HP27 Restoration (Lots of Corrosion)
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05-16-2022, 05:52 PM
(This post was last modified: 05-19-2022 03:06 PM by calc-calcs.)
Post: #30
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RE: HP27 Restoration (Lots of Corrosion)
Keyboard problems.
I have been using the calculator quite a bit to put it through its paces with the manual in hand, and started noticing that the [2] key was sometimes behaving weirdly. Most times it would put a 2 in the display, but sometimes it would be a 3! There was clearly still something amiss so I disassembled the calculator and took another look at this key. By putting the keyboard side onto a strong light source I could see there are some blue-green areas inside the keyboard area shimmering though a bit but that are completely inaccessible. Unless the keyboard is taken apart, which I understand is something that needs to be avoided unless there is really no other alternative, as the end results of the reassembly are not always optimum. So instead decided to put some Deoxit red into the small hole above the key. I had done this on one of my HP25 that had quite a few bouncy or non fully responsive keys with excellent results which brought the whole keypad back to like "new" operation. So while I was doing this for the apparently faulty [2] key in the HP27, why not put a bit of Deoxit into every key as a preventive/proactive manner? Well what happened next told me that this was NOT a good idea. The whole keyboard became inoperative. I confirmed this by swapping in the keyboard from my test HP21, and sure enough it was working fine. Swap back the HP27 keyboard and nothing, not even a single key. Furthermore I found that the resistance between most rows and columns was way bellow 1M ohm, sometimes just 200-300K. The Deoxit must have reacted with the corrosion and had created conductive paths that completely obliterated the keyboard scanning lines. As a reference, on the untouched HP21 keyboard there are around 3-5 mega ohms of resistance between most columns and rows, although one or two are actually just above 1 mega ohm. So now what, disassemble and clean the mess? Before the "nuclear" option I decided to give it a try by connecting the ohm meter between one row and column to monitor the resistance of one key, and then putting a bit of IPA into the hole of the key and then twist a small piece of fine multi-strand wire in there to see if it would clear up the problem. But it did not, zero progress. So now one step closer to having to take the whole thing apart. Ugh. One last try, use a can of compressed air and blow some into each key hole, what the heck. And that did the trick. Now the keyboard is working again, and even a few tiny bits of blue stuff came flying out the side of the keyboard but most is still in there. So I guess time will tell if this will hold or a keyboard disassemble will still be needed if the problem returns, but I am aware this keyboard has very likely a finite lifetime before it needs to go into major surgery. Calculator Battery Contacts. One of the battery contacts on the board is really bad (- side as usual). I did of course fully clean off all the corrosion but that did not help too much. While testing the calculator with another battery pack, it was still not making a reliable contact and often the display would flash some random digits, or just turn off and on midway into calculations, etc. Sometimes the flash was rather bright, so needed to take care of this before I damage any LED segments or even worse one of the driver chips. There is no way to restore the round metal head of the negative contact, it was too corroded and completely lost its plating. So for now as a stop gap solution soldered a bare wire (leftover from a resistor tail) on one side of the contact where it attaches to the board, bent it over and in front of the metal head of the contact and soldered it onto the other side of the board as well. This should provide a much better new mating surface with the battery. Did the same for the other contact as well, so both would be even. The battery is now a bit tighter to insert, but the springs inside the battery pack had enough travel to take up the additional required clearance, and actually helped as contacts are now being held much tighter. In any event battery contact problems now seem to be a thing of the past. Probably not the best long term solution, but for now it seems to work quite well. Battery pack restoration. Well finally managed to open up the very stubborn battery pack. Had to brute force it a bit and paid a price with an unwanted crack, but there was simply no other way. I ended up having to fully cut through all the seams where if was possible but even so there are some inaccessible areas. The side of the contact spring that goes towards the negative battery side was a bit tarnished, but otherwise ok, so I just cleaned it up with the Dremel and a small rotating brass metal brush until it was shiny again. Two new Eneloop cells and temporarily close the pack up with a couple of strips of magic tape on the sides. So far it seems to be working just fine, no contact issues with the springs so far. Return some of the gold inside. Lastly, I decided to reinstall the RAM/ROM chip, which is no good outside the calculator anyhow. And out of curiosity also wanted to test it again, and check its checksum. The Panamatik ACT easily allows for this, and to switch between using its internal ROM or the original code on the external ROM chip. Turns out that with the external ROM all 6 trigonometric functions still stop working just as they did at some point when the original ACT was still operable. Also PI just returns zero as it also did before. Needless to say the checksum between both ROMs are wildly different. Out of curiosity I could also have gone over each line of code and directly compare between both ROMs line by line by switching back and forth to find the defective blocks. But given that there are just too many lines of code, it might end up being an unnecessary heavy burned on the two keys used to scroll up and down. And in the end there is nothing further I could have done to correct the problem anyway. So far so good. |
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