HP 17b Series Calculator Repair
DESCRIPTION
OBLIGATORY WARNING - I AM NOT RESPONSIBLE FOR DAMAGE TO YOURSELF OR YOUR DEVICE. THESE INSTRUCTIONS ARE AT YOUR OWN RISK.
I have had 3 calculators in my career of the same series. One HP 17b and two HP 17bII's. All three have failed in the same way. The buttons on the right side of the calculator fail to function after a few years. All of the other buttons work. I decided I wasn't going to buy another, that I wanted to see if I could fix the calculators. I had kept two of the calculators, having thrown away one. I figured they were broke when I started, so I couldn't do much harm!
SYMPTOM - some or all of the buttons on the calculator do not respond. If you "pinch" the front and back of the calculator together underneath the display and the malfunctioning buttons start to work, the you likely have the same problem I did.
I did not have to figure out the basics of dis-assembly on my own. Thanks to Paul J Brogger and is explanation on the Pioneer series on the HP Museum site. Check it out here.
This same approach should work with many different keypad devices using a ribbon connector. Functionally, all you have to do is open the case, pad the ribbon connector and reassemble. So, here goes for this one.
Remove the battery cover. You can then see four holes where the "heat stakes" are. These are a plastic posts protruding from the back side of the front half of the case and can be seen in subsequent photos. After assembling the two halves of the case at the factory, the heat stakes are peened or spread out via heat, I assume, to hold the case together.
The top four heat stakes are visible under the battery cover. There are several more between the bottom two rows of the case. The bottom heat stakes protrude from the back half of the case and could only be drilled out by removing the face plate of the calculator. It would be difficult to remove the face plate without damage.
Separating the case is the hardest part of this job. I used a #28 drill bit, because that is what I had handy. You need a drill bit that is just smaller than the hole the heat stake is in. Per Mr Brogger's suggestion, I would not use a drill. In fact, the plastic is plenty soft and can be drilled out easily. I just held the bit with a paper napkin and had no problem drilling the holes. This works for the top four holes.
As the bottom heat stakes are not easily drilled, I just pried the two halves apart. The top separates easily after drilling the holes. The bottom takes a fair amount of force, pulling the halves at different angles. I would not use a screwdriver as the circuit board could easily be damaged.
The number of heat stakes on the bottom differs by model of calculator. There may be four to six on the bottom.
This photo shows the metal tabs that hold the circuit board to the front half of the case. In both models I have repaired, the top and bottom rows of tabs were twisted opposite directions. So, be cautious about which direction to twist the the tabs to line them up with the slots in the circuit board so as not to twist them off.
These tabs are what maintain the pressure of the pcb against the ribbon connector to the keyboard and the conductive display connector.
If the circuit board is not coming off easily, gently pry the problem tabs back and forth while applying upward pressure on the board.
After the circuit board is removed, the tabs need to be straitened. For this project I have used flat head pliers. The keypad conductive strip can be seen just below the back of the display and is white with black strips on it.
While straitening the tabs, I used the pliers to squeeze the bottom heat stakes so that the case will snap back together.
Underneath the keypad conductive strip is a foam insert that presses the strip against the contacts on the circuit board. Rather than look for a new piece of foam with the right density and try to cut it the right size, I thought I would try just adding a little thickness. I ended up cutting a strip from a disposable cup, which proved just the thing to do.
Cut the plastic strip about the same width and length as the foam and insert the strip between the conductive piece and the foam. I did not have to retain the strip. It held itself in place. This added enough extra pressure to keep the contact between the pad and the circuit board.
I cleaned both the keypad and display contacts with a swap and alcohol. Also clean the circuit board pads where the display and keypad contacts mate. I used just plain isopropyl alcohol.
After several attempts with no luck, I found that I had to press the circuit board down with one hand while twisting the tabs a quarter turn.
Replace the batteries and test the keypad before reassembling the case. If the buttons or display do not work, then there is likely not enough pressure from the tabs.
I was able to twist the tabs at least 5 times and didn't have any fail and break off.
In all three of mine 17b's, the function keys down the right side failed.
Success! The buttons work again. The second calculator went much faster. I hope your calculator can come to life again.