HP41CV Repair



#11

Finally got brave enough to remove the rubber feet and carefully remove the screws. Theory is that key contacts may need cleaning. I have separated the two halves, and in doing so a strip of plastic the width of the case, but only about 1/8inch wide, dropped out. I can't figure out where it goes!

Also, I'm not sure how to remove the circuit board to clean key contacts, or if that is even necessary. Symptoms were random oddities--sometimes wouldn't turn on, sometimes wouldn't turn off, other times screwy displays.


#12

The little right angle strip of plastic is a dust shield that fits above the LCD display module. It just lies between the top of the module and the case.

The keyboard cannot be removed. It is basically melted into place by those zillions of little heat stakes.

To clean the keys you need to make a tiny wire brush out of three small wire bush bristles chucked into a pin vise or Xactio knife holder. Twist the bristles together and trim the end square with a pair of cutters.

Notice that beneath each key disk is a tiny plated through hole. Place a drop of 91% isopropyl alcohol on the hole of the offending key, dip the brush in some more alcochol and gently "scritch" around in the hole. While holding down the key scritch some more. Be careful not to break a bristle off in the hole. Repeat as needed.


#13

David, thanx much for the info. I will work on that. Did the symptoms I described give you any other ideas??
Tom


#14

Tom,

I had a similar experience with my 41, and it appears the major problem is with the flexible gold connector connecting the CPU board to the main (keyboard) circuit board. Make sure there is enough pressure on this connector, or all kinds of screwy stuff can happen.

The other flexible interconnect (the one to the batteries & external ports) can also be problematic. Bad connections here can cause one or more ports to stop working.

While you have it open, check all the screw posts for cracking, and also check the back cover to ensure there are no cracks where the screws go - this can also lead to insufficient pressure on the contacts.

Despite all I said about sufficient pressure, don't overtighten the screws, as that can lead to cracked posts.

Use 91% isopropyl alcohol or a contact cleaner pen to clean the contacts while you are in there.

One last pointer - if you get some crazy results after you put everything back together, try shorting the battery contacts or leaving the batteries out for several hours, as stray charges can cause problems.

Good Luck.

Ed


#15

ED, DAVID, I really appreciate the guidance. As I look at the open case what I presume is the circuit board (since it has a couple of capacitors thereon) slides over the two bottom screw posts. Is there any wiring beneath this or can it be removed by carefully sliding it off the posts? It seems to have a slight rocking motion. Now, for the larger board which seems to be the backside of the key contacts, it is mounted on all four posts and has numerous wires going to the display module at the top. Can this also be removed (partially--I don't want to get into soldering!) by sliding off the four posts and tilting upward??? Maybe that is not necessary if the problem is on the underside of the circuit board.
Tom


#16

The contacts (zebra connector strips) for the cpu board to keyboard pcb is under the cpu board. The contacts are of two type. The first is extremely fine wire wound around a soft spongy material. The other type is a connector with fine wires impregnated into a silicon strip.
In any case, connections to the cpu board and keyboard pcb are made through these two soft flexible connectors. Be very careful when removing the board from the soft zebra connectors because in the case of connector with fine wire wrapped around the spongy material, you can break the wires if they are stuck to either the keyboard or cpu board.
Look for corrosion or dirt on the cpu board or keyboard contacts where the zebra strips make contact.


#17

Erik, in a previous msg. David M. had suggested to make sure
there was sufficient pressure on the cpu board to make good contact with the main board. Any ideas (from anyone) on a means of increasing pressure on that board. I am thinking more and more that's where the problem lies.
Tom


#18

Tom,

To answer your first question - yes, the CPU board is the small board that slides over the two posts. You can safely remove this. Make sure all of the contacts are clean. I wouldn't do too much to increase pressure other than ensure the screws are tight. I suppose you could place very thin nylon spacers over the screw posts to increase the compression if nothing else works.

The main circuit board is heat staked on, and is not removable (my mere mortals with my skill level, at least).

- Ed


#19

ED, I believe you "cracked the code"! I placed a short strip of rubber, about 2mm thick, along the bottom of the circuit board. With the case closed it exerts enough pressure on the circuit board and the connector beneath to solve the problem! I am delighted.

I have three "old" HP's that I bought when they were new, in the 70's and early 80's. An HP65, 41CV, and 41CX. I've had to build up a battery pack for the 65, and the CV you know about. The CX has never had a problem (yet!)

Needless to say I appreciate the help that you and the other experts provide on this forum.


#20

Tom,

I'm glad I was able to help. I'm not sure I would call myself an expert, certainly not compared to some of the talents in this forum. Nevertheless, I am flattered and happy that I could be of some assistance.

- Ed


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