The Sinclair Inferface 2 adaptor plugs into a ZX Spectrum computer and adds rom cartridge support and two joystick ports. Unfortunately this one had issues with the player one port, left and right were moving very slow and erratically. I opened it up and reflowed the joystick port and connector that goes to the spectrum and also cleaned everything as there was a lot of flux left over from when it was manufactured.
This didn't fix it, meaning the only possible cause could be the custom MT62001 chip. It doesn't look like this chip has a gal or any other type of replacement available or was used in anything else so I thought that was that.
After a little more research though it turns out that Amstrad continued to use this chip in later computers but renamed it to 40057. As luck would have it I have a scrap +2 (grey) motherboard that's been mostly stripped but still had this chip intact. After installing it into the interface 2 it now works perfectly.
Repair complete.