Spannernick and jj0 have setup all the tools and files to rescue a C64 Maxi if you have messed up the firmware with dumb changes like editing the games inside the firmware a bit too much. (I know I did). The info on how to rescue is in several posts and I thought I should give the QUICK version here – please find the details in the respective threads. I just rescued my C64 Maxi exactly the same way:

You need:
* A Windows PC (Linux will also work but these instructions are for the lowest common denominator)
* A USB A to USB A type cable that you can plug into a USB port on your PC AND on your C 64. You most likely do not have this and will have to buy one.
* A copy of “NandB” – at the time of writing provided by Nick here in the folders named “Nand Backup” DOWNLOAD NAND

* The FEL Rescue boot software – at the time of writing provided by jj0 here DOWNLOAD

* USB Drivers from here:

* A Windows Disk Imager, the one here is confirmed as working:

So this is the shortened step by step:

* Remove all cable from the Maxi.
* Take a screwdriver, take out the three screws at the bottom of the C64.
* Plug the USB A cable into your PC
* Press the “FEL mode” button on the board of the C64 Maxi, it is a very small button directly under the empty white socket on your left. Keep it pressed
* Insert the USB A cable into the Maxi on the BACKSIDE (next to HDMI)
* Start the Zadig software you downloaded and have it install the driver for the new unknown device. CAN TAKE SEVERAL MINUTES, be patient
* Start the “boot.cmd” inside the Windows folder of the FEL Rescue boot software and WAIT
* After a while you will see two additional drives on your PC with unknown filesystems. One is ~16 MB large, the other should be at around 180 MByte but that can vary based on how you mangled the ROM.
* Get the drive letter of the larger drive.
* Start Win32Imager and write the Nandb file straight to that larger drive (BE CAREFUL not to overwrite any real drive on your PC)
* Once this is done, “eject” the extra hardware on the USB port via the Windows taskbar command for ejecting stuff. (Not really necessary, you can simply unplug the Maxi).

That’s it. Your Maxi should now boot with original 1.30 firmware again. If you also messed with Nanda (most likely you did not), you need to image the smaller drive with the nanda file.

THANK YOU, jj0 and spannernick – nothing in this here is new, I just wanted to collate the info. All the details are in the forum, if you need more background, please check the respective threads.