HP27 Restoration (Lots of Corrosion)

[calc-calcs]

Just got this HP27 Woodstock off ebay. From the pictures on the listing there was obvious corrosion present on the battery contacts which would probably (almost certainly) mean there would also be internal corrosion. Typical calculator with its battery pack left installed, give or take 30-40 years ago until someone found it and put it on ebay. Oh and on top of it all the original charger was also (unfortunately!) in the drawer, so the seller must have decided why not just try to make it work... and probably roasted one or more ICs. It was listed as having an intermittent display. Hmmm, were did I hear that before...

After having restored two HP25 (one 25 needed a Panamatik ACT) and three HP21 calculators, last two mostly purchased for parts but ended up getting them all to work, and having dealt several times with moderate internal corrosion I felt confident (or hoped) I could also handle this one without too much work. And maybe, just maybe the ACT might still be alive. Otherwise there is always Panamatik to the rescue.

The calculator really looks quite nice on the outside, no scratches or blemishes, label is fine, and all keys still have a good feel. But after opening it up I realized this one is bad inside. Really BAD. It will need a lot of cleaning, and might even need to rebuild some traces as a few have really been eaten away by the blue stuff. At least the corrosion seems to be only very heavy in the ACT and ROM chips area. There is also slight corrosion around the power supply components, and around the charge limiting resistor on the opposite side. At least the display and driver chips part of the board seems to be clean. Fortunately the keypad traces are mostly (golden) clean, and only a few will need attention, hope that the contacts of the switches are ok. If not there is always a drop of Deoxit that might help.

Oh well, lots of work to do. First will be removing both the ACT and RAM chip (or is it a ROM chip?) from the board. Experience has taught me that corrosion likes to hide under those chips as well. Going to clean around the pins before attempting to desolder them.

Will be updating this thread as I move forward. Any comments or suggestions welcomed.