I have done many mods such as Atari Lynx 5v mod, SNES jr S-Video support and power LED, PSOne modchip, 2600 S-Video and composite, etc.
but this is the only one I did with photo and details
Duo-R (and all Duo) has only 1 LED for drive activity. I wanted power on LED and I wanted it separate from the existing red LED so I don't get purple glow. Mounting the second LED would have worked but I wanted to leave the exterior looking stock so I had to figure a way to make blue LED come on only when red LED is not on. Simple schematic I found worked:
I wired it dead bug style. With total of 4 parts, it wasn't worth wasting pcb. I mounted the LED on top of the transistor so the blue LED would sit about the same height as the red LED. Conveniently the collector and emitter leads of 2n3904 are spaced the same as LED leads so nothing special there.
Here's a video of it in work:
https://youtu.be/v7awBIm9M3g
When I did the LED mod, I also swapped the stock 5 pin video connector with an 8 pin one so I can add RGB support later. Just need RGB amp, and to build custom cable with sync stripper so it works on Commodore 1084 monitor. (it expects separate sync and likely won't work with straight composite video as composite sync)