Canon FT QL Light Meter problem

I recently got my hands on a Canon FT QL and I think the light meter might be broken, but I’m not 100% sure if that’s the case and, if it is, exactly what is wrong with it.

Behaviour: so, when I don’t put a battery on the camera it’s stack on de lowest position (as it should) and it changes it’s resting position depending on the shutter speed and ISO, now when I put the battery on it’s always in an overexposed position (up, past the circle) no matter how low the ISO, shutter speed and aperture. Now the truth is that sometimes it seems to work, but it’s on extremely rare ocations and it looks to be completely aleatory (unless I’m doing something wrong?) and also the needle does move with the change of light but how much it moves it’s insignificant compared to the light differences I expose the camera to. I also tested this with two different batteries I have, they are both 1.5V replacement batteries but one is really discharged(my multimeter says 1V) and the other is brand new, but nothing changes with this.

So, up to now my main theory is that the light sensor of the circuit is faulty and that is why it rarely work and why it barely moves with such a low sensitivity, but I’m here mostly to see if this theory is wrong and what else could be the reason.