Problem with HP 25

[Geoff Quickfall]

On some occasions when the ACT is getting two inputs, for example, two key presses held down (engaged)at the same the display will be strange.

The fact that 4 and 5 key do not respond, combined with the display might, MIGHT indicate a short due to contamination or corrosion within the keyboard or the connections (pins).

Before analyzing anything. Undo the screws at the top. While pushing in on the battery contacts, pivot the calculator bottom shell away from the keyboard shell using the base a the pivot point.

Once apart, carefully separate the PCA and the LED boards from the keyboard PCA.

QUESTION: does your keyboard PCA have tiny through holes above each key? If so, then introduce a tiny amount of IPA (not the beer, isopropyl alcohol) and exercise the 4 and 5 key. If you have DeOxit in your cupboard then that is even better for the keys. Last resort on the keys is to introduce a single strand of copper wire to act as a swizzled stick.

Next, clean the through holes for the pins as well as the pins for both the LED block and PCA using a fiberglass pen or your choice of brush or eraser. Clean with the IPA or DeOxit.


Reassemble. If that does not cure the problem, then it is a logic fault on the board.




The upshot is, clean up the connections and PCA to remove the easy fault of poor connections, a dirty keyboard or contamination shorting or changing resistance between components.




You may have multiple faults so ensure good batteries and clean battery contacts. NEVER, NEVER use the wall wart (adapter) unless you can confirm the batteries are good and the battery to calculator contacts are 100% effective.

Rechargeable batteries may not leak contaminants BUT may outgas. This is not perceivable as visible contamination but has the same affect as liquid leakage.