How to reassemble a HP15C ?

[Nicolas]

I looked at all the posts but no answer for this question.

I have a HP-15C. I disassembled it for repair but how to get the electronic card back in and fix it (keyboard problem).

If anyone could help me, i'll appreciate.

Thanks a lot.

Nicolas

PS : sorry for the mistakes, message from France ;)

[Ellis Easley]

I have disassembled two Voyagers: a 12C and a 15C, both with the single PCB. I cut off the tops of the heat-staked posts in both cases to remove the PCB. On the 12C, I built up a pad with layers of duct tape to fill the space between the case back and the PCB. This works but gives the keyboard a spongy feel. On the 15C, I glued the PCB in place with some superglue (cyanoacrylate) that had become thick with age. I applied a small drop to each post and pressed the PCB with my fingers for a minute or more till the glue set, one post at a time. The thicker kind of superglue that is sold that way (non-running) might work just as well - I feel it is important not to use the regular thin glue because it could run down and interfere with the keyboard contacts.

(I think it was) Randy Sloyer suggested using a hot knife to soften the mushroom shaped caps of the heat staked posts and then form the softened plastic back into a cylinder, freeing the PCB and allowing the posts to be re-heat staked to reassemble the machine. He suggested using a wood burning tool with a variable auto-transformer to control the temperature of the tip. He said it is important to have the temperature just high enough to soften the plastic, if the temperature is much higher the plastic is burned and cannot be re-used.

Here's an idea that just occurred to me: I think one reason why a space filler gives a spongy feel (other than the softness of my duct tape!) is that the pressure on the filler is transmitted through to the back of the calculator case, which is free to bow out in the middle. If instead of the pad there was a thinner sheet of very stiff material (with a layer of insulation between it and the PCB if necessary) and then an "X" shaped sheet metal spring pressing from the corners of the case back to the middle region of the stiff material, the force would be transmitted to the corners, where the screws are. Or maybe four small, stiff coil springs in the corners. Or how about a piece cast from acrylic resin to exactly fill the space - would that be stiff enough to do the job without springs?

A combination of these ideas would be to cut a sheet of stiff metal to fit snugly inside the case back and then glue onto it a number of little blocks of hard rubber (not foam) to push against the PCB. As I recall, the space that needs to be filled is not of a uniform thicknes - I remember using strips of tape in different numbers of layers for the front and the back - unless I'm just thinking of making clearance for the IC's. But whether uniform or not, each rubber block would be the right thickness for it's space. A sheet of aluminum 0.050" or 0.0625" (1/16") thick would be stiffer than the case back without adding much weight - certain alloys or annealing might provide more stiffness.

This approach ignores the bowing out of the case front (full of holes!) as a contributor to the the sponginess, or just looseness, of the keys. I like Randy's idea but it is too late for my calculators. Also, it would be nice if there was an effective way to reassemble the unit that could be undone and re-used easily. After I glued my 15C, I found the machine had a tendency to give a double entry on the "4" key. I think this is because I didn't completely dry out the keyboard swith sandwich (the calculator had to be disassembled to clean PCB corrosion from battery leakage, which was causing it to go through a set of batteries in about a week). The multiple keyboard enry has gotten better over time.

[Vieira, Luiz C. (Brazil)]

Hi, Ellis;

just to add one of my own concerns about reassembling the Voyagers.

Every solution that would use the back half of the calculator's case as a "shield" to hold any sort of material that will press the main PCB against the keys leads me to wonder about this fact: the four screws that hold the back of the calculators and their plastic threads were not design to retain that much pressure. I am afraid they will be loose with the time, for as many times as they are unscrewed to repair the calculator.

Is it too much concern or not? When I look at the HP41's screws and I realize they were design to hold not only the back case but to keep flex circuits in contact with each other, I see the difference: they are bigger and deeper.

Any comments?

Best regards

[Nicolas]

Thanks a lot to you both.

I already tried to fill empty space on the back of the PCB but not with one single aluminium plate, what is not a bad idea. I just have to find or build it :/

I think I will try to glue it, another idea I had a long time ago. I didn't do it 'cause I was not sure it could work. I think also I'll have one chance and only one to success (cyanoacrylate doesn't excuse any fault).

I thought also to find very little screws, like watch ones, and try to screw the PCB. But without success, all I found were too big.

I'll keep you informed of my reassemble operation I called 'what do you want to glue today' :)

Thanks again.

Nicolas

[Ellis Easley]

Luiz,
I agree, the screws won't stand up to a lot of force. Any spring or hard rubber spacers would need to be just the right size so they are just barely compressed when the machine is screwed together. Then together they would provide a solid backup when keys are pressed, and the force of the key presses would be distributed among the four screws.

I want to try Randy's idea next time I need to open up a Voyager.