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Dell's may tend to flog the 12V rail, locally. I've replaced un-bulging 12V rail caps at some distance from the PSU connector on the MB to revive a not-booting unit.(sfter making sure PSU was OK)

Have also replaced motherboards in two units with no issues.

RL

When I force-start the PSU by shunting the green wire to ground all voltages come up as expected but the PC won't do anything other than run its fans and hard drives.

What I have is that the 5V standby voltage is fine and when the power button is pressed (it essentially pulls down two pins on a connector) you can here a very faint wine but the motherboard does not pull down the green PSU wire, so no start. It won't start the PSU.

I could do that but it would only make sense if I knew for sure that the big i7 processor isn't the culprit and is ok. Problem ist, you get zero documentation for motherboards these days so a decent on-board diagnosis is next to impossible.

Did you remember to plug in the 12 volt cable?

Yup :-)

A lot of stuff hangs on the 12V - it's usually the 12V inside the PSU that shows bulging caps - placing stress on external caps at same time as they go hi-Z. Replacing PSU or those internal caps won't correct for premature aging of the external parts, which are usually less rugged to begin with - they're in a place where they're needed - possibly only at spin-up.

I wouldn't have thought much about it, but I had one bulging 12V cap on one Dell motherboard that clued me in for a later bulge-free failure-to-boot on another.

The motherboard replacements were for; dead usb port and; a video chip with a hole blown through it's package top. issue on a second board.

However, for that to cause a non-start the PSU would have to come on and try to deliver the 12V. The problem in my case is that there is only 5V standby power as is usual on modern PCs and then when the power button is pressed the motherboard does not react. It does not pull down the terminal with the green wire so the PSU isn't even commanded to start.

If I start it by hand (BIST button or shorting the green wire) it comes up fine but, of course, the motherboard keeps everything in the processor area dormant, so no POST or boot screen.

What I can hear before pressing the power button is a ticking noise coming from the little piezo beeper. When the power button is pressed a very faint whine comes from somewhere on the motherboard as if some switch mode converter wants to start but can't. Courtesy of an army accident my hearing ain't good enough to find where, despite trying with a plastic tube. Maybe I should use a small directional microphone and a waterfall FFT diagram on a laptop.

I've looked all over for electrolytics but they all look pristine and no bulging. I went through there with a strong light and a little mirror on a stick.

If I'd only get to POST or a hanging BIOS screen, but I don't :-(

Replacing a QFP or BGA on-board video chip would be tough, that I'd call a total loss. In my case it may be something simple but without a schematic or at least some clues it won't be fixable. Main thing is, I need to know whether it's the CPU or something else on the mobo. If CPU then spending money on a used mobo makes no sense.

Meantime I have replaced this PC with a HP M01. It's AMD Ryzen 3 processor will probably only give me half the horsepower of the i7 but being semi-retired that's ok. I just would like to use the old i7 machine for something else and not just toss it, also for environmental reasons. It could become a nice lab bench PC because it has VT-x.

Well, replacing the MB will probably fix it up enough - you'll want to stick the HP in some background role, unless it's the I7.

RL