HP-33E Missing Pin on Memory Chip

[brouhaha]

The older thread suggested "dummy" chips. None of them are dummies. There were two generations of NMOS ROM/RAM chips used in 3xE series, 1MA1 and 1MA3, which had differing capacities. In some cases the two were mixed, so every 3xE model had at least two different combinations of chips, often with different numbers of chips. I believe the 1MA1 has 1536x10 ROM and 4x56 RAM, and the 1MA3 has 1024x10 ROM and 16x56 RAM, but I have not yet verified these details. Some 3xE calculators in some or all configurations contain more memory than actually used, but none contain entirely superfluous chips. Unused memory might be disabled by mask options, but more likely it is just programmed to all-zeroes (ROM) or present but never accessed (RAM)

The 00031-80008 chip appears to be a RAM-only part. The 00031-80012 may be RAM-only at a different address. These are not exclusive to the 31E, and in fact the 00031-80012 should never be in a 31E.

The missing pin still seems quite mysterious. I believe that the fully working 3xE units with NMOS chips missing a pin are inadvertently powering themselves using phantom power from other pins. This phenomenon is commonly seen in CMOS, but in this case the NMOS chips were designed for low power consumption and high ESD protection, so they may do it also.

The CMOS RAM/ROM chip, of which exactly one is used in each 3xC model, is the 1LB9-nnnn. The 1LB9 is used in conjunction with NMOS parts.