I have acquired some Don Lancaster designs that use RTL logic and I am looking for spare parts. The MC789 hex inverter is used in all of them and I have 2 chips. Does anyone have a reasonable source for the MC789, I can get all I want for $20 a chip. I also have 2 MC799 buffers that are use in the Microlab. I have found two each of these (MC725, MC778, MC785, MC792, MC853, MC855) that I will part with. SWTPC Digital Logic Microlab http://www.swtpc.com/mholley/PopularElectronics/Apr1970/PE_Apr1970.htm Early SWTPC keyboard http://www.swtpc.com/mholley/PopularElectronics/Apr1974/PE_Apr1974.htm I think that Don Lancaster's TIC TAC TOE machine from 1971 is a very clever design. It uses 5 ICs and 60 diodes to play the game. I may build one. http://www.swtpc.com/mholley/RadioElectronics/Dec1971/RE_Dec1971.htm Michael Holley www.swtpc.com/mholley