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Precision M6700 No-POST issue

Dear Dell Community,

I just can't tell you how happy I had always been with my Precision M6700 mobile workstation!
That is, until recently, when it stopped working. I am now stuck at No-POST. The problem is that I don't observe a stable No-POST behavior and deal with conflicting interpretations of the same diagnostic information. Because of that, I need your help to be able to repair it.

Here is what happened.

While the laptop was running, the screen went completely dark, with a slight sign of being back-lit. It happened during a routine task that was not at all resource-intensive (I had Firefox and an office app running). The laptop was powered through a power surge protector, and when it happened, there were no thunderstorms or any other signs of possible power surge, anyway.
It has never happened before.

After a while, I switched it off and tried switching it on again but wasn't able to get to BIOS screen. Since then, I haven't been able to start the computer. I am stuck at the no-POST stage and need some directions on how to repair my laptop. Here are the two issues that I need your help coping with.

#1 Not stable behavior

The no-POST start-up behavior is not stable. Sometimes the screen is a little bit back-lit and sometimes it is completely black. After querying the devices the laptop gets stuck with nothing going on --- that's the stable part. In a while the fans would kick in, if the temperature has increased enough.

The diagnostics is not stable, either. While I never had a problem starting the LCD test by pressing 'D' while powering on, the other test, launched by pressing 'Fn' while powering on, is much less stable. Sometimes I am able to launch it and sometimes I am not. And when it starts, it sometimes shows only the blinking diagnostic LEDs (blinking-blinking-off) and sometimes, after they stop blinking, continues with a more elaborate sequence, including the LCD cycling through red-green-blue-white-black colors (very much like during the 'D+PowerOn' test but with accompanying single-time beeps) and then, after the LCD goes off, just firing beeping the 3-3-3 sequence forever (each 3-3-3 beep sequence is separated by a longer pause and melody beeping with a varied pitch).

The only stable variant of 'Fn+PowerOn' test is when I remove all the memory modules, in which case it always starts and consistently shows solid-blinking-blinking.

#2 Conflicting interpretations of the diagnostic information

Different sources on dell.com provide conflicting interpretations of the diagnostic information that I have. Thus, the table located at

https://www.dell.com/support/article/us/en/04/sln295101/a-reference-guide-to-the-precision-mobile-di...

interprets the blinking-blinking-solid LED combination as "The keyboard is preventing the system from completing POST".

However, M6700 Owner's Manual, located at

https://downloads.dell.com/manuals/all-products/esuprt_laptop/esuprt_precision_mobile/precision-m670...

interprets the same LED combination as "System failed on hard drive initialization OR System failed in Option ROM initialization."

Finally, the beep codes reference guide located at

https://www.dell.com/support/article/us/en/04/sln309623/understanding-beep-codes-on-a-dell-notebook-...

interprets the "3-3-3" beep sequence as "System board, Chipset Error, Clock failure, Gate A20 failure, Super I/O Failure, Keyboard controller failure"

Given the above discrepancies, I have difficulty in deciding what shall I do to repair my beloved machine.
Service tag and exact system cofiguration are available in PM.
Thank you in advance.

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