HP 41CX loose screws (no not the posts!)...



#2

As a few of the Corvallis members might remember, my tried and true 1984 HP41CX decided to have display problems at the conference. Probably the shock of returning to it's roots!

Here is a shot of my prize which has worked since university days 1984 to present in the cockpit (figure 1):

FIGURE 1

ABSTRACT

Of course the problem was due to the two top screws being loose and not securing the two halves of the case together. I thought to myself, ah! the cracked posts syndrome. Upon disassembly I noticed the posts, all four were in perfect condition, but that the two 'washers' prefabricated as part of the inside rear shell had sheared off almost in one piece (figure 2). More on why at the end of the post.

FIGURE 2

As you can see the washer part of the bottom case has completely sheared off. I decided that the two fixes I have seen, both which work well, were not suited to my ideas. One involves the inelegant method of securing longer screws with metal washers from the outside of the rear shell. The other listed here

HP 41CV repairs

is much nicer and would fix the problem. The only downside as described by the author is that the battery/block is also glued into place with epoxy. Heat will soften epoxy and therefore the block could be removed at a later date for repair if required.

METHOD

I decided to use a different method as follows:

FABRICATION:

1.  source a sheet of hard plastic for about 2mm in thickness.
This must be stiff plastic that will crack when bent and not
give. I used a battery cover from a disused tv remote.
2. cut the plastic into two rectangular pieces. Sand the edges
and shape the rectangles to loosely fit the inside ends of the
battery/module block (figure 3). The rectangle must fit
loosely for gluing purposes.

FIGURE 3

PREPARATION:

1.  prep the inside rear shell for gluing by removing the
protruding edges of any remaining washer, I used an
exacto knife.
2. sand the inside rear shell for two reasons,
a. to make sure the rectangles will fit flush to the
inside case back.
b. for better glue adhesion purposes.
3. sand the gluing side of the two new rectangles for better
adhesion.

First a little theory on why I chose the rectangles. For best long term results the area available for glue adhesion must be as large as possible. Due to the design of the case the largest size of new washer can only be as big as the rectangular recesses at the bottom of the battery/module block (figure 3). You could make a larger rectangle but this would entail modifying the bottom of the battery/module block, something I was hesitant to do.

Leveling the surface of the inside of the case also allows for greater contact between the new washer and inside rear shell. Roughing both sides with 400 grit sandpaper also allows for greater surface area for bonding. Remember strength is important here.

ASSEMBLY:

Now that the two rectangles have been shaped and fit inside the two openings at the bottom of the battery/module block we have to find a way of positioning the rectangles in place. Here comes the reason for the loose rectangles.

1.  using liquid plastic cement, place glue as per the
instructions aroung the new rough opening on the inside
rear shell.
2. then placed the new washers in the battery/module block in
the recesses.
3. place a small amount of glue on the new washers careful
not to add glue to the battery/module block.
4. holding the battery/module block in one hand with the loose
new washers in place, take the rear shell and position the
battery/module block in place, (should be obvious but the
battery/module block should be upside down during this
manuever, else the new washers would fall out ;-) (figure 4)

FIGURE 4

5.  the two glued surfaces will contact each other and the 
battery/module block can be removed leaving the washers in
place (figure 5).

FIGURE 5

6.  apply liquid glue to the edges of the new washer-
rear shell interface. This will wick into the
recesses and allow for complete saturation of the glue.
7. apply pressure, careful not to move the new washers, as they
now sit in the perfect position to allow the battery/module
block to be installed once the glue has dried.
8. now that the glue has dried, drill to small holes in the centre
of the new washers (figure 6)
9. assemble the calculator.

FIGURE 6

CONCLULSION

Firstly, I have assumed you all know how to access the inside of the 41C series. There are few wonderful articles replete with photos showing this. Of course, don't hestitate to ask questions!

Now to the reason the calculator washers sheared. The original battery pack has spring battery clips that have a weak spring to them. Consequently the pressure on the battery/module holder and therefore the stress to the case is low. I recently replaced an aged holder with a new aftermarket version. This is an excellent reproduction but with much stronger springs. The battery module fits tightly and in my case exerted a large force on the screw heads. This is a twisting moment as the battery/module block wants to rotate out of position.

I have subsequently replaced the aftermarket springs with some much weaker versions found in many tv remotes. These can be picked up very cheaply ($1) at junk stores.

The battery pack now slips in and out without much force at all and the stress to the top end is much less now. And of course the calculator now works perfectly. Time will tell if this method survives!

Hope this helps.

Cheers, Geoff

Edited: 4 Nov 2008, 5:04 p.m.


#3

Very nice work Geoff. Thanks for taking the time to put all this detail together!

Mike


#4

check that you battery holder slides into place without much force"

The reason I say this is that the calc ran for 24 years without mishap until this spring when I purchased an unchipped after market battery replacement.

The battery pack was extremely hard to get into place and remove. All due in retrospect to the strength of the springs. They are visibly thicker and physically much stronger than the originals.

One can replace them as described or shorten the length by removing them and pushing them in the reverse direction until the batteries barely protrude above the holder. This also has the effect of reducing the twisting moment on the case and battery/module block.

Cheers, Geoff

#5

Excellent! Now to work on a decrepit 41cv...

Thank you for the detailed writeup,

sdb


#6

let me know how it works. My cx is functioning perfectly.

Make sure the battery pack springs are not to strong. Another problem is leaving the card reader in place as this too flexes the case. I install the reader only when required.

Cheers Geoff


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