You disconnected the GI connectors to make sure it is not the board itself blowing the fuses? After that, look closely at all the sockets in the bad string looking for bent contacts, solder globs, etc. Next remove all the bulbs in that string to increase the resistance between the +/-. Measure across a socket with a meter to see if it is a short. If it is a short, inspect all the sockets again and find a socket half way down the string and desolder the wires from it. At that point you will have the string isolated into two halves of 6 or 7 sockets each. Measure resistance across a socket in either half. Hopefully one half the sockets will be good at this point and you can now concentrate on the other half. Divide the string in half again and again and measure with the meter each time until you isolate the bad socket.