HP 41CX Annunciators Become Visible with Long Inputs

[akpoff]

I have an HP41CX whose display causes the annunciators and other elements in the segments to become visible as you type longer inputs.

If I start with FIX 4 and the x-register is 0.000, the display looks just fine. As I begin to type in a longer number, particularly 8s, various annunciators become visible. It also affects the number segment.

You can see in the image below. I typed all 8s. The batteries are good and I did not have ALPHA selected, yet you can see them almost as clearly as the primary segments in numbers. As I type numbers the displayed annunciators change. E.g., if I stop at nine 8s, the PRGM and ALPHA annunciators are both visible, but typing the tenth 8 causes PRGM to disappear. 1s and 7s have very little effect on the display, 8s and 9s the most.

   

The calculator is a halfnut (edit: fullnut, see below) with nuts to hold the board to the zebra connector. I took the calculator apart and cleaned the zebra connector to be sure. The battery compartment is clean, no damage. I removed all modules and reset the memory. No change.

Any idea what's going on with this display?

--Aaron

[aurelio]

(01-21-2023 07:05 AM)akpoff Wrote:  I have an HP41CX whose display causes the annunciators and other elements in the segments to become visible as you type longer inputs.

[omissis]

The calculator is a half nut with nuts
--Aaron

Hi Aaron, I've not yet idea of what is going on with your calculator, still thinking about it..
anyway the calculator seems to be a fullnut and not "halfnut"as you wrote
regards

[akpoff]

(01-22-2023 02:41 AM)aurelio Wrote:  Hi Aaron, I've not yet idea of what is going on with your calculator, still thinking about it..
anyway the calculator seems to be a fullnut and not "halfnut"as you wrote
regards

Hi Aurelio,
You're right. I misspoke, as it were. It's a fullnut. Thanks! Here's a photo if that helps.

   

Further update for anyone else following along:

I took the board off and cleaned under the zebra connector. I also tested continuity from the soldered connections on the display to the board. I tested both where they connect near the display at top of the board, as well as tracing the connections as far as down they go and tested. Continuity is good AFAICT.

I exchanged battery packs with another calculator to exclude power issues. Both calculators worked as before with the swapped battery packs.

The problem persists no matter what I do.

I have a 41C with a fullnut board in it. I could try swapping them to see whether the issue is unique to the board or the display. Does anyone know whether swapping between models will work?

Cheers,

--Aaron

[brouhaha]

(01-22-2023 06:28 PM)akpoff Wrote:  I have a 41C with a fullnut board in it. I could try swapping them to see whether the issue is unique to the board or the display. Does anyone know whether swapping between models will work?

Fullnut displays are mostly interchangeable, but they did go through multiple vendors and revisions, so there two known issues, one mechanical and one electrical.

The really old revision F display driver, with the hybrid covered by a plastic cap, needs C2 on the logic board to be a 0.01 uF capacitor. The revision G driver needs a 470 pF capacitor. Having the wrong capacitor is only a serious issue if using the revision G driver with the .01 uF capacitor, which reportedly causes display flickering. This is from the 41C/CV/CX service manual, service note 41C-13, dated 11/21/79.

The mechanical issue is that some versions of the LCD use different clips and locators than others. This is covered in the same service manual, in service note 41C-58/41CV-19, dated 7/23/82.

[akpoff]

(01-22-2023 11:53 PM)brouhaha Wrote:  Fullnut displays are mostly interchangeable, but they did go through multiple vendors and revisions, so there two known issues, one mechanical and one electrical.

I should've been clearer. Rather than swapping displays, can I swap the two circuit boards between the 41C and 41CX to see whether display issue in the 41CX body is present with the 41C board, or whether the display issue appears in the 41C body with the 41CX board?

Cheers,

--Aaron

[brouhaha]

(01-23-2023 12:13 AM)akpoff Wrote:  I should've been clearer. Rather than swapping displays, can I swap the two circuit boards between the 41C and 41CX to see whether display issue in the 41CX body is present with the 41C board, or whether the display issue appears in the 41C body with the 41CX board?

Sure, but the capacitor issue is still relevant if one calcuator has a really early display, and the other does not. That won't damage anything, though.

[akpoff]

(01-23-2023 12:15 AM)brouhaha Wrote:  Sure, but the capacitor issue is still relevant if one calcuator has a really early display, and the other does not. That won't damage anything, though.

My evening is planned, then.

Thanks,

--Aaron

[brouhaha]

In my opinion, there's not a lot of reason to swap the logic board. I don't see any way in whcih a defective logic board could cause your display issue. The logic board doesn't drive the display segments. The serial data bus (phase 1 and phase 2 clocks, SYNC, ISA, DATA), power, ground, and a few other signals (e.g. DPWO) go to the display, and the two chips mounted on the display hybrid are the only things that connect to the LCD segments. That's why I previously thought you were talking about swapping the components of the LCD assembly, which could reveal whether the problem is on the driver PCB or the compression connector.

[akpoff]

(01-23-2023 12:20 AM)brouhaha Wrote:  In my opinion, there's not a lot of reason to swap the logic board. I don't see any way in whcih a defective logic board could cause your display issue. The logic board doesn't drive the display segments. The serial data bus (phase 1 and phase 2 clocks, SYNC, ISA, DATA), power, ground, and a few other signals (e.g. DPWO) go to the display, and the two chips mounted on the display hybrid are the only things that connect to the LCD segments. That's why I previously thought you were talking about swapping the components of the LCD assembly, which could reveal whether the problem is on the driver PCB or the compression connector.

Ha, success! It's fixed.

Based on your comment I began desoldering the connectors, right to left. As I was desoldering the 3rd connector from the keyboard side it popped off the display as well. I was using a Hakko desoldering gun so I wasn't leaving the heat applied for long, probably a cold solder joint. I had tugged on each one to be sure they were well attached and looked at them closely. All were shiny and passed a continuity test, but perhaps the connection was just secure enough to pass a mild tug test.

I reattached the 3 connectors, put the calculator back together and voilà, it worked.

   

I had suspected this unit had prior surgery because a couple of the traces on the keyboard had solder on them. Looking more closely at the display I see the same thing.

Thanks, brouhaha, for pointing me in the right direction.

--Aaron

[brouhaha]

(01-23-2023 01:24 AM)akpoff Wrote:  Ha, success! It's fixed.
...
Thanks, brouhaha, for pointing me in the right direction.

Congrats on the successful repair. I'm glad I could offer a little help!

[aurelio]

Aaron, I'm glad too reading of your success
Cheers
Aurelio

[akpoff]

(01-23-2023 08:50 PM)aurelio Wrote:  Aaron, I'm glad too reading of your success
Cheers
Aurelio

Thanks, Aurelio. I'm very happy about the result.