41CV corrosion puzzle



#2

To any 41 repair experts:


I recently bought a 41CV (Fullnut) which, other than the battery terminals, is in really beautiful condition. And it has a set of manuals etc.


I got a decent price on account of the terminals, but here is the problem:


After surface cleaning of the negative contact (left side, looking from the back towards the front), the unit would not turn on. even with a little ball of wire smooshed into the contact, no power.

So I carefully took apart the (Fullnut) calculator, and saw that the corrosion had leached down the contact, and dirtied the contact at the printed circuit board. So, I carefully cleaned all the parts that had to touch, using a q-tip and a bit of alchohol. After drying the unit and reassembling, I put the little ball of wire in the negative contact, installed the batteries, and Voila!--it worked!


Unfortunately, after a blissful 10 days or so, she (the calculator, like a boat, is a vessel) locked up--with numbers in the dispay, she would not turn off or respond. So I took the batteries out. A few days later, I opened her up, re-cleaned her, and with more wire, put the batteries in, but the instant the batteries made contact, she let out this horrendous piezo squeek, and the display filled up with segments. Again, no keystroke response.

I am afraid for the worst, but I hope for the best. I took some pictures, but am not sure how to get them up on this site. I read a really great article (on MOHPC i think) about soldering a new terminal on. I suppose I could go for it--but I wonder if I have already "fried" the guts with my impetuous first repair.


regards,


Bill


#3

I just found two other facts which must be important.

1. one of the screw posts at the bottom of the calculator is cracked through (did I do that when re-assembling?!!)

2. there is very light but noticeable green corrosion visible on the contacts that are in way of the lower screw posts.

I also have a piece of plastic that fell out just now, that I can't for the life of me figure out from whence it came. It is 2.75" (69.9mm) by 1/8" (3.2mm) x 5/32" (4 mm)and shaped like a very long angle iron with caps on each end.

I have pics of this stuff, too, but again, don't know about posting (method, bandwidth, etc.).

a note for Luiz--see, I did need help with a calculator fix-it job--thanks again for your suggestion--but I saved up for a "good one" instead of the lowly 48G--though that will surely come around to the bench, too!

-Bill Platt


#4

In my recent "41CV Gone Wild," I had a similar problem, in that the display filled up, and the keyboard did not respond - Had to remove the batteries to turn it off.

The problem was a slightly cracked post. Per help received here, I fixed the post using Tenax "model adhesive"/solvent from Hobby Town (this is great stuff, by the way). Just drop a little in the crack and squeeze the crack closed for a few seconds, and it's a done deal. Some have suggested using small wire to wrap the post while the adhesive cures, but there's not time. The Tenax bond is complete within about ten seconds, so I just used a pair of needle-nosed pliers to squeeze the crack closed, and then held it for about 20 seconds (10 recommended, plus 10 for good measure). You have to work fast though, as the Tenax evaporates faster than you can say "Jack Sprat."

Hope this helps.

Can't help with the corrosion though.


#5

Although the Tenax appears to evaporate and set quickly this is just the surface material. You should wait 24 hours before reassembling. This gives time for the solvent locked up within the bond time to evaporate and gives the joint maximum strength.

#6

Hi, Bill;

gimme a few minutes and I'll find the pictures I need to compose a complete answer, O.K.?

I'll add a new post after this one with a new tittle soon.

Best regards.

Luiz C. Vieira - Brazil


#7

Hi, Bill;

With the calculator opened, look at the LCD (top). There is a small gap between the LCD assy and the calculator's case. It's where the small plastic piece fits in. Look at the picture bellow and you'll have a better idea.

About calculator's complete malfunction, you wrote:

Quote:
1. one of the screw posts at the bottom of the calculator is cracked through (did I do that when re-assembling?!!)

2. there is very light but noticeable green corrosion visible on the contacts that are in way of the lower screw posts.

These are reasons enough to freeze calculator's keyboard and allow the "horrendous piezo squeek" come to life. I'll find other images, but for now you might want to have a look in a zebra connector (two spare pieces or an assy) between the small mainboard and the keyboard. You should also carefully clean them, but do it only after being sure you can repair the broken post so it can hold both case halves together.

I replaced some broken posts with styrene plastic "custom" posts. Tis styrene is the same one used in plastic models (Revell, Tamiya, etc.). Some are too weak and canot hold a screw thread, while others are hard enough to hold it AND the screw inside.

This sort of repair procedure demands removing the keyboard and the keys, so you should be able to settle a new post without gluing keys or damaging the keyboard. I took some shots when doing it, but almost all photos were out of focus.

The images below show the final stages of the process, when the "new" post is placed with glue to "dry out", and later when a small hole (guide) is drilled by hand. What's the name in English of the tool used to keep the parts together? I know it's not wrench, but this is the name that comes to my mind...


Hope these help, too.

If you need more advice (I know these are not enough) let us know.

Good luck and success.

Luiz C. Vieira - Brazil


Edited: 13 June 2003, 1:52 a.m.


#8

Quote:
What's the name in English of the tool used to keep the parts together? I know it's not wrench, but this is the name that comes to my mind...

The word you want is "vise." Great pictures!


#9

Actually, just to confuse the non-native English speakers around here, I'll point out that 'vise' is the American English spelling; in British English, it's spelled 'vice'.

(Except when it's spelled 'clamp', of course.)


#10

Quote:
Actually, just to confuse the non-native English speakers around here, I'll point out that 'vise' is the American English spelling; in British English, it's spelled 'vice'

That's not the British English spelling, it rather is the Miami slang... ;-)


#11

I thought I should not mention "Miami Vice", but.... :)

#12

I really liked that old cops and druggies TV show... Miami Clamp...

#13

Hey Wayne, Frank;

thank you both.

About pictures, I'm taking a few time to draw what could not be shown by the original images (the others are pretty worst than these) and I'm thinking about to write an article. What do you think?

Best regards.

Luiz C. vieira - Brazil

Edited: 13 June 2003, 9:55 a.m.


#14

Luiz,

You HAVE to make a website with all your great pictures and procedures!

Meindert


#15

Hello Meindert, guys;

as it comes from you, Meindert, and I'd say the same if these words came from some other giants in here, I cannot decline to do. Your site and your work are kind of a reference, a model to all of us. In fact I thought about doing something about this, so I can share what I think that is useful. But it MUST be something I'm proud to show. For now, I can only store these images and post messages, maybe a few articles.

Anyway, I'll start the engines and start the tour for building some HTML and trying a few "looks". As I have something to show, I'm posting about it.

Thank you for the encouragement. I'm flattered.

Luiz C. Vieira - Brazil

Edited: 14 June 2003, 11:43 a.m.

#16

You're welcome, Luiz. I think an article is a great idea. Most people who deal with the 41 models are likely to run into the cracked-post problem sooner or later, and this would be a big help.

#17

Luiz et al,


Thanks for your help so far! I also went back and looked around a bit in the early May post about the "41 Gone Wild" and there are some distinct similarities to my situation. In fact I remember reading either there or somewhere else about how importatnt it is to avoid over-tightening the screw, as it can crack the post, and so I *thought* I was careful--but not enough!


So far all the comments seem good news--I haven't heard any ideas of the unit having been automatically "fried" from having corrosion.


Your pictures are very helpful---I need to upload mine so that you can see them, too.


I had not thought of the LCD being a potential problem, but I will look at those connections, too.


Regarding the broken screw post: mine is cracked right off--right at the plane of the PCB for the brain. I will try to glue it back on, and hope it holds.


Somebody posted some weeks ago about actually replacing the contacts with soldered connections. I am not only mystified by the thought of doing that, but also I do not understand how the contacts work. I don't see any insulators on the intermediate part (the part that looks like two long coils, or two combs).


I guess I better post my pictures to explain (in my work, I deal with drawings and sketches as much as I do with numbers and words, so I always want to make a picture..:-)


Best regards,


Bill


P.S. I haven't had this much fun communicating since I first got my HAM radio license and made contacts all over the world with a tiny little transmitter.....


#18

Bill,

I would advise against trying to solder permanent wires between the main board and CPU board. I tried it and failed. I'm not saying it can't be done, but it's tricky business and very hard to do well without oversoldering or actually desoldering the traces from the main board.

The zebra connector consists of a bunch of tiny "loops" that connect the main board to the CPU board. They are all isolated from each other, and typically more than one will make contact to each pad on the board. However, because they are isolated from each other, they won't short across to the wrong pad.

I suspect your display problems and squeaky piezo are the result of a bad zebra connection, which is in turn due to lack of sufficient compression because of the broken post. That connection is particularly fussy in fullnuts.

My advice is to clean all contacts and remove any corrosion using isopropyl alcohol, then repair the post as advised in an earlier post (I couldn't avoid using the word "post" twice ... err ... three times). Replace the small plastic LCD support as advised in Luiz's post, reassemble, and good luck.

On another note, I think Luiz's idea to contribute an HP-41 repair guide would be an excellent addition to the Forum. The 41 is a great calculator, and very durable, but it does have some very specific weaknesses, many of which can be quickly diagnosed based on the symptoms. If Dr. Vieira provided us with the appropriate medical texts, we could maintain our machines in better health !

- Ed

#19

While the green heebie-jeebies under the logic board area are enough to cause the problem, check/resolder/clean the LCD module connections. Check for green fuzz growing there as well. They have to be pristine with no flux or fuzz.

This is another common cause of @@@@@@@@@@@@ displays.

#20

To all who have posted,

Thanks much for the advice! I have now gone through round two of the repair. For a brief moment, I thought I might have the repair successfully completed, but alas some more thinking is required.


So, here's how it went. I carefully removed the connection between the board and the keyboard, and cleaned the green gunk off. Then, I thought it would be best to test the contacts before fixing the post, and so I re-asembled and merely squeezed the system back together. And for a brief second or so, I had a happy "memory clear" message, but then it is hard to hold it together.


So at this point I was fairly confident that merely applying some plastic model cement to the cracked off post and reasemblig, and clamping externally, would do the trick. But it did not work.


I did not use Tenax--rather, I used "Testor's".


I tried to do more dry fits, but had no "memory clear" messages.

So, two issues to be explored, next>:


1. Try Tenax this time (have to find some)


2. Figure out how critical the alignment has to be. It seems that the alignment has to be very precise.


3. NOTE: I do not have a merely cracked poset; rather, the post is broken right of at its root.

To Luiz: thanks for the photos of the repair you did. Have you ever had a completely broken of post, or just a cracked one? How would you test the alignment is right? I don't want to glue it back together in a permanently non-workable alignment.


Best regards,


Bill


#21

Hello, Bill;

I'm glad you made it!

The "MEMORY LOST" is the best sign that everything is fine. There is a small, tiny chance of failure, but let's first check the post.

This is what I need to rebuild a post. Small screwdrivers (jewelers type), drill, cutter, glue (not shown), the "clamp" and a small plastic tube, a piece of styrene found in plastic models. The small black plastic piece at the right side of the cahlcualtor's case is the piece I used to build the missing post in this HP41 (down, right). I also rebuilt two posts with a similar technique in my 82143A, but I had to use small, custom metal "gloves" to hold the plastic tube in place. Look at the drawing below.

1 - Representation of a "sane" post

2 - an "X-ray" view sowing where the screw fits (no thread, sorry...)

3 - this is how would it look a base remaining from a broken post, say, completely removed; in this case...

4 - I homogenize the surface and drill a hole a bit larger, so I can build...

5 - a new post that fits in the hole...

6 - like this.

7 - After placed (with the glue of your choice), you should drill a new hole that is enough to allow a hew thread when using a screw for the first time.

8 - An X-ray view of the final settling.

About alignment: after settling the new post and before drilling the hole for the screw, put both halves of the calculator's case together and you'll be able to "mark" the precise location of the screw by using it's own hole on the back case. I cannot see a better and faster way, unless you have precision instruments that allow you to precisely locate the place for drilling.

I hope this helps. Success!

Luiz C. Vieira - Brazil

Edited: 15 June 2003, 4:40 p.m.


#22

Hi;

I recovered this image, where two rebuilt posts are shown in the same HP41 case, the ones in the bottom that you can easily see (the ones that go in the top are in the shadow and out of focus). You can see they don't have a regular surface, they are not "pretty" well finished, but they can hold the case. This HP41CV is perfectly fine.

The remaining small plastic and the cutter (what's the actual name for this sort of knife, please?) are also shown.

I think this is all I have for now. The remainign data is about measurements, but I cannot find a precise meter.

Let me know if there is anything else I should have posted here.

Thanks.

Luiz C. Vieira - Brazil


#23

Luiz,

Thanks for your posts! What sort of "glue" are you using---(glue or is it a solvent type cement). Does the regular plastic modelling glue such as "Testor's" work for you? Another person mentioned "Tenax" which I am not familiar with.


I tried "Testor's" but it wasn't the regular stuff in a tube--it was like a felt tip pen. Perhaps the thicker stuff in the tube will work?


#24

Heed David Smith's advice on solvent cements in his previous post int this thread, he knows of where he speaks.

There many brands available, DO NOT get the citrus based products. Tenax and Pro-weld are two that come to mind. They should be clear, of a water like consistency and smell pretty nasty.

Regular styrene model cement will not do the job, period. Nor will cyanoacrylates, not to mention the white bloom mess they make. Using anything other than a solvent cerment will only make matters worse. You want to fuse the pieces together, not glue them.

If you can't find any, let me know and I can UPS you a bottle.

Edited: 17 June 2003, 10:27 a.m.

#25

Hi, Bill;

I have access only to super glues like SuperBond (Loctite®), I have not found those solvents mentioned by Randy. In this case, to keep the "new post" in place, I sculpt it tight enough to firmly sit in previous post's hole (draws #4 and #6 in my post) and I keep them together with the clamp (vice). Remaining super glue between the new post and the calculator's hole is a thin coat.

I believe solvent will be a lot better, but I could not found it so far. As the (negative) pressure is not too big, super glue is holding bravely.

What I must warn is about glue/solvent quantity: in some cases, a drop is too much. I you can use something that allow you to put a small amount of glue and spread it in the base of the hole (avoid the walls so you can press th new post till the bottom) and let the new post itself to finish spreading it trhough the rest of the surface. I strongly recomend having the clamp already in position and enough opened. And remember using all necessary safety protecion for your eyes, skin and hands. And don't breath too deep while doing it...

Let us know how things are going. If you have the chance, take some pictures so they will help illustrate for the others.

Success!

luiz C. Vieira - Brazil

#26

I think that qualifies as a "box cutter". Primary purpose is to cut open cardboard boxes.


#27

I though the primary purpose of a box cutter was to get you busted at the airport... It's amazing how many idiots still try to fly with these things. My next door neighbor is a pilot of American Airlines. DFW still carts away a barrel of such things every month.

#28

We call it an X-acto (TM) knife


#29

Really? It's not the regular X-Acto blade holder with the round aluminum handle - but that's another kind of knife X-Acto makes?

#30

Glues will NOT work. They will eventually give way. Tenax, etc is a plastic solvent. It disolves the plastic and the joint it makes is a weld.

I have fixed several units that have had sheared off bottom posts just by using a longer screw. The case plastic below where the post shears off is quite deep and reinforced by ribs in the keyboard construction. I carefully drill out the hole in the sheared off plastic post so that it is now just a bushing that keeps the CPU board aligned. The screws that you need are 2-28 self tapping sheet metal screws (preferably with a type "B" point)... good luck finding them though.


#31

David,


how far below the bottom of the post can you drill?


-Bill


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