If you are looking for the actual PDF to operate or repair the machine, you can find it through the following sources:

This is often the most referenced page. It shows exactly which terminals are for input, output, and ground.

Finding an official digital copy of this vintage manual can be challenging, but several reliable sources host it:

: Only use straight, high-quality needles; bent needles can cause damage to the needle plate or machine timing. Brother USA specific troubleshooting step for this model, or are you looking for a direct download link for the PDF? Sigma - mimaquina.es Manual Sigma 161 ... ¡Disponible sólo en Internet! mimaquina.es ANTIGUO MANUAL MAQUINA DE COSER SIGMA MOD. 161