HP27 Restoration (Lots of Corrosion)

[calc-calcs]

The keypad problem of the 3 non functional keys is solved. There was a very small but obvious gap in one of the traces that I had overlooked where corrosion did its thing. Fixing this gap solved the problem.

However still having lots of display flashing issues, pushing or moving the top area of the board where the display and drivers are located seems quite sensitive to trigger these issues. Will have to check around there to see if there is a bad contact somewhere in the mating pins, or even an overlooked faulty trace. I am concerned that sometimes when only one or a few separate digits flash they seem overly bright. Hope this will not burn out segments or digits so just in case I am going to swap the display module with the one in my test HP21 that is already missing about half the A segment of the rightmost digit to lessen stress on the HP27 one which seems to be fully operational.

Also noticed that aside from the storage registers always returning zero (saving anything to any location seems to return Ok), and the E+ count also staying on zero after each key push. Now also noticed that doing a 10 [1/X] operation always returns "Error" instead of the expected 0.10 result. I suspect that RAM memory might be toast, or somehow the chip is not being addressed properly. Perhaps I missed some broken around the connections to U3/U4? One of my HP25 had almost identical problems due to an intermittently working RAM chip. That one ended up with the Panamatik ACT just to solve that problem as it has its own internal RAM and ROM.