HP27 Restoration (Lots of Corrosion)

[teenix]

(04-07-2022 03:30 PM)calc-calcs Wrote:  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.

It maybe the interconnect pins that supply power to the CPU board from the keyboard via the power switch. Maybe corroded pins, PCB mating holes, and or poor tensile contact from the pin.

This rapid power on/off cycling may be upsetting the ACT and/or ROM0 chip (5061-0430) causing them to stop working. This can be damaging to the LED segments, and maybe the drivers, as they will be getting continuous high currents flowing through them which is the brightness that you see. This high current normally flows but only for around 300uS through each segment during the display refresh process.

If you have a soldering iron, you could try re-soldering the IC connections. They can be corroded also, resulting in dry joints and poor connection. You have to be careful as the new solder might not take and you will be tempted to keep applying the iron causing overheat damage.

It is better to unsolder them and then clean things up and re-solder.

As you mention the RAM chip seems to be inoperative. (5061-0459) If so, then only a replacement will fix things.

These chips are sensitive to static electricity damage so proper handling of them and the circuit board should be observed.

cheers

Tony