EL-804 reverse engineering

Documents: Schematic

     I love VFD displays especially when they are coupled to old calculator tech so I bought an EL-804 calculator from EBay and reverse engineered the schematic, stripped the original board and built all the parts on to a new board with an integrated keyboard.  The only thing that did not go on the new board was the power transformer which is now connected via a 6 pin cable.  This has a smaller footprint that the original even with the integrated keyboard as it uses a modern 2-Layer board vs the original's single layer board with jumpers.

     This board was made to take all the ICs, displays, diodes, resistor networks and the pass transistor from the old board.  All of the discrete resistors and capacitors are smaller through hole or 1206 SMT parts.  The display footprints are the same as the original so they will directly drop in.

     The schematic differs somewhat from what I could find online but it accurately reflects the calculator I bought.  There is a good reference for this calculator at the Old Calculator Museum.