HP 41CX won't turn on without banging it.



#2

Just bought a replacement for my CX that sprouted legs. I got one on ebay, and it is in great shape. Except for this little problem. It will not turn on after it has sat in the off position for 5 or 10 minutes. The only way I can get it to turn on is to drop the display end 1/4 to 1/2 inch onto a hard surface. Then it turns on fine and the weird thing is that you can turn it off and on as many times as you like, as long as you don't let it sit for a while. Then you have to drop it again. I have not opened it up yet as it is useable, and I wanted to get some second or more opinions about the possible cause. Everything else seems to work as new.

TIA for the help.


#3

I would recommend that you open up the calculator and have a very close look at connections between boards.

Is the calculator case "loose" at all? This may be an indication that the calculator was dropped in the past. This can break the posts inside tha calculator that hold the screws that hold the two halves together. All sorts of weird things happen when the case gets loose (pressure holds things in place inside).


#4

!

#5

Thanks for responding so quickly. There is not any sign of damage on the case and it does not feel loose at all. Squeezing the case does not help. I think I go ahead and open it up and have a look at the connections. Anything in particular to look for other than cracked posts and bad solder joints?


Thanks


#6

What I've done succesfully is to tae out the connecting stuff (a long and thin parallepipede, there are 2 of these) and enlarge it with a very thin screwdriver. This ensures a better connection.


#7

I opened it up last night and took a look. Sure enough the posts are cracked as one and one half sides of the plastic holder that positions the board connectors. The screws were very tight, especially the bottom two. I cleaned everything with alcohol and reassembled it. I think the problem is in the upper connection where the battery power comes in. It worked perfect for a little while, but it is now back to its old trick. Squeezing the battery holder just a little and then pressing the on switch will cause it to come on.

I plan to fix the posts this weekend, but are there any tricks for the flexible battery connector where it touches the keyboard pcb? There is no sign of corrosion on any of the contacts. I thought about sliding a thin strip of plastic under the foam that pressurizes the connection. Maybe the foam has relaxed over time and a little more pressure is all I need.


#8

The foam idea may help ( I did with a strip of flat cable, 2 insulated conductors). In my case there was corrosion, so I opted in favor of wire wrap wires soldered to some well researched component leads on the CPU board, and connected to the appropriate batteries terminals. (Please do not try this if you are not VERY sure on what you are doing!!)

#9

I'd start by fixing the posts. That may fix the problem.

If you don't fix the posts, the extra pressure you exert by replacing/adding foam will just make the problem worse.

It's better to fix cracked posts than try to reassemble broken posts.


#10

Yep I agree, fix the posts first. The foam usually has more than enough pressure to make good contact, adding more pressure will only increase the stress on the posts.

#11

I'd start here too.
Use a very strong glue, and do put some around the posts to make extra pushing surface, so that you don't concentrate all the pressure on a tiny place.

Then clean all contacts. Do not screw too tight, otherwise you may break the posts again.

The 2 hhalves must be well stucked together, but it's useless to tight them too much, you will only create damage.


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