Are you confused because you have two HP-25Cs and one contains a single RAM chip whereas the other one has two? Apparently, earlier models had two RAM chips (HP part number 1820-1843), whereas later models used a double-capacity chip (1820-1886). If you look closely, you'll notice that the two circuit boards are otherwise identical, or nearly so (there's a diode that may appear in two different places, but it is just a matter of layout, the circuit remains the same) except for a 100 kOhm resistor connecting pins 8 and 14 of the double-size memory chip. So you can use one double size memory chip in place of the two single-size ones, but you need to add this resistor to ensure that the chip's contents are properly preserved when the machine is turned off. Haven't got any 1820-1886 chips? No problem... as it turns out, another HP part number, 5061-0469, the RAM chip used in HP-19C and HP-29C calculators, is a pin-compatible replacement. But, you still need that 100 kOhm resistor.

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