HP 41 not working

[AndiGer]

Hi all,
I have here an HP 41 CV that shows a weird behaviour:
When I put 5V to the battery ports for a very short time it shows starbursts but only the upper half and sometimes only in the left or right half of the display, sometimes thru the whole display.
What makes me wondering is the same behaviour when I disconnect the power source.
Anybody encountered this problem and has a solution to bring the machine back to work?
Capacitor problems? Anything else?
Thanks
Andi

[Frido Bohn]

(12-10-2017 09:27 PM)AndiGer Wrote:  Anybody encountered this problem and has a solution to bring the machine back to work?
Capacitor problems? Anything else?
Thanks
Andi

Before you delve into the electronic parts make sure that it's not a mechanical problem. The HP41 series is especially sensitive to the "broken-post" problem. There are tons of material in the forum (including old forum) about this.
A first check would be to look if there is any change in the display if you press the calculator at different places just below the display. During this maneuver, the HP41 should reside firmly, e.g. on a desk.

[AndiGer]

Today fiddled around with a 5V DC power source in the calc.
When I put power only little life appeared in the display for a very short time.
When power switched off the display shows some signs of life.
Switching on and off more often and fast changes from upper half of starburst in the right half of the display to full width of the display's upper half starburst.
Waiting with no power for about 5 seconds I can repeat from the start with only little signs of life.
Any ideas?
Thanks
Andi

[cruff]

What current will your 5V supply provide? The 41 will not require very much, unless you are trying to use the card reader, but if the supply is limiting the current too much you could see strangeness. Have you verified that it isn't an issue with broken posts yet?

[jebem]

Any reason to use just 5VDC?
This 41 series operates with 6 Volt DC from 4 x "N" 1.5VDC battery cells connected in series.
I would test the machine with a 6V battery holder (any 4 x AA or AAA will do).

[AndiGer]

I use 5V power source from an USB hub giving 1A. 5V because the rechargable battery HP82120A also gives about 5V - fully loaded maybe 5.2V.
Always use that to test powering up of HP-41's also with the card reader that is enough to operate the calc as it is from the HP82120A.
That calc had one broken lower post that I repaired (or tried to repair). The case is closed well now.
Measuring resistance from battery plus (anode there) to the lower hole in the tunnel (kathode there) gives 1880 ohms which is comparable to another CV (1820 ohms).
Battery minus to upper hole is near 0 ohms as in another CV.
What makes me wonder is the display behaviour when I put and especially remove power for several times in a fast sequence. If I hold power to the battery ports for a longer time (some seconds) and remove most of the time only right-hand side upper parts of starburst appear. No idea ...
Andi

[jebem]

If you are sure about your 5vdc power source quality, then most probable cause for this is the very well known problem of broken posts that are very common on the full nut models.

I have a few of these machines abandoned in a drawer with this issue and to be honest i don't bother anymore to fix them.

No matter how good these 41 series can be on features count, that design choice of using press contacts combined with poor choice of plastics results in a major fault that is hard to fix unless you just go an replace a now costly front cover/display/keyboard assembly.
There was a very good reason why HP finally changed that design on the later half nut models after all.

[Frido Bohn]

(12-18-2017 07:38 AM)jebem Wrote:  There was a very good reason why HP finally changed that design on the later half nut models after all.

This statement made me curious.
What did HP change in the design to make the half nuts less prone to malfunction, especially with regards to the broken post problem? (I'd like to introduce a new abbreviation for the HP41 enthusiast: 'BPP' aka broken post problem )

[jebem]

(12-18-2017 11:40 AM)Frido Bohn Wrote:  

(12-18-2017 07:38 AM)jebem Wrote:  There was a very good reason why HP finally changed that design on the later half nut models after all.

This statement made me curious.
What did HP change in the design to make the half nuts less prone to malfunction, especially with regards to the broken post problem? (I'd like to introduce a new abbreviation for the HP41 enthusiast: 'BPP' aka broken post problem )

as far as i know, the half nuts have all the processor components integrated in the keyboard assembly, therefore reducing (but not eliminating) the probability of this broken posts issue.

Edit:
To be more detailed:
Both full and half nuts still use the power/IO block that is a cause for BPP issues. However the full nut have an additional BPP issues on the processor daughter board.

IMHO, this BPP syndrome is aggravated because not only the case plastics crack easily, but because HP decided to use 4 metal self tapping coarse pitch screws to make pressure over the processor pca and power/IO block.
If they spent more money to have used a more resilient solution with metal fixed pitch thread screws and metal nuts, this issue would probably be drastically reduced.

While we are at it, this kind of bad contact failure is not only about the BPP syndrome.
Even with good posts, bad contact can happen on both the Processor board and on the Power/IO block, due to the pressure contact design choice as an alternative to the conventional connectors. These pressure only contacts will lose the pressure over time due to its dependency to plastic/foam based elastic materials.
Conventional connectors would increase the production costs but would guarantee a much more reliable electrical connection as it maintains a constant pressure between the metallic parts involved.
There is a good reason why that old pressure only contacts have been abandoned in modern artifacts in favor of the good old traditional connectors, despite using updated materials and miniaturization.

[d b]

Jose; There's a positive fix for your drawer of treasures.
In the early 2000s I found a few tiny springs that wanted to be just a little tighter than the 41's post o.d. I could barely get them on by rotating them so they would open up as I pushed them on. Rounding off the top of the post a bit helped. A couple of them also got a drop of thin glue to seep into the now-closed cracks, but I had no failures on any I did this to. Keep your eyes open. They're cheap if you can find them.
They should have came that way stock.

[AndiGer]

Can you descibe the spring solution in more details?
Maybe you have some pictures?
Very interested in
Andi

[mfleming]

Neat idea! I measure the post diameter as around 0.171" or roughly 11/64". A quick search turns up an appropriate spring here:

https://www.mcmaster.com/#9657k75/=1aquan6

You'd obviously want a cheaper source, of course! Perhaps a trip to a hardware store in search of a 3/16" OD spring?

~Mark

[d b]

(12-18-2017 05:08 PM)AndiGer Wrote:  Can you descibe the spring solution in more details?
Maybe you have some pictures?
Very interested in
Andi

AndiGer;
mfleming's answer was quicker and just what I'd have said. I got mine from a regular seller at the flea market. I came back with a couple of drill bits for sizing the next week and bought an assortment that were about right. Obviously the thinner wire ones were easier to push on and plenty strong enough. The ones that mfleming found will have to be cut and the cut end cleaned up a bit on the inside anyway. At about 6 piecesX12; Get the right ones and you'll be the go-to guy around here for post repair kits for a while.

[AndiGer]

Didn't try the spring solution right now.
The calc actually had a broken post problem.
Resolved and working perfectly now
Andi