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HP 41C TALL KEYS repairing questions
09-27-2014, 11:24 AM (This post was last modified: 09-27-2014 11:29 AM by aurelio.)
Post: #1
HP 41C TALL KEYS repairing questions
Hi all, I've a 41C which needs to be restored.

When I bought it on TAS was defective: the internals were dirty and there was a principle of corrosion on the PCB, the block was completely damaged and also the zebra connector. I found both the spares (thanks to Diego and thanks to Dave).

I cleaned all the internals and replaced the faulty parts and reassembled the calculator and it worked, at least for a week..........altough:

a) the toggle "ON/USER" key was lazy, I mean that was necessary on the ON/OFF side to keep it pressed for a while to work
b) the ENTER key was broken (the hinge), but anyway working

I decided to keep it "as is", thinking that at least it works, but today, when I plugged in the batteries the calculator didn't wake up, Tried also with another pack (rechargeable): the same result.
Opened it and checked. externally the PCB seems ok both the viewable sides (main and keyboard), tighted properly the four screws while reassembling (there are no broken posts), so that I think the problem could be a bad contact in the toggle, finally gone.

Now I would like to disassemble the keyboard and check better, a chanche to reach and repair (or replace) the enter key too.

May you tell me if to remove the keyboard needs to cut just the plastic rivets (unfortunately a lot) and how may I restore it (glue?); then i ask if somebody has a enter key for replacing or knows where may I buy it; on TAS were for sale month ago but I think there aren't now.
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09-27-2014, 03:31 PM (This post was last modified: 09-27-2014 03:46 PM by jebem.)
Post: #2
RE: HP 41C TALL KEYS repairing questions
(09-27-2014 11:24 AM)aurelio Wrote:  May you tell me if to remove the keyboard needs to cut just the plastic rivets (unfortunately a lot) and how may I restore it (glue?);

Hi Aurelio,

From what I have been reading, these machines are a pain to maintain and that's why I didn't got one to join my collection.

On the positive side, You mentioned that you had the batteries removed from the calculator, perhaps for long time.
This is good news (for me at least) because there are some nasty little things inside the calculator called electrolytic capacitors that may degrade themselves with time, specially if they stay long time without energy.


1. KB assembly removal:

I was checking the Service Guide (from the MoHPC DVD set) and in fact HP do not offer a procedure to remove the keyboard Sad
This 41 series shares this fixing plastic rivets manufacturing technique with many other models like the 20's, 28's, 48's and so forth.

Apparently the only solution is to destroy the original rivets shape by heating them (maybe a iron solder?) so they melt enough to remove the KB assembly.
In this way it should be possible to reuse the rivets by heating them again, even if we would need to add some strong glue to help the rivets to fix the assembly in place.

I'm sure there are experts here that may offer help based on previous experience on this model.


2. Power On delay/laziness/not working

From what I read in the service guide and in the old forums, this symptom can have several possible causes, starting by defective contacts on the connection between the kb assy and the processor assy, or a possible power supply failure.

First thing, make sure there are no excessive current consumption from the 6V batteries.
And check CR1, CR5 and CR6 protection diodes, just in case.
While you are there, check C1 as well.

Next, verify the cpu reset circuit around U2 Power Supply IC.
Check proper voltage at Vin/Vbat (input voltage to U2), and at Vcc (output voltage from U2).
Page 4-13 of the service guide can be followed here for details.

Check the capacitor C9 and Transistors Q1 and Q2.
And check C3 and C10 just in case.
If nothing found, I would check all the components around U2 IC, including the T1 inductor for good continuity.

Just a word of caution, it is easy to damage a good component when removing it (using a iron solder) by overheating. Good soldering technique is a requirement here.


EDIT:
Luiz Veira was a very active MoHPC contributor in the old forum. Here some nice photos concerning the LCD removal.


Good luck with your repair, and please share your work here.
Thanks!

Jose Mesquita
RadioMuseum.org member

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09-27-2014, 06:31 PM
Post: #3
RE: HP 41C TALL KEYS repairing questions
(09-27-2014 03:31 PM)jebem Wrote:  From what I have been reading, these machines are a pain to maintain and that's why I didn't got one to join my collection.

You don't know what you are missing... :)

A pain to mantain? Not true; at least this is my impression, and I have lots of them.
A thorough cleaning (dismantle and immerse in distilled water, preferably using an ultrasonic cleaner) usually solves a lot of problems.

Greetings,
    Massimo

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09-27-2014, 07:02 PM (This post was last modified: 09-27-2014 07:05 PM by jebem.)
Post: #4
RE: HP 41C TALL KEYS repairing questions
(09-27-2014 06:31 PM)Massimo Gnerucci Wrote:  
(09-27-2014 03:31 PM)jebem Wrote:  From what I have been reading, these machines are a pain to maintain and that's why I didn't got one to join my collection.

You don't know what you are missing... Smile

A pain to mantain? Not true; at least this is my impression, and I have lots of them.
A thorough cleaning (dismantle and immerse in distilled water, preferably using an ultrasonic cleaner) usually solves a lot of problems.

Washing will solve some problems if you have the right tools to do it. It will not solve all the problems, however Smile
But the 41C is in my short list to be acquired one of these next months... I just need to find a good one at a good price, and that is the issue - this calculator is overpriced in the usual buying places IMHO.

Jose Mesquita
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09-27-2014, 08:09 PM
Post: #5
RE: HP 41C TALL KEYS repairing questions
(09-27-2014 07:02 PM)jebem Wrote:  Washing will solve some problems if you have the right tools to do it. It will not solve all the problems, however Smile
Not all indeed, that's why I wrote "a lot of"

(09-27-2014 07:02 PM)jebem Wrote:  But the 41C is in my short list to be acquired one of these next months... I just need to find a good one at a good price, and that is the issue - this calculator is overpriced in the usual buying places IMHO.

I found many of them "in the usual buying places" for a fair amount of money.
Some sold in "unknown state" and some "not working" turned out to be fully working after inserting fresh batteries or - as I wrote - a dismantle and cleaning.

Good luck to you!

Greetings,
    Massimo

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10-02-2014, 10:26 AM
Post: #6
RE: HP 41C TALL KEYS repairing questions
Hi Aurelio,
I've all the keys you need, let me know.

Ignazio
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