptops_Notebooks/A_3430-How-to-test-your-laptop-battery.html
Pay attention to the AC adapter section.

I think you will need a service manual for the mother  board .
If you had service manual you could check voltage points etc.

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nobusCommented:
i would test first with a known good battery, or test this one on another PC
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nobusCommented:
and note that most batteries are bad after 2-3 years...
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milnerfloresAuthor Commented:
Thanks again for your comments. I have done what jmaggio322 suggested but nothing changed (I also disconnected the BIOS battery for that purpose), so it is not possible to update the BIOS, because the laptop is still shutting itself off in less than two minutes.

The ac adapter is definitely working because of two things: 1) I have measured the voltage with a multimeter directly from the adapter and inside the motherboard, and 2) the laptop behaves exactly the same when I plug in a different adapter that I am positive is working.

About the battery, I haven't had the opportunity to confirm that the one that I'm using as a test replacement is fine, nevertheless, I think that this is not so relevant at this time (please correct me if I'm wrong) since the laptop doesn't work at all if It has only the ac adapter connected. Even the DC jack LED is off (it needs the original battery inserted in order to be lit, and only the original battery). And as far as I know, if the battery were the problem, the laptop should turn on with a good ac adapter and the battery out, but this doesn't happen. That's what makes me think that something is wrong with an internal IC, but since I don't have a circuit diagram of the MoBo with me, I'm kind of lost.
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jmaggio322Commented:
sounds like a bad dc jack in that case that either needs to be replace or resoldered
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milnerfloresAuthor Commented:
The DC jack is not soldered to the motherboard. It is connected to the board through four cables, and I've checked the voltage on them and they're Ok. Thanks anyway.

Picture of it:

http://store03.prostores.com/servlet/electronicclinic/the-4147/Compaq-Presario-CQ40-fdsh-CQ45-Series/Detail
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nobusCommented:
did you test with all devices  removed yet?  eg, disk, cd drive- only 1 ram stick ?
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nobusCommented:
if you did so, and it is still shutting down, the only other test i know is replacing the cpu (also the heat paste)
that would leave only the mobo ..
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milnerfloresAuthor Commented:
This is what I just did.

* Took out the motherboard.
* Unscrewed the fan from the case and plugged it to the mobo.
* Cleaned the CPU and put some new thermal paste on it.
* Reinserted the RAM module.
* Connected the DC jack to the mobo.
* Connected an external monitor to the VGA port.
* Put everything on a table made of wood.
* Verified the voltages on the adapter (18.6V)
* Plugged in the adapter.
* Shorted with a screwdriver the two contacts to emulate the ON button.

Results: Nothing happened. Not even the LED on the DC jack was on.

* Managed to place the battery in (the original one) without disconnecting the AC adapter.

Results: The charging LED and the DC jack LED turned on after about half a second I "inserted" the battery.

* Emulated the ON button again with the screwdriver.

Results: The ON indicator LED went on and the fan began to spin. Compaq's welcome screen showed up on the external monitor, then changed to a black screen saying something about the devices. Not more than 30 seconds later, everything went off, LEDS, fan, screen...everything. But after 1 second, the charging LED and the DC jack LED went on again. I tried to turn it on again and the fan began to spin. Also the ON indicator LED was on, but now the screen didn't show anything. And once more, everything went off.

* Unplugged the ac adapter. Detached the battery and took out the CPU.
* Inserted a compatible mobile AMD CPU and put some new thermal paste on it.
* Retested.

Results: The exact same thing happens. Goes on for a little while with the original battery attached, but nothing happens without any battery or with a different one.

Definitely the problem is in the mobo. But what in the world is causing that just with the original battery the laptop shows life signals? My guess is that some sort of power management IC is defective, but without a circuit diagram of the mobo, it could not be more than that, a simple guess.
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milnerfloresAuthor Commented:
Here's an update. I found the way to get a good charged battery, so after getting the laptop together again I gave it a try with this new battery. The laptop worked like a charm. It started the OS with no problem and showed on the system tray the right amount of charge (41%). After a minute or two I decided to plug in the charger and to my surprise the system tray icon changed to the charging state (with animation included). The problem is that, after another two minutes of "charge" the battery level showed 40%,and kept going down as minutes passed, even though the message on the icon showed "XX% and charging" all the time.

Now I know that the problem is that something is broken between the AC adapter connector inside the MoBo and the battery port, could it be only a fuse? (I've already found a fuse named F2, but cannot find any F1 fuse)
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nobusCommented:
in most, there are no fuses afaik.
it can be a broken wire, or solder, or simply a bad charger circuit (which often happens)
unless it is a separate board, it is not "repairable" in a normal way
if you want the same motherboard, look on e-bay (you may be able to find the same model laptop with a broken screen)
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milnerfloresAuthor Commented:
Still looking for that piece of hardware that's avoiding the battery to get some energy.
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nobusCommented:
post results !
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milnerfloresAuthor Commented:
It is so difficult to find the problem without the circuit diagram of the motherboard. So that's my request now. If anyone has it, or a trustworthy link where I can find it or buy it will be extremely helpful.
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nobusCommented:
you can google the chips - and see if their functionality tells you something
but right now - ther's only 4-5 chips left imo...
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milnerfloresAuthor Commented:
I tried to follow the most logical-that-I-could-think-of-without-a-diagram path for the current and found I chip of MOSFETs that might be the culprit. Googled for the datasheets and tried to test them but since I couldn't desolder the chip (it is very close to other tiny chips) I really don't trust on my tests' results.

Now I think the work ahead is so much it will no longer be worth to do it, so I'm just gonna close the question with the idea that the motherboard needs to be replaced.
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nobusCommented:
that is what is done in most cases; it costs more to repair the board - even if you have parts - than to replace it
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