My 12 year old Black & Decker 7308 Type 7, 230 Volt A.C corded circular saw is struggling to rip 12 mm plywood; I have changed the blade and it still struggles to rip. Can you help me diagnose the problem?
I tried to cut a 30 mm thick wood, but it was struggling to cut and so I stopped. Afterwards it even struggles to cut 12 mm thick plywood. (not enough torque)
Does the motor need rewinding to improve the torque?
Honestly, a 12 year old black and decker probably needs to go to the scrap yard. Low end saws are not built to be rebuildable, at least since the late 20th century (some 50 year old "low end" saws were a lot less "low end" in construction) - a 21st century low-end saw was usually built with "planned obsolescence" from the design phase forward.
Provide the exact model of your saw to check some details.
The one thing (if that's been left serviceable on this model) it might be worth trying is to replace the brushes if they are excessively worn. Beyond that, replacing the saw is far more likely to be cost-effective.
This could be a bearing problem where the main bearings have dried out and are starting to seize up.
If the saw has seen lots and lots of use over its life time it is possible that the motor brushes are failing and need replacement. (Note that this would primarily apply to an AC powered tool).
If the saw is a battery powered unit the problem may be that the battery pack is so far gone that it has built up an internal resistance that limits the ability to deliver the needed current to the saw motor.