HP27 Restoration (Lots of Corrosion)

[calc-calcs]

OK both chips are out, and as anticipated, there was indeed LOTS of corrosion on the traces bellow them. Much more than was initially visible. I wonder what effect this corrosion had on the chips when the seller tried to test the calculator with the charger. Still hoping for a miracle and the ACT might still be good (saved by corrosion?).

And now the clean-up process will be next. After that will have to verify continuity of ALL traces in that area, and also the vias on all the IC pins and contact fingers. This will easily be so far my most challenging calculator restoration for sure. But given the model at hand, well worth the effort.

To be able to properly heat up the solder joints, washed down the IC pins in the the affected area before removing the chips. First used some alcohol and Qtips, then hand soap and a soft toothbrush and gently brushed some of the stuff away and let the board dry for an hour. Some of the harder spots just used a Xacto knife to very carefully flick/scrape them off and then another wash with soap. I tried to be careful not to touch the top of the ICs, unfortunately it seems that during the process a bit of the labels came off just by getting them wet. But at least I have them documented in previous pictures, not that the labels where completely intact to begin with. Very fragile ink after all these years, or perhaps the weakening was also related to exposure to the corrosion chemicals.

I'm thinking to remove as well the two transistors of the power supply and the yellow and the small black tantalum cap with the green beads to be able to clean under both of them. Might even need to remove most components of the power supply as well to properly reach all the traces and remove all the CuSO4. At least its way less corrosion in those areas, mostly just some spots here and there, but not all of it is directly accessible.

Given the state of the traces at the ACT/ROM, does anybody have the schematic of the HP27, or perhaps the PCB layout? Probably a long shot, but that would be a big help. I assume the PCB layout might be similar to the HP25? or at the very least it might be a good general guide. As said before, I anticipate quite a few traces will be gone or close to that and will need to be repaired after I'm though cleaning off all the blue stuff, which really takes a bit of watchmaker precision and a good magnifier to get to the very last bits as they can really be almost microscopic. No point leaving behind any of it.

Going to get some rest and continue with this tomorrow.