|Re: Repairing HP-41 keyboards|
Message #2 Posted by Vieira, Luiz C. (Brazil) on 18 June 2003, 8:15 p.m.,
in response to message #1 by Michael
the worst part is actualy cleaning the contacts. Under each key there's a small metal cap that bends everytime you press a key and it "snaps". This way, the inner top of the cap touches the surface of the keyboard. It happens that the cap's border are already touching one of the poles of the key contact, and when it is forced down (key pressed) it touches the other pole and closes the key contact.
The most common problem is related to dust. If dust gets inside the cap, it will be between the cap and the mid contact. I did not find the time to draw it, but I hope my description helps.
The good news is that there is a small hole exactly aligned with the middle of each cap. If you open the calculator and locate the hole under the key with bad contact you'll have a chance to clean it. I use a few brush bristles soaked with some sort of cleaning liquid (isopropyl alchool) and tied together as a bundle. I carefully insert them inside the (mentioned) hole, then I rotate the bristles and remove them. I repeat this operation about three to four times. If it does not recover the key's contacts, maybe you have another sort of problem.
About keypad compatibility: there are two basic types of HP41 if we consider their inner parts. Look at the image below (MoHPC original image)
The left LCD refers to the first series, called fullnut. Their keypad is fully compatible with each other.
The right LCD reffers to the halfnut series, newer ones. They have everything built in one single board: LCD, chipset, keyboard... Like an all-on-board computer mainboard. In this case, there's a need to remove and resolder parts if you want to use a "sane" keyboard with bad chipset: you need to remove the bad chipset and resolder another. The worst of this all is taht the "chipest" for the HP41CV and the HP41CX are not exactly compatible. They are electrically compatible, but there is only one chip that is identical: the processor itself.
Have you checked which HP41 is (are) yours?
I hope this helps for now. If not, let us know.
Luzi C. Vieira - Brazil
Edited: 18 June 2003, 8:18 p.m.