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Post replacement that works! Thanks
02-06-2017, 10:25 PM (This post was last modified: 02-24-2017 04:38 PM by dingebre.)
Post: #1
Post replacement that works! Thanks
Thank you aj04062!

See:
http://www.hpmuseum.org/cgi-sys/cgiwrap/...ead=184344

and especially:
http://www.hpmuseum.org/cgi-sys/cgiwrap/...?read=1174

****************" EDIT
I found the thread with the Delrin posts:
http://www.hpmuseum.org/cgi-sys/cgiwrap/...789#226789

Thank you Bruce!
***************** END EDIT

I thought I'd try to give back a little and share my success with this method. After some fails with superglue, I decided to try this permanent fix.

Here are the Mouser part numbers:
1. Aluminum standoffs from Keystone Electronics Corp PN 1801B, Mouser PN 534-1801B
2. 2-56 x .25 lg Pan Head screw, Mouser PN 534-9355
3. 2-56 x .75 lg Pan Head screw, Mouser PN 534-9357

I purchased some phillips head (I like them better) stainless steel machine screws, a little more money, about 0.40 more per screw. Yes, I know they have an "O-ring" under the head for sealing, but it doesn't hurt and I prefer the stainless steel and Phillips head.
1. Screws & Fasteners SEELSKREW STD PH REC 2-56 THD .25" LG, Mouser PN 608-R2-56X1/4
2. Screws & Fasteners SEELSKREW STD PH REC 2-56 THD .75" LG, Mouser PN 608-R2-56X3/4

You also need a hand drill or drill press and a 5/32" (0.156) drill bit or end mill.

WHAT I DID (see photos below, from a couple of different calculators):
1. Carefully disassemble the 41.
2. Check the height of your posts, the two bottom should be 0.2 inches give or take from the circuit board surface. The top posts by the ports/battery are a little shorter.
3. Trim the old post reasonably flush with the circuit board using some flush cutting diagonal cutters or similar.
4. Apply some masking tape to prevent dust from getting into anything like the back of the key switches. (Note: since I was holding my calculator vertically, I wasn't worried about the dust or chips).
5. Put a 5/32" (0.156) diameter drill in your drill. A drill press might be handy, but in the end I chose to use my cordless drill and rest the handle on my table. This gave me a good, steady hold on the drill and then I could hold the calculator vertically and feed it into the drill.
6. Carefully, slowly drill into the old post, letting the hole guide the drill bit. Take your time and go a little at a time. As you can see from my photos, the drill is just smaller than the old post and actually left a thin wall.
7. Drill just to or just below the opposite side of the PCB. If you look at my photos (a bit blurry, sorry), you can see about how far.
8. As you drill, take the time to insert your spacer into the hole (it's a nice snug fit) and measure how far it extends above the PCB. When you are at or just under 0.2", you're good. You can see my one post was just over 0.19" This works fine. As aj04062 said, it's better to be just a little shorter than too long.
9. If you haven't already, put tape over the contacts. I don't know why, but epoxy and silicone RTV always seem to get everywhere when I use them. You don't want epoxy on your PCB contact points. A little obvious, but use the masking tape.
10. Put a little epoxy in the hole to help with the bonding. Don't put much in here, just enough to ensure the bottom of the spacer is bonded to the plastic. If you put too much, it will fill the threaded hole and you will be left trying to drill and tap your brand new mounting post Smile
11. Put a little epoxy around the bottom of the spacer.
12. Carefully insert the spacer into the hole, watching for epoxy coming up the threaded hole in the spacer.
13. When the spacer feels inserted, measure the height again and make sure you are at or just under 0.2".
14. It should be a tight fit (see below). Make any small adjustments to make sure the spacer is straight up and down. You can temporarily re-install the bottom to make sure the threaded hole in the spacer lines up with the hole in the bottom.
15. Carefully clean up any excess epoxy around the base of the spacer. You need a flat surface here for the plastic piece that holds the connector strip. It's way easier to do it while the epoxy is still workable. Just be careful not to disturb the threaded spacer and check its alignment after you are done cleaning up.
16. LET IT CURE! Epoxy doesn't "dry", it cures and even the fast set epoxy needs 24 hours to fully cure. You'll be glad you waited.
17. Remove the tape, clean, and reassemble your 41!

NOTES:
a. The post by the ports/battery are a little shorter than the bottom posts. I have not had to replace these ones yet, but I believe there is enough depth to drill the hole a little deeper to accommodate the 0.25" spacers.
b. Be sure to mix your epoxy well and be careful with the measurements.
c. Use a good epoxy that will bond well to both the plastic and aluminum.
d. I had one calculator where the post broke off below the PCB. This method still works. After removing all loose pieces, use the drill VERY carefully if needed to clean up the surface of the plastic or to break off loose plastic. You may need a bit more epoxy in the hole to act as a filler. You may also need to build a "jig" ahead of time to hold the spacer vertically and at the proper height above the PCB. In my case, the fit was still tight enough to hold the spacer and the hole depth was ok.

CONCLUSION:
What a great fix! I saw somewhere else (I can't find the page) where someone did the same thing using nylon spacers. I like the aluminum because I think the epoxy bonds better to aluminum than to nylon and it is harder to strip the aluminum.

I've done several of these now, and it works great. You can get a snug fit between the top and bottom case without worrying that you are breaking the posts or stripping them. The only possible downside is this is a pretty permanent repair that can't really be undone unless you are willing to drill out the aluminum spacer. Possible, but much more difficult than drilling our the plastic ones. It also makes it nearly impossible to remove the keyboard from the case. But if it has come to that, it may be a better idea to get a "new" used one with a working keyboard.

Happy to answer questions about my experience with this and many thanks again to aj04062.

I have some other photos, but I could only attach 5 small ones here. Email me and I can send others.

   
   
   
   
   

David

David Ingebretsen
Collision Forensics & Engineering, Inc.
dingebre@CFandE.com
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02-07-2017, 02:40 AM
Post: #2
RE: Post replacement that works! Thanks
Excellent write-up! The manufacturer & Mouser part numbers are especially appreciated.

Now, let's see . . . I think I have a calculator drawer to rummage through!

~ Mark
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02-10-2017, 03:54 PM
Post: #3
RE: Post replacement that works! Thanks
Congratulations!! Good work. I am going to test it immediately.
One question: Why two different lengths for the screws (.25 & .75)? Are the longer for the upper posts?
Thanks again and congrats again!!

Kind regards

Ignacio
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02-10-2017, 04:25 PM
Post: #4
RE: Post replacement that works! Thanks
(02-10-2017 03:54 PM)isanchez Wrote:  Congratulations!! Good work. I am going to test it immediately.
One question: Why two different lengths for the screws (.25 & .75)? Are the longer for the upper posts?
Thanks again and congrats again!!

Kind regards

Ignacio

I think that's right. The upper posts use a longer screw, but like I wrote, I haven't tried to replace the upper posts yet. If you do, make sure to measure the ones that remain before you drill them out and make the new posts are the same height.

David

David Ingebretsen
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02-24-2017, 04:40 PM (This post was last modified: 02-24-2017 09:47 PM by dingebre.)
Post: #5
RE: Post replacement that works! Thanks
I found the thread with the Delrin posts:
http://www.hpmuseum.org/cgi-sys/cgiwrap/...789#226789

Thank you Bruce!

David

********** EDIT
The above link seems to be broken. Here are a couple of other ways:

Add a "www" to the front of this address and enter it into your Browser's address line:

hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv021.cgi?read=226789#226789

Go to the old forum archives, Archive 21, search the page for "HP41 post replacement experiment" or a subset of good keywords. I searched on replacement and found it was the #8 of 11 matches.

http://www.hpmuseum.org/cgi-sys/cgiwrap/...chv021.cgi

For good measure, here is the link again. Hope it inserts properly:
http://www.hpmuseum.org/cgi-sys/cgiwrap/...789#226789

These all work in the preview, sorry for the problem.

**********END EDIT

David Ingebretsen
Collision Forensics & Engineering, Inc.
dingebre@CFandE.com
http://www.CFandE.com
http://Analoguerealities.com
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02-24-2017, 09:34 PM
Post: #6
RE: Post replacement that works! Thanks
(02-24-2017 04:40 PM)dingebre Wrote:  I found the thread with the Delrin posts:
http://www.hpmuseum.org/cgi-sys/cgiwrap/...789#226789

Thank you Bruce!

David

FYI - This url does not work, brings up an unusual error from the MoHPC Forum system. At least this is true in US on Win-7/IE11.

--Bob Prosperi
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02-24-2017, 09:48 PM
Post: #7
RE: Post replacement that works! Thanks
(02-24-2017 09:34 PM)rprosperi Wrote:  
(02-24-2017 04:40 PM)dingebre Wrote:  I found the thread with the Delrin posts:
http://www.hpmuseum.org/cgi-sys/cgiwrap/...789#226789

Thank you Bruce!

David

FYI - This url does not work, brings up an unusual error from the MoHPC Forum system. At least this is true in US on Win-7/IE11.

Thanks Bob. Updated my post above. Let me know if there are issues.

David

David Ingebretsen
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02-25-2017, 01:46 AM
Post: #8
RE: Post replacement that works! Thanks
(02-24-2017 09:48 PM)dingebre Wrote:  ...Updated my post above. Let me know if there are issues.

Works for me, thanks for prompt update.

--Bob Prosperi
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