Thanks for your quick response bryce!
So what you suggest is replace the 6845 (which i already ordered anyway) and if i still have problem, then go for ram right?
To tell you the truth,i already made the "classic" temperature "finger-sensing"

and i found out that all ram chips seem to heat the same (and that's why i ordered first the 6845...).
I had a small experience in the past, by discovering a faulty ram chip from an amiga 500, using this method, one of the 4 ram chips was heated more than the others 3,i changed it, and amiga was back to life again!
As for the desoldering gun, i dont know if you noticed, but this is not the usual cheap desoldering pump. It is actually an electric iron with a hollow tip in the end (i didn't even know that these kind of special iron exist!), which gives you the unique ability to "wrap" the soldered pin,heated very fast and quick, and as soon as solder becomes liquid, you press the button and all solder is sucked away!This way you minimize the time of heating and also you will get a very "clean" hole! It's like having both soldering iron and a pump in one tool!
At least this is how it should work, "in theory"...