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Stone Dead Laptop

tk3000

Jun 19, 2022
4
Joined
Jun 19, 2022
Messages
4
Hello Folks,

Ones of my notebook computers, a Toshiba model P75-A7200 which uses the motherboard da0bdbmb8f0 rev. F. was left on overnight, and when I checked it out in the morning it was stone dead: nothing, no standby or charging lights and would no turn on. Tried the trivial stuff, such as turning the computer on without the battery, etc, to no avail.

In the past, I dealt with electronic and have some basic understanding of circuitry, components, etc; but it was many years ago and I only assembled circuits using the old style of components and parts (not smd). Of course, the circuit board of a modern computer are very complex, but if one can isolate and pinpoint the circuitry area and the fault component (feeling its temperature, checking for shorts, etc) it may be possible to fix a problem without any thorough understanding of the circuit. Thereby lays my quest.

So far, I took the thing apart removing the motherboard. Whenever I connect the original power supply and test the voltage at the jack wherein the power enters the system the readout is fluctuating between 0.5V to 3V (should be about 19V, and the power supply itself is outputting about 19V when not hooked up to the computer, used other power supply just in case with the same results). Then, I connected a variable bench power supply directly to the board's jack (did not bypass the built-in circuit protection of the board) with voltage adjusted to 15V and amperage maxed up at 1amp; and when measuring the voltage in the power entry point (jack) I got only about 4 volts (even if I would increase the voltage in the power supply, the voltage measure at the jack would still be about 4 Volts), also if I would increase the current available in the bench power supply the board seemed to take it without prejudice (I did not go too overboard with the amps since a motherboard in stand-by should not use anything close to 1 amp) Near the power jack on the other side of the board some components seemed to get rather hot, one such components looks like a smd power mosfet and other a diode (diode with 0.5v by multimeter's diode testing feature); and I also checked check some components in the vicinity of the power jack for shorts and there is an smd electrolytic capacitor and it seems that thence (including the capacitor) it is shorted (at least a partial short).

Any insights would appreciated.
 
Last edited:

Martaine2005

May 12, 2015
4,345
Joined
May 12, 2015
Messages
4,345
Don’t bin it just yet, a shorted capacitor is the most likely culprit. You seem to have an idea about voltage injection so continue with your bench supply. There is probably three voltage rails, 19v main rail, 5V and 3.3V or 1.8V. Trace the main 19V rail to the other onboard power supplies. Look for areas with a mosfets, inductor and grouped capacitors. Check the caps for a direct short to ground. Apply some isopropyl alcohol liberally over those areas and then turn your bench supply on.
But first, for ease, just check all the caps for a direct short. Find a good ground and then probe away. Take photos.. and photos.. and photos. Remove shorted cap, check board again to see if short has gone. If your lucky, the laptop will happily plod on without that cap.
Too much to write here so try and report back with photos and indicate where you are testing and how.


Martin
 

Erstwhile

Apr 12, 2022
43
Joined
Apr 12, 2022
Messages
43
You might find a (used) replacement motherboard for it online.
A few years ago, I found one for my ThinkPad R400 for ~$25 at Walmart.com (of all places!).
 

tk3000

Jun 19, 2022
4
Joined
Jun 19, 2022
Messages
4
Hello Fellows,

Sorry about the delay in my response. I would like to thank you for the responses. Also, at the moment I am embroiled with some problems and can not work or do anything on the motherboard. Hopefully, in a few days I will back working on the computer again and will post updates.

-tk3000
 
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