My Macintosh Classic II displays something like a chessboard (just black and white squares) and it stays there, after sometime the squares turn into bars, then sometimes it starts up sometimes not. any help?

(This happens inmediatly after I turn it on, it is not a software problem)

Try a new Motherboard battery as this stores the PRAM settings for the MACintosh. It uses a 1/3 AA size 3.6 Volt type and is easiest found from local Radio Spares suppliers.

DO BE VERY CAREFUL OF "STATIC" .

Ground yourself using an antistatic strap and be carefull to replace it with the correct polarity (the tip is the + terminal.

I've changed the old battery with a new one, and found that there is some problem with the motherboard it starts up or shows the squares randomly, maybe something is not making a good contact internaly (in some "floor" of the motherboard).. :C( and i think that's imposible to fix.

Thanks anyway.

This is something that happens with classic 2's, unfortunately. Often dust related shorts can cause it , among other things.
The best thing you can do is clean out all the dust with a (cool temperature) blowdrier etc. (carefully,more to avoid deadly shocks from the picture tube than static...), and ,if they look real corroded,clean the contacts of the ram sticks by inserting the ram and removing them several times (this tends to scrape the corrosion off...).
lastly, you can always wait..often after a few hours the classic will just start up on it's own (but you might need to turn it off and on quickly first)...This is what usually worked with me..after a few hours of the checkerboard or dark lines, mine would start up,(sometimes requiring the previously mentioned cold reboots though)..I eventually just left it on all the time...despite being on for up to two months at a time, it never got hot or complained otherwise..but it's not a pram problem, though a good battery never hurts either.

Thanks for your help, i'm going to clean it up and give this old mac another chance. by the way I have another Classic2 (my brothers one) that runs well, but for some reason the internal SCSI bus doesn't work anymore, i thought it was the the disk but i connected it externaly and it works, how is this possible? it seems that the external port it's part of the internal one, so if the external works the internal should work too.
Thanks for any further help.