Check the continuity of the PCB tracks with a multimeter. It's common for many devices of this era (not only MSX) to have broken tracks on the joystick connectors due to the stress.
There are technics to restore broken tracks, depending on the extension of the damage. For most cases, it's enough to scrap to expose the copper and bridge the fissure with solder.
Yes I did that and managed to get it working again 
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