overhead projectors

Q) how to use a lamp to test a cct which has blown a fuse.

I dont really understand this question. can anyone help me.

I don't really understand this thread!! Please explain your question in better detail. First off, by "overhead projector" do you mean the transparency projector type whereby a sheet of clear acetate is used on a glass platten/Fresnel lense? Or are you referring to a data/video projector that uses LDC or DLP technology? I gather that you are trying to substitute a lamp for a blown fuse? If so, typically a 60w incandescent lamp is temporarily wired in place of the fuse.

A fuse blows due to over-current. If you bypass the blown fuse with a light bulb but the over-current problem isn't fixed then it might cause more damage.

I think he means that when the fuse had blown to fit a new fuse of the same amperage and put a lamp in series with the supply.
This is actually a good idea because if the fault is still there the lamp will absorb most of the the surge and the damage to the cct board may remain minimal.
Start with a 60 Watt lamp first.
May have to use a higher wattage depending upon the load from the cct under test.
If it lights up to full brightness there is a dead short in the load behind it.