HP-41CV power-on weirdness

[Adam Vaughn]

Hello everyone. I recently picked up a HP 41CV with no included modules at a hamfest:
   
The seller told me that other than a small amount of corrosion in the battery holder, it was working. I cleaned as much of the corrosion off the battery spring as I could, put in four N cells, and tried to power it up.

The first thing I was met with was a continuous tone from the speaker, which stopped after I removed the battery holder. I then did some more cleaning of the spring, made sure I'd inserted the batteries into said holder (I had), and tried again. This time, I got the expected "MEMORY LOST" message (the only time I've seen it) on the display, which went away after I pressed the "ON" button, but I couldn't get the display to come up again after any amount of pressing "ON".

After doing some Googling, I found an archived post from this forum with ideas, which I tried. Nothing worked until I played around with holding down the Backspace key while pressing the ON button several times, which brought up "CLX" on the display, followed by 0.0000, and the calculator seemed to work normally. Pressing the "ON" button again turned it off, but instead of the problem being fixed, I have to press one of the keys on the keypad (except for the yellow SHIFT key, see below) every time I want to turn the calculator on.

In addition, there's some weirdness in conjunction with this already weird power-up sequence. For example, let's say I hold Backspace long enough that the display shows "CLx" then "NULL", release Backspace, then press "ON" to turn it off. If I then press "ON" again without pressing any other buttons, the display shows "NONEXISTENT", which then goes away if I press "ON" again, or Backspace brings it back to 0.0000. This same sort of thing also happens if I press SHIFT+4 for the BEEP function; pressing "ON" to turn the calculator off, then pressing "ON" again sounds the BEEP tones again, but nothing else happens. Speaking of SHIFT, it nor the other buttons under the display next to the "ON" button bring the calculator up when pressed along with "ON".

I'm sure I haven't found all the weird things associated with this calculator's operation as of yet, but figured I'd throw this info out there as a start. Any ideas as to why it might be acting this way? Thanks in advance!
-Adam

[mfleming]

Internal contact problems are usually the source of failure in 41's. A quick check would be to place the calculator on a flat surface and then press down evenly around the top of the calculator with one hand while pressing the ON or other buttons with the other hand. Two fingers on the top corners and thumb at the center bottom should press the two halves of the case together enough to compress the contacts inside. If that works then the fun begins...

[Adam Vaughn]

(11-08-2022 09:52 PM)mfleming Wrote:  Internal contact problems are usually the source of failure in 41's. A quick check would be to place the calculator on a flat surface and then press down evenly around the top of the calculator with one hand while pressing the ON or other buttons with the other hand. Two fingers on the top corners and thumb at the center bottom should press the two halves of the case together enough to compress the contacts inside. If that works then the fun begins...

Thanks for the reply. I tried what you suggested, but it has yet to cause the "ON" button to work as it's supposed to. It definitely seems to be related to pressing a non-SHIFT key on the keypad as I press "ON"; I tried using a pen-cap to apply pressure between keys as I pressed "ON", but got no results. I thought there was a sensitive area near the SIN key, but I think it was just getting pressed down slightly during some of said pressure tests, as there's no response from "ON" if no keys are being pressed down at all.
-Adam

[rprosperi]

Internally, there are screw posts at both the top and bottom of the case (under the 4 rubber feet), and any one of the posts being broken can lead to the erratic behavior you're seeing.

Instead of trying to hold down the entire case at once, try these:

* Short the left- and right-most battery terminals
* Insert battery pack with new batteries (check voltage to be sure)
* Put the 41 onto a hard, flat surface (table)
* Press down on the side rails on both sides of the top row of keys, and while holding those try pressing [ON], etc. Don't press so hard as to deform the plastic or risk flipping it on it's side; firm, but not hard.
* Press down on the side rails on both sides of the 2nd row of keys from the bottom, and while holding those try pressing [ON], etc. Don't press so hard as to deform the plastic or risk flipping it on it's side; firm, but not hard.

Also, try [ON]+[<-] to cause ML, it may help.

If it will not power on, there is likely some internal damage...

[Adam Vaughn]

(11-08-2022 10:34 PM)rprosperi Wrote:  Internally, there are screw posts at both the top and bottom of the case (under the 4 rubber feet), and any one of the posts being broken can lead to the erratic behavior you're seeing.

Instead of trying to hold down the entire case at once, try these:

* Short the left- and right-most battery terminals
* Insert battery pack with new batteries (check voltage to be sure)
* Put the 41 onto a hard, flat surface (table)
* Press down on the side rails on both sides of the top row of keys, and while holding those try pressing [ON], etc. Don't press so hard as to deform the plastic or risk flipping it on it's side; firm, but not hard.
* Press down on the side rails on both sides of the 2nd row of keys from the bottom, and while holding those try pressing [ON], etc. Don't press so hard as to deform the plastic or risk flipping it on it's side; firm, but not hard.

Also, try [ON]+[<-] to cause ML, it may help.

If it will not power on, there is likely some internal damage...

Thanks for the reply. I tried your suggestions, with the following results:

• How long are you supposed to short the terminals? I tried holding a piece of foil between them for 30 seconds
• Checked the N cells, all four test good in a RadioShack battery tester
• No message appeared on the screen when the battery holder was re-inserted
• Held the side rails on both sides along the top row, pressed "ON" several times, no results
• Tried holding the side rails again at the second row, pressed "ON", no results. Did the same at each other row, again, nothing
  

Pressing another key definitely seems like a pre-requisite to getting this unit to power up. Unlike what I stated earlier, the "ALPHA" button is one of the buttons I can hold down as I press "ON" to get it to power up. Holding Backspace and pressing "ON" has yet to result in the "MEMORY LOST" message; I get "CLX" on the screen, followed by "NULL" if I hold it long enough. Sometimes I do get "DATA ERROR", though that might coincide with trying to power it up while holding down "PRGM", which has at least once resulted in a "PRIVATE" message along with the "PRGM" flag.
-Adam

[rprosperi]

That was quick, but unfortunately confirms the problems are probably deeper than simple broken posts, however there is a chance it is only dirty connections, as seeing instructions like CLX followed by NULL means the CPU (and display drivers, etc.) is working, at least to some extent.

You're going to have to open the unit and minimally clean the zebra or ribbon connectors; it's likely it's not that simple but without opening it, you'll never get to the bottom of the problem.

Are you handy with electronics? If not, I'd say find someone who has experience working on a 41, as even the act of opening the unit can lead to post damage and you could end up worse off. If you want to do the work, get a copy of Jose's excellent book "HP41 Repair: A beginner's guide to repairing your HP41 calculator", available from Amazon, and also check out TheCalculatorStore, which carries a wide selection of quality repair parts for almost any 41 repair issue.

[AndiGer]

Hey guys,
Did you see it is a halfnut? So: no zebra, no lower post problem (at least operating the machine).
I suppose dirty (corrosion) on the I/O block connections inside.
Andi

[BillBee]

For Half-nut issues I always suspect the case back first (if the corrosian is not too bad). Take off the top two feet and see if the screw lugs have sunk. Usually this causes a movable gap in the sides of the calc. When the case back breaks pressure on the ribbon cable is eased off and can cause the issues you describe.

If the back is good then I move on to the ribbon cable and mainboard contacts.

Be careful if you open up the calc. The buzzer is glued on the case back and the wee wires are not very long.

-Bill