Post by Alex Liggett on Feb 10, 2024 20:27:10 GMT
Following on from the HP-11C a year or two back, a mate down the pub recently sold me his 'buggered' HP-42S (I did offer to repair it but he prefers using his phone, the weirdo). The symptoms were that the keys (especially on the right hand side) either required a very hard press or were completely unresponsive. This is usually symptomatic of a known issue relating to the connection between the keyboard and the main PCB, verified by applying gentle pressure to the face of the calculator beneath the screen. Sure enough in this case the keys worked fine with said pressure applied.
So with my purchase safely home, and with sober and steady hands it was time for surgery. The HP-42S is not designed to be opened. The case is held together by eight plastic 'heat stakes'. Four of these are inside the battery compartment:
These are carefully (ie using a 4mm drill bit between fingers, no power tools required) drilled so that the heads are just removed. The other four are at the bottom of the calculator under the keyboard bezel. Removing the bezel in a non-destructive way can be done (see the HP-11C thread) but not without a great deal of difficulty. Reading a few online guides and watching a few videos suggested that it is possible to separate the lower stakes without collateral damage, by simply forcing the two halves of the case apart (working from left to right to separate the four stakes in turn), and this turns out to be true, although the process is real heart-in-mouth stuff, with a lot of care required not to bend the PCB or keyboard frame. The machine separates into its two halves with the lower stakes arrowed on the left:
The PCB is attached to the keyboard by 6 twisted metal clips around the display (arrowed on the right in the pic above). These are gently straightened out (noting which way they were bent and taking care to avoid damaging the tracks on the PCB) , then the main PCB with its beautiful gold tracks can be lifted off:
This reveals the keyboard connector. This is a flexible plastic ribbon connector:
Which is pressed against the main PCB by a piece of foam rubber - the villain of the piece. The fault is caused by this rubber gradually compressing to the point where it no longer provides adequate pressure. The ribbon can be gently lifted out of the way:
I removed the original foam and substituted a thin strip of double-sided adhesive foam tape for the foam (some folks have used rubber bands, it just needs to be about 1.5mm thick). Reassembly is simply lowering on the PCB onto its clips and twisting them to their original position. Time for a quick keyboard test:
Success. The plastic shell then clips back together, and while it isn't possible to re-stake the heat stakes, I found that a couple of countersunk #2 brass woodscrews acquired from my 'other' project sufficed to hold everything together nicely:
The 42S is a bit modern for my tastes, but one of my colleagues had frequently lamented losing hers in the field way back, and she was happy to buy it off me for the acquisition price. These machines go for around $350 on EBay so if you have a 'dead' one in a drawer, please don't throw it away!
So with my purchase safely home, and with sober and steady hands it was time for surgery. The HP-42S is not designed to be opened. The case is held together by eight plastic 'heat stakes'. Four of these are inside the battery compartment:
These are carefully (ie using a 4mm drill bit between fingers, no power tools required) drilled so that the heads are just removed. The other four are at the bottom of the calculator under the keyboard bezel. Removing the bezel in a non-destructive way can be done (see the HP-11C thread) but not without a great deal of difficulty. Reading a few online guides and watching a few videos suggested that it is possible to separate the lower stakes without collateral damage, by simply forcing the two halves of the case apart (working from left to right to separate the four stakes in turn), and this turns out to be true, although the process is real heart-in-mouth stuff, with a lot of care required not to bend the PCB or keyboard frame. The machine separates into its two halves with the lower stakes arrowed on the left:
The PCB is attached to the keyboard by 6 twisted metal clips around the display (arrowed on the right in the pic above). These are gently straightened out (noting which way they were bent and taking care to avoid damaging the tracks on the PCB) , then the main PCB with its beautiful gold tracks can be lifted off:
This reveals the keyboard connector. This is a flexible plastic ribbon connector:
Which is pressed against the main PCB by a piece of foam rubber - the villain of the piece. The fault is caused by this rubber gradually compressing to the point where it no longer provides adequate pressure. The ribbon can be gently lifted out of the way:
I removed the original foam and substituted a thin strip of double-sided adhesive foam tape for the foam (some folks have used rubber bands, it just needs to be about 1.5mm thick). Reassembly is simply lowering on the PCB onto its clips and twisting them to their original position. Time for a quick keyboard test:
Success. The plastic shell then clips back together, and while it isn't possible to re-stake the heat stakes, I found that a couple of countersunk #2 brass woodscrews acquired from my 'other' project sufficed to hold everything together nicely:
The 42S is a bit modern for my tastes, but one of my colleagues had frequently lamented losing hers in the field way back, and she was happy to buy it off me for the acquisition price. These machines go for around $350 on EBay so if you have a 'dead' one in a drawer, please don't throw it away!
