I have not encountered this issue before, but I can give you a start point.
The GBA SP has 2 CPUs, one to play GBA games and another one for the GB games. If you're only getting the GB screen maybe the GBA CPU is damaged. Maybe a pin has broken.
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I'd suggest you to check if you can get any signal from all the pins.
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I'd suggest you to test all the pins and check if they can pass signals.
Hello [link|https://es.ifixit.com/User/3338052/brain+box+jelly+fish|brain box jelly fish|new_window=true].
I have not encountered this issue before, but I can give you a start point.
The GBA SP has 2 CPUs, one to play GBA games and another one for the GB games. If you're only getting the GB screen maybe the GBA CPU is damaged. Maybe a pin has broken.
I'd suggest you to check if you can get any signal from all the pins.
Cheers!