Check Valve Assembly, Exchange, for the 3900 Oligo Synthesizer Drain. This is the assembly that is screwed into the bottom of the synthesis rotor head. There are 5 drain check valves in each instrument and they are located on the bottom of the synthesis rotor. There is one for each bank and one for the purge well. As this is an exchange item, you must return a complete check valve to us. If you order this exchange part and you do not return your old one to us or the one you return is missing parts, we will charge you for a new one.