I have a similar/same issue (only now i seem to be making backward progress)
Platform :
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Old model Dell Optiplex 755 with an core 2 cpu, no GPU and booting/running off a 32gb usb
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6.1.1 dragon blaze. x86_64 (also had x86 running on it yesterday)
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wired keyboard (but did not work with PowerA Mini xbox controler)
with a fresh install i can hear the game and navigate it, but not see anything. While i suspect that it might be the lack of a dedicated graphics card on an old machine, i followed this https://github.com/recalbox/recalbox-os/wiki/N64-%3A-return-to-ES-(EN) i had found in the readme int he rom folder. I was able to follow everything in the instructions (not the first time by any means) but the TV (and monitor) i have these connected to do not offer a list of options, like the write up suggest. so i am stuck with a higher than desired resolution without being able to change it.
Have tried the different cores as well.
Since messing around with it i am now unable to even get it to run (with black screen) it just boots me out.
Suggestions?