HP 21 Woodstock Missing Decimal Point

[Sanjeev Visvanatha]

Hello,

I have a HP-21 that is not displaying the left-most decimal point. All other digits and decimals appear to be working correctly.

Could I get your thoughts on whether this is repairable? I have not opened a Woodstock before.

Thank you.

[Harald]

(01-06-2016 02:17 AM)Sanjeev Visvanatha Wrote:  Hello,

I have a HP-21 that is not displaying the left-most decimal point. All other digits and decimals appear to be working correctly.

Could I get your thoughts on whether this is repairable? I have not opened a Woodstock before.

Thank you.

This could be either a contact problem or display problem.

Disasembling a woodstock is very simple:
- Remove the battery pack
- Pull out the two rubber feet under the display
- Remove the two screws under the feet
- Slightly push the PCB upward at the battery terminals, the keyboard should lift slightly so you can grab it at the display end
- Pull the keyboard away towards the top of the calculator, lifting it just enough to clear the back case
- Now you should have disassemble the calculator

Have a look at the contacts between the LED Display and the main board. Unpluge the display and plug it back in. If it was a connection problem, it might be working again. If not, do you have a Multimeter with diode tester? That is perfect to check if all display segments work.

[Sanjeev Visvanatha]

(01-06-2016 09:54 AM)Harald Wrote:  Have a look at the contacts between the LED Display and the main board. Unpluge the display and plug it back in. If it was a connection problem, it might be working again. If not, do you have a Multimeter with diode tester? That is perfect to check if all display segments work.

Thank you for the instructions on dissembling the calculator! That was easy enough.

I unplugged the display from the board, cleaned the contacts, and then set it back again. Problem persists.

I do have a DMM with a diode tester, although I have not used that function to-date.

[jebem]

To test the LED segments I use a 3Volt battery holder (2 x AA or AAA) and a 470 Ohm series resistor to be on the safe side and not burn the LEDs.

[Sanjeev Visvanatha]

Just to clarify, from the pinning of the display (http://www.keesvandersanden.nl/calculators/led.php), the absolute left-most digit is only used to display the minus sign (although it has all the segments the other digits have). The left most digit that is visible when using the calculator is digit 11. My Digit 11 does not display the decimal point.

Thank you for the suggestion on using the AA battery pack. As I said, my DMM has a diode function which I now know how to use (very simple).

Decimal 12 (where the minus symbol is located) - shows decimal lights up




Decimal 11 (non-working decimal)




Digit 11 (non-working decimal)




Digits 11 and 10




Is this fixable, or do I need a donor display?

[jebem]

I can't find now the references on how to repair these 7-segments displays, but assuming it is failed you may have nothing to lose by trying to repair it.

One possible cause for this failure is related to battery leaking corroding the fin LED wires.

If I remember well, it is possible to remove the plastic lens (with great care!) in order to access the actual LED wires and try to repair it.
A microscope is in order here.

Another much easier option would be to ask Panamatik for a display replacements here.

But of course if you really are into diy (like myself), you can buy 3 HP QDSP-6064 7-segemnt bubble LED chips, then totally remove the old leds from the PCB and hard-wire thee 3 new chips in place.

I got my HP QDSP-6064 from this German supplier recently at a excellent price, thanks to Harald who has shared the supplier's link.

Please note that I'm not sure if they will work on the HP-2x series as I had not the chance to validate it.

And thinking speed of repair and lower costs, Panamatik offering is a safe bet and very competitive price against any other options that I mentioned.

[emece67]

(01-07-2016 08:44 AM)jebem Wrote:  One possible cause for this failure is related to battery leaking corroding the fin LED wires.

If I remember well, it is possible to remove the plastic lens (with great care!) in order to access the actual LED wires and try to repair it.
A microscope is in order here.

Sometimes, inspection with a watchmaker's lens can give a clue if the tiny aerial wire connecting the offending dot to the PCB strip is damaged. If it is still attached to the dot, but otherwise broken or desoldered from the PCB, it may be repairable by soldering a new (thicker) wire to the PCB and soldering it to the remaining tiny wire. Use old Sn/Pb mix to solder, as the tiny wire does not support well the high temperatures needed by modern mixes.

I've repaired some segments in a 25 in such way.

Regards.

[Sanjeev Visvanatha]

3-2-1 Detach!

I had some time, and luck to get the plastic bubble cover off the PCB holding the LEDs. With a very sharp Exacto blade, I lifted the very thin 'mushroom' head from the stakes holding the bubble cover in place.

While working, the best optical magnification I could get was about 5x. It was sufficient for me to confirm that the tiny wire is connected to the PCB, but detached from the LED.

I took a digital photo, which shows it rather well. The offending digit is the second from the top of the photo (click to enlarge):

(01-07-2016 12:03 PM)emece67 Wrote:  Sometimes, inspection with a watchmaker's lens can give a clue if the tiny aerial wire connecting the offending dot to the PCB strip is damaged. If it is still attached to the dot, but otherwise broken or desoldered from the PCB, it may be repairable by soldering a new (thicker) wire to the PCB and soldering it to the remaining tiny wire. Use old Sn/Pb mix to solder, as the tiny wire does not support well the high temperatures needed by modern mixes.

Indeed, I think I would have more confidence in repairing a situation as you describe. As it is, it appears the tiny wire is detached from the dot, which I think is beyond my skill level to repair

(01-07-2016 08:44 AM)jebem Wrote:  Another much easier option would be to ask Panamatik for a display replacements here.

But of course if you really are into diy (like myself), you can buy 3 HP QDSP-6064 7-segemnt bubble LED chips, then totally remove the old leds from the PCB and hard-wire thee 3 new chips in place.

And thinking speed of repair and lower costs, Panamatik offering is a safe bet and very competitive price against any other options that I mentioned.

Panamatik's replacement display is a very good option in this case. I was not aware that he had devised this neat solution!

If there were single digit, 7 segment LED's available, I may be able to affect a repair with my skills. Doing the tiny soldering several times over is probably going to end up badly for me!

As it is, I have a personal machine. When there is no decimal point displayed, I know it is after the first digit!

[PANAMATIK]

(01-08-2016 01:58 AM)Sanjeev Visvanatha Wrote:  Panamatik's replacement display is a very good option in this case. I was not aware that he had devised this neat solution!

If there were single digit, 7 segment LED's available, I may be able to affect a repair with my skills. Doing the tiny soldering several times over is probably going to end up badly for me!

As it is, I have a personal machine. When there is no decimal point displayed, I know it is after the first digit!

When considering using my display solution you will get back all digits, but you will have slightly smaller digits and they have a minimal different look than the originals. If you like to find a donor display you can use any woodstock model from HP-21 to HP-29C, all have the same display. However you should try to get a non working non repairable model, otherwise you transfer the display problem just from one calculator to another.

Good luck
Bernhard