Electrical – Black & Decker KS850 Circular Saw pinning sporadically

Here's a first-stab at a troubleshooting guide:

With the unit unplugged and/or the battery removed:

1. Visually inspect the tool...

   1. Is anything potentially jamming a mechanism?
   2. Are the air vents clogged?
   3. Has the unit been exposed to too much dust, ash, water and/or fine sawdust?
   4. Is the case discolored or cracked at any spot?
   5. On the power cord, is the insulation cracked at any point or is one or more strain-reliefs damaged or missing (especially where the cord enters the tool)?
   6. Are the plug contacts reasonably straight and untarnished?
   7. Is a fuse or reset button present? Is it blown or popped out?
   8. Was the tool used until it might have become too hot? Many tools have a safety, over-temperature shutoff; wait an hour for it to reset.
   9. Is the safety switch/lock, if present, installed and/or on?
   10. If the unit came with a grounded plug, be sure that it's undamaged and that the power plug really is a properly wired grounded outlet. Use one of those circuit checkers you can get for $2.
   11. Has an oily, greasy, or varnishy fluid dripped or oozed out of any part of of the unit? Grease could indicate bearing failure. Other oozings would point to catastrophic motor damage, or sometimes electronics failure or starting capacitor failure.

2. Does the tool rotate/reciprocate relatively normally when moved by hand?

3. Does the the power switch feel normal? Usually a busted switch will feel "off" (Pardon the pun.)
4. If you are comfortable with an ohmmeter, you can use it to do basic checks on the cord and switch. Note that it is possible for a circuit to look okay with the meter and still be bad under load -- or for control electronics to be bad.

With the unit powered:

1. Does the motor hum or smoke when turned on? (Of course, Immediately switch it back off if it does this.)
2. Does a circuit breaker trip?
3. Do you feel an electric shock?
4. Does the motor arc excessively?
5. Any unusual noises? Humming, or squealing?

If the visual inspection is failed, it should be obvious what to do.

1. For a damaged cord, replace it. Do not attempt to repair it.
2. If the case is plastic and is discolored anywhere, it has likely degraded to the point that the tool is either unsafe or it's just not worth repairing. Some uniform case discoloration due to age and UV exposure can be okay -- as long as the case is not also cracked or brittle.

If the switch feels funny and/or nothing happens when it is turned on, then it could well be a busted switch -- which is pretty common. You can sometimes get replacement switches at a reasonable price.

If the motor arcs excessively (a little visible arcing is normal), the brushes may need replacing. Do not run the tool if you suspect brushes, you can permanently damage the motor.

If the motor hums or smokes, it's typically one of these problems:

1. Something is mechanically jammed.
2. Bad switch.
3. Bad power cord.
4. Bad control electronics, if any.
5. Bad, broken, or burnt internal wiring.
6. Bad brushes or damaged commutator plates.
7. Bad starting capacitor (if used).
8. Bad starting coil.
9. Improper outlet voltage and/or grounding.

I had this same situation. I made a (written) list of issues (with pictures) and gave it to the landlord. When it became clear that the landlord would not fix them, I called an inspector. It turned out that there was a completely different department overseeing residential safety inspection vs. building code inspection, but after getting that sorted, they gave the home a free inspection and gave the landlord a fine and deadline to fix the issues. The landlord was understandably upset and asked us to leave, and we happily signed the lease-termination. I maybe should have told the landlord that I was going to refer the issues to the city by such-and-such a date and this may have avoided involving the city and angering the landlord. We probably would have been let out of lease under better terms, but there would have been no incentive for the landlord to fix anything and the cycle would have begun again with the next tenants.

I recommend you give the landlord every opportunity to fix your concerns, and seek a mutual lease-termination agreement if/when this fails. Pay your rent! If you don't and this goes to court, then It'll just end up looking like the you wanted out all along and used the safety issues as an excuse. If the inspector condemns the place, and you are forced to move, then you can stop paying your rent.