HP41C with very touchy keyboard

[steve]

I have a very early HP41C (with all the bugs) that seems a little touchy.

Pressing some of the keys causes the display to blank (sometines with dim characters appearing incompletely on the right side if the display).

The calculator does not turn off and the display will usually reapear with a little fiddling.

Connecting a card reader to the calculator to either not turn on, or the display to fail (I havn't checked this out completely)

The calculator seems to "flex" a little more than any of the others I have and I'm suspecting that the internal structure that's supposed to hold things together has partially failed.

The calculator is otherwise in great condition and I would like to get it into a fully working condition once more.

Other observations include:

Memory corruption sometimes occurs after one of these display "events", suggesting that power to the at least part of the memory is interrupted

Occasionally an on/CLX (to try to get Memory Lost) will result in NONESISTANT. I'm wondering if this may be due to bad connections also (can't find parts of ram or rom?)

Pressing the user key firmly can often result in a "display event".

I would appreciate suggestions on how to tackle this one.

[DaveC]

Check out: http://www.hpmuseum.org/guest/41repair.htm

[steve]

Thanks for the URL. It provided me with some hope, but my fault appears to be different. Before I go about "fixing" it I'd like to be sure it's broken :-)

My very early C differs from the one in the article in that

1) it seems to be made out of gold :-)

2) the processor board is held down with what look like gold nuts on the bottom posts.

3) The connectors between the display and the keyboard PCB appear to be in a poor way...

With the calculator face down and the display closest to me, there are three sets of connectors.

the first has 6 connections, the next 7, and the last 5 or 6??

The furthest right of the right most set is missing. From the look ogf the board under a loupe, there may have been a link here at one stage. The images on the page I was referred to are a little difficult to make out, but it appears that the connectors on the CV pictured may be a little different????

Two of the other connections have lifted from the board, but are still connected to the tracks (which have also slightly lifted

One further connection has lifted from the pad.

All of the connections on the keyboard PCB appear to be OK, except for the missing link.

I am planning to resolder the link that has lifted from the display board, and replace the missing link. The tracks that have lifted can stay as they are.

Before I attempt this:

Is the last link really missing, or was this link not installed?

Has anyone done a repair like this, and if so, do you have any advice.

Oh, and the posts appear to be in great condition, although the screws at the top were quite slack (i.e. I could tighten them almost half a turn before meeting resistance (this tightened up the case nicely, but failed to fix the problem)

[steve]

I went ahead and fixed the one broken connector, but not the missing one.

The calculator had been without batteries for some time (about an hour), but didn't even MEMORY LOST on me!

The unit now works pretty much as expected, but I'm still concerned about the lack of the link.

The pads look like the link was once there (i.e. they've got that crystaline appearance of a broken or dry solder joint)

[Dave]

The "missing link" you refer to being the tab on the display at the far right when face down, display closest to you does not connect to the circuit board. I have seen many of these and none of them were connected.

[steve]

Thanks for the information.

For anyone who's having odd problems with their HP41. My symtoms also included the calculator beeping at odd times, and random corruption of memory, things that I've heard others complain of here.

To fix these small connectors I wrapped a piece of copper wire (about 1mm diameter) around my soldering iron and used it as the tip to solder the tiny connector. (and very fine solder (0.7mm))

I also had the soldering iron unplugged while fixing the joint as I wanted to minimise the risk of electrostatic damage.