I have a contractor repairing my 3 ton a/c who I have known for a long time. After replacing several parts over the last few days, we are running out of options. I do not want to reflect badly on my friend so I will release few details. No small debris in lines. Low pressure switch cuts off compressor about 10 seconds after startup. Low pressure guage goes down to about 20 psi. We believe that there is a solid blockage near the compressor. Can solder droplets come loose from a poor soldering job and block the condenser coil? Any other ideas or comments could help. Thanks.