Just found this sub and so excited to see people sharing about this stuff.

Basically my partner is a mechanical engineer and wants to learn to fix my machines so he can support me in my hobby but both of my machines are broken and I am going stir crazy not being able to make anything! Advice on where to start with these issues would be amazing.

First my Singer 99k (electric) the thread tensioner (little wire bit) isn't doing its job and the thread keeps jumping out of the plates or snapping. Everything else is working fine seemingly after troubleshooting.

Second my Singer 401 (I hope that's the model my manual is currently packed away as we have just moved house) needle position lever is broken. I heard an awful noise one day and now the position 5 is correctish but if I move the lever left down the settings the needle moves right instead!

We have serviced and oiled the moving parts and I do this regularly (they are well used machines so this needs to be regular).

Anyone else encountered these problems? Any advice or troubleshoots to try beyond the basics (not because the basics aren't good but because we have tried those already)?