HP 38C repair


I recently repaired a 38C with a corroded battery connector
by soldering a couple of wires onto the circuit board and attaching them to a battery holder in the battery compartment. The calculator now runs on two 1.2V NiMH rechargeable cells. I now find that sometimes some of the display segments disappear (for instance, the top segment of every digit may be missing), but that I can make the display return to normal by squeezing or twisting the case a bit. Does anyone have any idea what might be causing this? Also, would it be fatal to use 1.5V cells?


Older versions of the HP3xC and HP3xE machines were assembled with very few soldered connections. Instead the ICs and display pins were clamped againsta foam-backed flexible PCB using a plastic frame. And this design suffers from
bad contacts, causing faults ranging from missing segments to total non-operation.
Take off the back cover (ypu've already done this, right), and unclip the logic assembly from the top case (there are 3 plastic clips each side). Don't lose the keys, which are now loose
Then flip the logic assembly over (keyboard on top) and take off the metal clip strips on the 2 long sides. Lift off the keyboard/'PCB'. The display will probably fall off, if not, remove it.
Clean the PCB pads and the display pins with propan-2-ol (isopropyl alcohol) and put it all back together. That should fix the fault.

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