This HP-41c doesn't work --- what next?



#2

Hi all,

I've recently come into possession of a non-functioning 41c. I have read a good amount of the available online repair guides and I feel like I'm getting my head around the issues. But I'm stuck and would be happy to hear your considered advice.

I have disassembled the unit. It took me some time to realize that those two 0.15" nuts (!!) screwed onto the lower CPU board posts were someone else's repair attempt. Both upper posts are cracked. The lower ones look alright (not cracked, that is) except for the threads cut by the 0.15" nuts. If you squeeze the two case halves together you can get the screen activated but it only (alas!) shows jibberish. Incidentally, the unit is in excellent cosmetic condition and the battery compartment, in particular, and contacts look very good.

I've considered rebuilding the upper posts and keeping the nuts on the lower ones to see if that would do it -- but I'm skeptical that it would. I would have thought that by squeezing the two case halves together (firmly but accurately) at the upper and lower post points I'd be able to see something sensible on the display when turning it on. Can anyone comment on this point? Shouldn't squeezing by hand do it?

The next step, I suppose, is to cut out the middle-man and just go ahead and solder wires between the pressure contacts. But this seems like some good amount of work. And, being a pessimist, I'm assuming the electronics are fried anyway.

My question is: does anyone know how to check the chips? Can't I use a digital multimeter to check point-to-point resistances or voltages and compare to a chart of expected values? I'm stabbing in the dark here, clearly. But I think there should be some way to do this.

Your advice and comments are most welcome. Let me know if I should try something else. (I'm hoping to fix it myself -- sorry FixThatCalc -- or chop it for parts.)

Thanks in advance.

Mark


#3

Upper posts being cracked - that's usually no big deal unless the screws don't tighten.

The nuts on the lower posts wasn't a repair attempt, it was the original method HP used to secure the logic board to the keyboard. It worked quite well until you removed and installed the logic board several times - at which point the post threads would fail and the nuts would no longer hold the board tight enough to make proper contact. HP did have undersized nuts (they were usually gold in color) for these cases but ultimately they would strip too.

Quote:
Shouldn't squeezing by hand do it?

No, it usually will not work. The logic board has to make (IIRC) 28 connections to work... and you simply can't apply pressure in the correct places.

Quote:
The next step, I suppose, is to cut out the middle-man and just go ahead and solder wires between the pressure contacts

DO NOT DO THIS!!! It is absolutely the wrong thing to do. There is NEVER a good reason to do this. It's a horrible mess which makes your 41 non-modular thus preventing any further testing by simple board swapping.

Quote:
My question is: does anyone know how to check the chips?

Yes, it's called the Service ROM. It's the only way.

Quote:
I'm assuming the electronics are fried anyway

That is *VERY* pessimistic.

Bottom line: Remove the logic board, clean both sides with isopropyl alcohol and dry well. Re-install and use the nuts to secure the logic board if they hold. If not, you'll need to fashion some spacers in place of the nuts in order to apply pressure to the board. Check the soldered connections between the keyboard and display driver, resolder any fractured joints.

You'll probably find it works...

Edited: 13 July 2007, 2:49 p.m. after one or more responses were posted


#4

Hi Randy,

Thanks for the advice. And for setting me straight on the nuts.

You said:
Bottom line: Remove the logic board, clean both sides with isopropyl alcohol and dry well. Re-install and use the nuts to secure the logic board if the hold. If not, you'll need to fashion some spacers in place of the nuts in order to apply pressure to the board. Check the soldered connections between the keyboard and display driver, resolder any fractured joints.

I can do the cleaning and reassemble. Just to be clear -- the "soldered connection b/w the k/b and display" are the 16 (I think) steel looking metal bars at the top of the calc, right?

I'll work on it tonight.

Another question -- any feelings about using cyanoacrylate to repair the posts?

Mark


#5

Yes, I was referring to the interconnects at the top edge of the keyboard.

Quote:
Another question -- any feelings about using cyanoacrylate to repair the posts?

Yes, definite feelings. Don't do it. It will not work and it is irreversible. IMO, there isn't one 41 repair that should be done with cyanoacrylates. In general, keep the stuff away from calculators, it's not suited to the task. Any and all 41 plastic repairs should be done with plastic model welding solvent. This is NOT the stuff found in hardware stores used to join plastic pipe. You'll find it in hobby shops, it is used to assemble styrene plastic models and is sold in small glass bottles. It's a clear liquid, you brush it on and hold the parts together for a minute, then let cure overnight. It's the only way to repair cracked posts. Any other adhesive will fail.

I have to mention again that if the upper post screws tighten, don't bother with repairing, at least at this stage of the game. Cracked posts are very common and do no harm, it's not causing the improper operation.

Edited: 12 July 2007, 4:17 p.m.


#6

Hi Randy --

Can't thank you enough for your astute advice. Turns out my pessimism was, as usual, unwarranted.

I checked the interconnects by moving them with the tip of my needle-nose pliers. Turns out that one of them was loose. Soldering it back in place did the trick. I guess this unit took a good tumble at some point.

The case exterior looks great but you can see scratches on the plastic under the feet where someone used metal (?!) to pry them up. No worries there really, I only mention it because it makes it obvious that someone has mucked around with this unit at some point. Point is, the screw-head seat in the upper left screw-hole has been cracked and some plastic has broken away -- I'm guessing from the mucker cranking down on the screw in a misguided effort to "fix" the unit. Any advice for rebuilding this screw-seat? Worse comes to worse I can use a washer but that'd be ugly.

Thanks again.

Mark


#7

Virtually all 41's that have been serviced have scars under the feet where the pads got lifted with small screwdriver... yours is in good company.

It's pretty common for the screw seats to get busted out or at least cracked. It probably happened when the unit took a dive off a desk which was in all likelihood why the display connection was fractured. Make sure that little strip of black plastic goes back into place along the top edge of the display. That prevents the display from moving when the case is closed. Leave it out and you'll find lots more of the connections fractured in a few years - due to the display flexing the solder joints.

This is where the plastic solvent cement comes to the rescue. You might have to piece it together in two steps but they always go back together. Just be sure to not rush the job and give it a minimum of twenty four hours drying time before putting screw pressure on the seat. Don't trust the label on the bottle that says a few minutes is all that is required. The solvent has to completely evaporate for the repair to reach maximum strength.


#8

Hi Randy,

I probably should have asked your hourly consultant rate before we got into this...thanks again for the help.

I didn't quite follow your last comments.

Quote:
This is where the plastic solvent cement comes to the rescue. You might have to piece it together in two steps but they always go back together. Just be sure to not rush the job and give it a minimum of twenty four hours drying time before putting screw pressure on the seat. Don't trust the label on the bottle that says a few minutes is all that is required. The solvent has to completely evaporate for the repair to reach maximum strength.

I understand about giving the solvent ample time to cure -- I'm in no rush. But what are the "two steps" that you refer to?

My conceptual difficulty here is that the broken out screw seat appears to require significant reconstruction. If the seat were exposed, like a post, I think I could manage it (if I could figure out somewhere to get some (same color?) filler plastic and melt it down with some solvent...). But this screw-seat here is in a recessed hole. And to complicate matters, the cracked out part isn't exposed from the backside either. Any advice? (I could send a pic if my description is lacking.)

Mark


#9

I think I understand...

The bosses are exposed, from the inside of the case. I had assumed you had removed the port i/o block... I forget that sometimes they can seem as though they are one with the case bottom, but they're not. Remove the four port fillers or any modules and you should be able to lift the block off the case.

If the block is stuck in place, use a small flat screwdriver slid in from the module side under one corner of the block. Work the screwdriver over to middle and twist the screwdriver such that it lifts the block up and off the four retainers. It may give way with a loud snap but it should be okay, it had just fused itself to the retainers.

Once the block is out, it will expose the bosses and you should be left with the broken out pieces and the proper side to repair from. With the new perspective, all should be self-evident.


#10

Randy -- I haven't gotten a chance to follow your good advice. I'll have to get to it after vacation. Thanks, again. -Mark


Possibly Related Threads...
Thread Author Replies Views Last Post
  emu48 - copy stack doesn't work (as expected) Thomas Radtke 2 317 11-11-2013, 02:19 PM
Last Post: Thomas Radtke
  Another Prime mystery - templates don't work anymore Michael de Estrada 2 233 11-02-2013, 11:34 AM
Last Post: Michael de Estrada
  Does the HP Prime software only work with Win 7/ 8 ? Michael de Estrada 3 284 10-12-2013, 02:52 PM
Last Post: John Ioannidis
  Back to School/Work/Other Calculator Poll Eddie W. Shore 63 3,153 09-12-2013, 06:03 AM
Last Post: bhtooefr
  Anyone got a 39gII and doesn't mind memory loss? bhtooefr 4 269 05-19-2013, 03:57 PM
Last Post: bhtooefr
  How I use my HP50G at work. David Bengtson 7 374 05-06-2013, 02:57 PM
Last Post: Ingo
  Does IR printing not work on your converted WP34s? Then read on... Harald 3 288 04-04-2013, 05:46 PM
Last Post: Harald
  No luck getting flash disk to work in 95lx Harald 7 530 03-18-2013, 08:11 AM
Last Post: Harald
  Repaired HP-97 card reader doesn't show any signs of life jbssm 6 362 02-17-2013, 03:15 AM
Last Post: Alberto Fenini
  35s refuses to work Silvio A. Bensi 19 1,145 10-18-2012, 12:48 AM
Last Post: Reth

Forum Jump: