HP48S strange behavior

[Daniel Sole]

Hi all,

After many years having my old HP48S in the drawer, I missed using it again.

I have put new batteries and although it starts up perfectly and apparently all the buttons are good, it shows a strange behavior.

For example, if I press 1 + ENTER, it enters the stack as -1 (as negative). If I press 7, it seems to do the SIN function. If I press 0, the calculator enters the algebraic mode.

I dont know. Seems strange to me. It has always been stored in an appropriate place. When I found it, the batteries had made some salt, but nothing serious.

What do you think? Should I say "the king has died"? Tks!

[BartDB]

Hi,

You can try the following ways to reset the calculator:

https://www.support.hp.com/us-en/document/bpia5166

Although the title mentions the 48G, the procedures should be similar on the 48S

-Regards, Bart

[Daniel Sole]

Thank you very much, Bart.
I already checked all these with no result.
I think there must be some "hardware" issue. But it's quite a weird misbehavior. From an electronic point of view, I can only guess some kind of short inside between circuit tracks.
Maybe I will dare to try to open the calculator and take a look inside. I've got a technical background and this doesn't scare me at all.
Many thanks!
Daniel

(06-26-2019 12:50 PM)BartDB Wrote:  Hi,

You can try the following ways to reset the calculator:

https://www.support.hp.com/us-en/document/bpia5166

Although the title mentions the 48G, the procedures should be similar on the 48S

-Regards, Bart

[mfleming]

(06-26-2019 01:41 PM)Daniel Sole Wrote:  I think there must be some "hardware" issue. But it's quite a weird misbehavior. From an electronic point of view, I can only guess some kind of short inside between circuit tracks.
Maybe I will dare to try to open the calculator and take a look inside. I've got a technical background and this doesn't scare me at all.

The most common failure mode for old 48 series calculators is a loss of connectivity between the keyboard ribbon connector and the cpu board. The failure usually manifests itself as a loss of a row or column of keys. Try pressing down on the center of the calculator just below the screen. If the weird behavior goes away, you have a connector problem.

The fix is fairly easy though opening the case is not. There are instructions (with pictures) posted in the old articles forum and in hpcalc.org directories. I followed member Han's detailed directions with success. With a little care you can have your old calc operating again.

~Mark

[Daniel Sole]

Thank you very much, Mark. It didn't work but that was a good try indeed.
I'll look for Han's instruction and carefully follow it. I'll keep you posted upon fail or success.
Tks & Bst Rgds!

Daniel

(06-26-2019 02:31 PM)mfleming Wrote:  

(06-26-2019 01:41 PM)Daniel Sole Wrote:  I think there must be some "hardware" issue. But it's quite a weird misbehavior. From an electronic point of view, I can only guess some kind of short inside between circuit tracks.
Maybe I will dare to try to open the calculator and take a look inside. I've got a technical background and this doesn't scare me at all.

The most common failure mode for old 48 series calculators is a loss of connectivity between the keyboard ribbon connector and the cpu board. The failure usually manifests itself as a loss of a row or column of keys. Try pressing down on the center of the calculator just below the screen. If the weird behavior goes away, you have a connector problem.

The fix is fairly easy though opening the case is not. There are instructions (with pictures) posted in the old articles forum and in hpcalc.org directories. I followed member Han's detailed directions with success. With a little care you can have your old calc operating again.

~Mark

[BartDB]

Hi Daniel,

It could be that the battery leakage has got onto the electronics or worse into the keyboard.

Attached is a PDF of Han Duong's opening the HP-48 guide.

  Opening the HP48.pdf (Size: 905.23 KB / Downloads: 20)

However, he uses a blunt metal tool, I usually use a plastic tool found in mobile phone repair kits:
Although they are prone to breaking I find they do less damage.
   

Once open, you'll have to undo the twist tabs to remove the PCB from the keyboard - the screen can be removed as well.

Check & clean the PCB. If the keyboard needs cleaning I suggest you not take it further apart as the plastic rivets are almost impossible to restore/replace. Instead I suggest you try the "wash and dry" procedure described by Geoff Quickfall (for a different calculator, but the principle is the same) in message #15 of this thread: https://www.hpmuseum.org/cgi-sys/cgiwrap...694#175788

As you have the calculator open, I suggest you replace the connector sponge. Just below the screen you will see the keyboard connector flex-pcb.
   
Underneath is a grey sponge strip that pushes it onto the main PCB. This sponge deteriorates and causes a bad connection. It is best to replace the sponge strip it while you can.

When re-attaching the PCB, the twist tabs have to be done up securely to prevent missing lines on the LCD (make sure the zebra strips and connections are squeaky clean). (doing this I always sweat a bit - the balance between doing it tightly and not breaking the tabs)
.

[Brad Barton]

(06-28-2019 05:03 PM)BartDB Wrote:  Underneath is a grey sponge strip that pushes it onto the main PCB. This sponge deteriorates and causes a bad connection. It is best to replace the sponge strip it while you can.

What is a good material to use to replace the sponge strip> I've tried a few different pieces of foam I've had lying around, but none see just right. Any pointers on the best foam to use?

Brad

[ijabbott]

(06-29-2019 01:59 AM)Brad Barton Wrote:  

(06-28-2019 05:03 PM)BartDB Wrote:  Underneath is a grey sponge strip that pushes it onto the main PCB. This sponge deteriorates and causes a bad connection. It is best to replace the sponge strip it while you can.

What is a good material to use to replace the sponge strip> I've tried a few different pieces of foam I've had lying around, but none see just right. Any pointers on the best foam to use?

Brad

Logan West used a couple of layers of Scotch double sided foam mounting tape in his repair videos for HP-32SII (https://youtu.be/Uo4EZG0NQbc) and HP-20S (https://youtu.be/G8wS95wK4bI). He cut the tape into thin strips with a craft knife and stuck two layers together to get the right thickness.

[mfleming]

I've used a strip cut from an old mousepad. The plastic phone repair tool is definitely the way to go, and will avoid marring the calculator case.

[Daniel Sole]

This is of great help, Bart. Thank you very much.
I've got those plastic pieces from iPhone repair kits.
I will start trying this repair right away. Keep you posted!
Tks & Bst Rgds
Daniel

(06-28-2019 05:03 PM)BartDB Wrote:  Hi Daniel,

It could be that the battery leakage has got onto the electronics or worse into the keyboard.

Attached is a PDF of Han Duong's opening the HP-48 guide.


However, he uses a blunt metal tool, I usually use a plastic tool found in mobile phone repair kits:
Although they are prone to breaking I find they do less damage.


Once open, you'll have to undo the twist tabs to remove the PCB from the keyboard - the screen can be removed as well.

Check & clean the PCB. If the keyboard needs cleaning I suggest you not take it further apart as the plastic rivets are almost impossible to restore/replace. Instead I suggest you try the "wash and dry" procedure described by Geoff Quickfall (for a different calculator, but the principle is the same) in message #15 of this thread: https://www.hpmuseum.org/cgi-sys/cgiwrap...694#175788

As you have the calculator open, I suggest you replace the connector sponge. Just below the screen you will see the keyboard connector flex-pcb.

Underneath is a grey sponge strip that pushes it onto the main PCB. This sponge deteriorates and causes a bad connection. It is best to replace the sponge strip it while you can.

When re-attaching the PCB, the twist tabs have to be done up securely to prevent missing lines on the LCD (make sure the zebra strips and connections are squeaky clean). (doing this I always sweat a bit - the balance between doing it tightly and not breaking the tabs)
.

[Daniel Sole]

Hi all,

Got the the calculator almost dismantled. I admit it's been a difficult work. Mine is one of those older ones from Singapore, as far as I can see engraved on the back cover. Iphones are much easier to dismantle!

I have also removed the PCB. What a piece of art, my lord! All circuits there, are perfect. No problem at all in there.

So my only possible try is that "wash and dry" process for the keyboard. But I haven't find the way to remove the display.

How do you remove the display? Do you remove it together with the metallic frame? Or you someway disconnect the display itself? How?

If I'm able to remove the display, then I'll sink the complete keyboard into water. If not, I will maintain the display out of the water, as Geoff Quickfall explains.

Thanks for all your help!

Bst Rgds from Daniel

(06-28-2019 05:03 PM)BartDB Wrote:  Hi Daniel,

It could be that the battery leakage has got onto the electronics or worse into the keyboard.

Attached is a PDF of Han Duong's opening the HP-48 guide.


However, he uses a blunt metal tool, I usually use a plastic tool found in mobile phone repair kits:
Although they are prone to breaking I find they do less damage.


Once open, you'll have to undo the twist tabs to remove the PCB from the keyboard - the screen can be removed as well.

Check & clean the PCB. If the keyboard needs cleaning I suggest you not take it further apart as the plastic rivets are almost impossible to restore/replace. Instead I suggest you try the "wash and dry" procedure described by Geoff Quickfall (for a different calculator, but the principle is the same) in message #15 of this thread: https://www.hpmuseum.org/cgi-sys/cgiwrap...694#175788

As you have the calculator open, I suggest you replace the connector sponge. Just below the screen you will see the keyboard connector flex-pcb.

Underneath is a grey sponge strip that pushes it onto the main PCB. This sponge deteriorates and causes a bad connection. It is best to replace the sponge strip it while you can.

When re-attaching the PCB, the twist tabs have to be done up securely to prevent missing lines on the LCD (make sure the zebra strips and connections are squeaky clean). (doing this I always sweat a bit - the balance between doing it tightly and not breaking the tabs)
.

[rprosperi]

(06-29-2019 08:45 PM)Daniel Sole Wrote:  Got the the calculator almost dismantled. I admit it's been a difficult work. Mine is one of those older ones from Singapore, as far as I can see engraved on the back cover. Iphones are much easier to dismantle!

That's true but at least the HP machines use parts that can readily be handled by actual human hands. I had an iPhone apart for a repair earlier today where one cover (over some connectors to keep them in-place) has 4 screws, one on each corner, all of different length, the longest being 2mm and the shortest 0.3mm. That's ridiculous!

[BartDB]

Hi,

The display lifts out of the metal frame. As I recall, it is stuck in with some double sided tape on the top and bottom of the display but with some careful pushing and prying it comes loose.

As you said you can wash with the display out of the water (but remove the zebra strips to prevent them falling into the water).

Regards
-Bart

(06-29-2019 08:45 PM)Daniel Sole Wrote:  Hi all,

Got the the calculator almost dismantled. I admit it's been a difficult work. Mine is one of those older ones from Singapore, as far as I can see engraved on the back cover. Iphones are much easier to dismantle!

I have also removed the PCB. What a piece of art, my lord! All circuits there, are perfect. No problem at all in there.

So my only possible try is that "wash and dry" process for the keyboard. But I haven't find the way to remove the display.

How do you remove the display? Do you remove it together with the metallic frame? Or you someway disconnect the display itself? How?

If I'm able to remove the display, then I'll sink the complete keyboard into water. If not, I will maintain the display out of the water, as Geoff Quickfall explains.

Thanks for all your help!

Bst Rgds from Daniel

[BartDB]

Hi,

Any suitably sized dense foam rubber. As Mark did, I cut a strip from a rubber mousepad.

(06-29-2019 01:59 AM)Brad Barton Wrote:  What is a good material to use to replace the sponge strip> I've tried a few different pieces of foam I've had lying around, but none see just right. Any pointers on the best foam to use?

Brad

[Daniel Sole]

Hi there, I think I must share my experience in an order others can know. I did the wash and dry process and, after assembling the calculator, I have found out that it has stopped to work.

My opinion is that I twisted too much the bottom part to be able to pry the rivets. In one of the attempts I accidentally entered the metallic piece I was using inside the keyboard with fatal result, I think. I don't think the wash&dry process has been the problem.

The truth is I regret having tried the repair. Anyway, calculator wasn't working right before my botched job.

I'll try to find an old calculator, maybe a not working one, and I will try to use the keyboard as a replacement part. I'm quite sure my problem is in there. If I have some kind of future success, I will post it here.

The funny thing is that on the while I've been working with this, I learned what HP did later: the 49G, the 50's, the Prime... I have purchased now a Prime and I'm going over my math!

Tks & Bst Rgds

[BartDB]

Hi,

Sorry to hear that the repair has not worked.

I would say that the 50g is the real successor to the 48, but unfortunately as production of it has stopped it has become quite expensive.

However with some patience you can get a working 48S/SX/G/GX on ebay for a reasonable price.

Also as you say a non working donor unit with good keyboard can be found too (and if you want to upgrade the screen to a higher contrast one, find a 39G or 49G as a screen donor)