Nooo, strong electronic burning smell...

[agami]

Just a black screen?
Try it without the Indivision.
Get the motherboard out and get your nose right down into it. Scan methodically from one side to another, top to bottom.
Has the motherboard been re-caped?
Get it down to basics; no Mediator, 2MB chip RAM only.
Is it still running on the original Commodore PSU?

[agami]

Wow, never seen this happen in the 32 years I've used computers.

I have. Only once. On an mega-expanded Amiga 1200.

[agami]

To all operators of an A4000 Desktop configuration.
The original Commodore integrated 150W PSU is built using late '80s/early '90s PSU knowledge and has also been procured with latter-day Commodore frugality.

You might think that the A4000 engineers took this into account when constructing the motherboard/daughterboard/expansions and made them that little-bit more resilient. That might be the case. But none of them envisioned that the A4000D with the original PSU would be running anything in 2015.

I can't emphasise the importance of a good power supply enough. To quote that guy from Apollo 13 "Power is everything".
I recommend you replace the old inefficient (by design and by age) PSU with a more modern SFX of TFX form factor PSU. Both use modern PSU designs that operated at higher levels of current supply efficiency and run much cooler and quieter. Both can be integrated into an A4000D case and connect to the the motherboard with the low cost Amiga 4000 ATX PSU adapter + switch.

Especially if you are running high end CPU cards and a Mediator with PCI cards, but also if you are just running a pure vanilla, unadulterated, classic Amiga A4000/0x0 with only classic Zorro expansion boards. Protect your investment in nostalgic computing glory.

[agami]

Nonsense, there is nothing wrong with the design or function of the original 4000 psu's....

I don't disagree with you. When the A4000 was designed it was a standard, if low cost PSU for the time. It's not like Commodore were breaking new ground with amazing PSU tech.

And I agree with the recapping of the original PSU and while you're at it, putting in a quieter fan.
I just found it a lot easier to put in a modern, manufactured in this century, high quality PSU. Only a few OEMs make a SFX and/or TFX PSU and they are reputable brands and the PSUs are generally of the 80+ certified variety, and run extremely quiet.

Whilst it is true that ATX PSUs do place more focus on 12V, the 80+ have solid 5V performance.

You clearly agree that good clean power is important. You can get it by improving the original A4000 PSU or by getting a brand spanking new SFF PSU.

For the record, the PSU is fine...

I wasn't implying that your PSU was dead. What on first glance is a regular running PSU can still be a culprit for many things that go wrong with one's rig.

I'm not saying you must buy a new PSU. But I would at the very least recap it as per mechy's inclination.