Modifying the HP42S?

[Ken S]

I recently picked up an HP42S in fairly decent shape except for the LCD which is bleeding a bit at the top. I have already bought an HP17B with a good display (identical bezel) for next to nothing for a donor.

Since I will have to open it up I figured I would do the 32kb ram upgrade, I have found the correct chip and ordered a couple. Now I am wondering if it is worth changing the crystal to a 65.5360KHZ for the double speed while it is open. Are there any downsides to this? Will it drastically reduce battery life?

Are there any other modifications you would recommend? Any one figure out anything interesting to do with the other 28 pin chip mount?

Ken

[Ken S]

Ok, after way too much searching it appears that the double speed crystal would kill the IR printing. Could be got around by having both crystals and switching between them but that would require finding space for both plus a switch and finding somewhere to mount said switch.

I will restrict my modifications to the 32kb ram and replacing the LCD display.

Ken

[mfleming]

(02-24-2018 01:20 AM)Ken S Wrote:  Any one figure out anything interesting to do with the other 28 pin chip mount?

From an earlier discussion I believe the extra mount was for a ROM chip. Its presence was sensed on startup and control transferred to it to provide language customization for the calculator. I believe someone else put a SRAM chip there in hopes of accessing the memory via peek & poke, but I don't recall any followup post about that mod.

~Mark

[Ken S]

Thanks Mark,

I appeciate the input. I want to only open the 42s once if at all possible.

Ken

[Sylvain Cote]

Takayuki HOSODA has a nice step-by-step HP-42S modification page.

[Ken S]

Thanks Sylvain,

The site is a great resource for the HP42s. I had found it earlier. I probably wouldn't have the guts to open this and replace the memory without such clear instructions.

Ken