It was free.
Seller said it doesn’t work, but at least something works:
Several R4 Minima example sketches ran without complaint.
Board Identify states:
Board Type: 0
Board Make: Unknown
Board Model: Unknown
Board MCU: Unknown
Hi @jg1. It is a derivative of the Arduino UNO R4 Minima board. Most likely the only differences in the hardware between this and the official board is:
- Pins are broken out to male headers in addition to the standard female headers
- The "love" pin is on the top of the board instead of the bottom.
- The barrel jack has not been populated (or perhaps desoldered?)
It is possible the designer of the boar might have done something similar to the "love" pin with the open source hardware logo pad on the board.
Sometimes people come to this conclusion when the problem is actually user error.
Or it may have been the case of a "soft brick". A bug (e.g., a divide by zero), or even expected behavior (e.g., putting the microcontroller to sleep), of a sketch program can put the native USB boards like the UNO R4 Minima into a state where they are not correctly recognized by the host PC. The board can always be easily recovered from this state by a double press of the "RESET" button, but the user is not always aware of that technique and so may come to the conclusion that the board is not recoverable.
I do find the missing barrel jack a bit odd though. But I also can't imagine a scenario where the user would have a good reason to remove it without that reason having left some evidence on the PCB. And the pads also look very pristine. So it seems more likely that the part was never populated by the manufacturer.
I think it is only because the author of that library hasn't added code to identify the UNO R4 Minima.
I removed the barrel jack. It was visually “twisted” i.e. poorly aligned / installed, which I thought a bit odd, but it was intact, and nothing else about it appeared to be damaged.
After determining the board worked with a USB-C power supply (its red “ON” LED illuminated but nothing else happened) I uploaded a simple sketch. The board still appeared not to function, but I stumbled upon the reset button double-press, after which everything began to work.
After some cursory testing I tried the barrel jack power input, but the 6V power supply I had been using with other boards failed to illuminate the power LED. I then tried a different, 9V power supply and still nothing, so I suspect something is wrong with its onboard regulator.
I’m not even sure the official Arduino schematic can be relied upon for this board.
Given the uncertainty I decided to remove the twisted barrel jack. Everything else about the board seems ok, but I have no need for the barrel jack power input anyway.
The abundance of header pins is a welcome feature.
That was an interesting read. Thank you!
If this is (or was) that onboard regulator it shall regulate no more:
It appears to have been a six terminal device, of which about 2/3 has long since been inhaled or otherwise scattered to the wind.
Without a schematic I can't even be certain it is / was a regulator. The barrel connector center conductor does not directly connect to any of its former pins. It first goes through a diode, directly to the right of that device shown in the first picture. That diode is intact and is probably ok. It in turn connects to the upper two pads on that failed device.
If I could identify that six terminal device I might be able to source a replacement. I can find pictures of that board (it seems to be known as "Arduino Minima R4 Black") but any markings are barely readable and have not led to any likely component. I certainly can't find any schematics of it.
That board isn't cheap!
To my knowledge official Arduino boards do not have a regulator in a package like that.
But what caused it o fail?
There are probably other bad components on the board.
It still works if you power it through the USB socket?
Looking at a similar picture and because of the proximity of the inductor and power jack, I guess that it is a buck converter.
It could be something like the device here ( DD4012SA ) shown in a module :
But, since the UNO MINIMA clones are so cheap on Aliexpress ( https://de.aliexpress.com/item/1005009892348911.html ), it is probably not worth spending too long repairing the one you have:
I believe you are correct. Where did you find that module?
Not gonna do that 
Whatever it may be is likely to be more expensive than the R4 Minima from Aliexpress, which "can't be shipped to your address". Not that it matters; I'm more interested in the challenge of fixing the one I have anyway.
Yes. Everything works.
That is the great thing about doing this as a hobby as opposed to work. You don't have to do a strenuous cost benefit analysis on such activities.
I found it here: https://de.aliexpress.com/item/1005009448987894.html . There, the dollar price (USD) is less than 1. Also here, somewhat cheaper: TZT DD4012SA 1A DC 5-40V to 3V 3.3V 3.7V 5V 6V 7.5V 9V 12V Regulator DC-DC Step-Down Buck Converter Module Board - Step-down Module - TUOZHANTENG HK co.,LTD. There is not enough information to say if the chip is compatible with your board but, of course, you could always put such a ready made (5V) module "in front" of the 5v rail of your Minima if your only power source is DC between about 7 and 40 volts.
The key search phrase is DD4012SA if you want to look locally. If the $ prices in your screen shot (post #8) are USD then you have my every sympathy.
That's a nice find. Ten for < $10 including taxes and shipping is more like it. I could find no US source that came anywhere close.
Exactly what I was thinking. Thank you!