Total Repair

[PANAMATIK]

No, this is not the story of a secret agent on mars, saving the world.

But it is the Total Repair story of my last non functioning HP-21 in my Woodstock collection.

This HP-21 calculator had a non functional switching power supply unit, measuring only 2 Volt instead of the usual 6,4 Volt. My repair skills for power supplies were not good enough at the time when I bought this calculator one year ago, and I decided to use it as a display driver and ACT spare part unit for three other Woodstock calculators, which could be repaired. At last all chips were removed, leaving a bare board as result, waiting for what to come.



HP-21 bare board. right: the display drivers occupying the space below the LED display, left: the ACT also removed.

Now after having made a replacement not only for the ACT but also for both display drivers, I thought I could replace all three chips and bring this calculator back to life again. The idea was to replace the ACT by the new ACT and anode driver and cathode driver by a modified ACT hardware, and let communicate them like the original parts.

Before I could be successful the power supply unit had to be repaired. Normally the two transistors had to be replaced by new ones. When I started to replace the transistors I recognized, that they were not original! Not the usual 4-071 and 4-055, but a normal todays BC337 NPN type. Then I recognized, that they were put in the wrong way, emitter and collector were exchanged. Somebody has tried to repair this unit and did it not well. Not knowing the condition of these transistors, I replaced them by BC338, which I had at hand. But still it didn't work. Then I removed the Zener diode by a new Zener 7,2 Volt. This made the power supply work again. And finally after having connected many wires to connect the three replacement boards I could repair this calculator.



Top and bottom wired with three replacement boards. Most of the wires are the 12 digits and 8 segments of the display, then the communication lines ISA, SYNC, RCD STR and PHI1 clock.

I used an HP-25 keyboard and programmed the HP-25E firmware, which upgraded the HP-21 to an HP-25, but I can go back to HP-21 or forward to an HP-29C as well.

Then I realized, that there were no more PMOS chips inside the calculator - all were replaced by todays semiconductors. And I realized, that the -12V and 6.4Volt were not longer necessary at all. The circuits could run with the single battery power. And further more, I could replace the replacements by my HP-01 board, which was already designed to drive a 9-digit display, and had three spare I/O pins for driving a 12-digit display. After some moments I started to completely remove all wires and solder new wires from a HP-01 board to all segments and digits. And after another half day of soldering I got this result.



Now this is the "Total Repair" of an HP-21, inside an HP-01 board, running the HP-25 firmware. And a surprising new feature!! As I could disconnect the internal switching power supply completely, the calculator drives only 20 mA, when display is on, compared to 180 mA original, and only Microamps when display is off.

Now I have to finish the software for my first Low Power HP-25E. Equipped with two AA batteries it will last for 100 hours and for years when using sleep mode and the processor register state is always saved. Still the software is not finished, but there is no doubt, it is the first Low Power LED calculator of the Woodstock series. Perhaps not the last.

Bernhard

[Massimo Gnerucci]

You're simply amazing Bernhard!

Hats off.

[rprosperi]

Thanks for details on another amazing rescue Bernhard! This is the most extreme recovery yet I think.

On the top photo, the 21 board left side has a socket where the ACT was removed. Did any/all Woodstocks come with socketed ACT chips or were they all soldered? Maybe this one had a socket from the original repair attempt?

[PANAMATIK]

(06-24-2017 01:47 PM)rprosperi Wrote:  Thanks for details on another amazing rescue Bernhard! This is the most extreme recovery yet I think.

On the top photo, the 21 board left side has a socket where the ACT was removed. Did any/all Woodstocks come with socketed ACT chips or were they all soldered? Maybe this one had a socket from the original repair attempt?

All chips were soldered originally. I myself inserted the sockets in the image.

Thanks for appreciating the Total Repair

Bernhard

[Dan B]

Amazing job and congratulations! Thank you for all the details also.

[PANAMATIK]

(06-24-2017 05:28 PM)Geir Isene Wrote:  This is pretty crazy :-)

Another idea: The RTC chip is not yet assembled on the HP-01 board, as seen in the image above. But this can be done, which will create a Woodstock calculator with integrated Real Time Clock. The On/Off switch has no function, because the calculator can be left On all the time, it could switch between the HP-25 and HP-01 firmware instead. Also the piezo buzzer can be attached for alarm.

This HP-21 will become a very special/crazy one.

Bernhard

[Dan B]

You are going to make me want one of these hybrids...

[Paul Dale]

Amazing work.


Pauli

[PANAMATIK]

(06-25-2017 03:35 AM)Paul Dale Wrote:  Amazing work.

Pauli

Thanks Pauli!

Success! After some more work this weekend to adapt the display scan and keyboard scan routines and the slide switch, I just got the HP-25E firmware with all its features running on this calculator.

The Total Repair is complete!

(06-25-2017 01:44 AM)Dan B Wrote:  You are going to make me want one of these hybrids...

I worked already significantly on a concept, which should make this Woodstock Low Power version available for everybody, it included a fast ARM processor with 128k ROM and a USB interface, but I cancelled the idea due to high manufacturing costs for the necessary multilayer board.

Bernhard

[PANAMATIK]

(06-25-2017 11:09 AM)PANAMATIK Wrote:  Success! After some more work this weekend to adapt the display scan and keyboard scan routines and the slide switch, I just got the HP-25E firmware with all its features running on this calculator.

The Total Repair is complete!

I'm quoting myself, because this statement was not really true. The repair couldn't be called complete at all. I had a lot of work ahead, not knowing how much, when I wrote the above lines. The hardware possibilities revealed an amazing new calculator.

Definitely this totally repaired and converted to an "HP-25 LP" calculator (LP for Low Power) will become my favouite LED calculator. It has fantastic new features.

1.) Power consumption is between 2-19 mA when display is on and 0.2 mA when in sleep mode (10000 hours with 2000mAh AA batteries).
2.) It has full alphanumeric capabilities.
3.) It can print on the HP82240B printer like the normal HP-25E
4.) I added the Real Time Clock chip, it provides accurate time, date and alarm time. Time, date or alarm time entry is very easy with the keyboard, just enter 6 digits and ENTER.
5.) The stopwatch can count upwards, downwards and you can enter values via keyboard. It is controlled by the RTC and hence very accurate.
6.) I added an alarm buzzer. The sound it very clear and you can use the HP-25 to wake you up every morning, the first ever red LED calculator with sound.
7.) I added the first ever alphanumeric program display of a red LED calculator. This changes the calculator completely. You can read your program steps. It works within PRGM mode and when SST BST R/S are hold down in RUN mode.
8.) All features of the HP-25E including 100 user programs, 100 constants, 88 registers can be stored in flash memory
9.) I made also a version "HP-29C LP" (Low Power).



The RTC (Real Time Clock) chip mounted and piezo buzzer attached, to be seen on these images of the prototype.



"HP-25 E LP" shows the actual time and date, like the HP-01 watch, but in time display it cannot display the semicolon.



"HP-25 LP" shows program steps in plain text, followed by a number or the key code. This is just an example. I never will miss this display in the future.

I also like to mention, that the ON/OFF switch is not longer connected. You can see in the images that it is in OFF position. The calculator is always ON and enters sleep mode when not used for some minutes or with f + division key sequence (like the pioneer LCD calculators, using the lower left button).

I will think about a way from the prototype to a small circuit board for everyone.

Bernhard

[PANAMATIK]

(07-15-2017 09:31 PM)Geir Isene Wrote:  Just WOW.

Thanks Geir!

But there is a drawback: It will not be compatible with GPS At least not this hardware configuration.

Bernhard

[BobVA]

(07-15-2017 09:12 PM)PANAMATIK Wrote:  I will think about a way from the prototype to a small circuit board for everyone.
Bernhard

Perfect! I love every feature on this and think I'd use most of them daily. I hope you can sort out a practical production design.

Suggestion for the ON/OFF switch: Perhaps make it a STANDBY / LOCK switch that disables power-up but keeps the memory state, to prevent the calculator from turning on accidentally while in the case?

Bravo!
Bob

PS I'd say you're going to bankrupt me, but there's the finite possibility this will cause the other Woodstock fans to buy up all the remaining ones at auction, saving me from financial ruin.

[PANAMATIK]

(07-15-2017 11:01 PM)BobVA Wrote:  Suggestion for the ON/OFF switch: Perhaps make it a STANDBY / LOCK switch that disables power-up but keeps the memory state, to prevent the calculator from turning on accidentally while in the case?

This is a brilliant idea! And it is feasible with this hardware! Thanks for the suggestion. I will test it tomorrow.

(07-15-2017 11:01 PM)BobVA Wrote:  PS I'd say you're going to bankrupt me, but there's the finite possibility this will cause the other Woodstock fans to buy up all the remaining ones at auction, saving me from financial ruin.

I wonder how many "Woodstocks" are remaining at auction. Does anybody know how many were produced? And how many are defective today. And how many were thrown away during the years.

Bernhard

[Paul Dale]

I see woodstocks coming up for auction quite a bit. I don't think there is a lack of supply yet.

I definitely want one of these

What character are you using for 'X'? There are more than a few of these in the command names. There is one 'k' and no 'm', 'v' or 'w' (m and w are easy with two characters but not present, k is kind of possible but not necessary here since CLST works instead of CLSTK)

Pauli

[rprosperi]

Amazing Bernhard, you've outdone yourself!

I generally prefer 'stock' machines, but this list of features is just too cool to pass up; I want one when they are ready.

Hopefully, the date function will no longer allow 51 days in July, but then again it will probably be past July 51st before the upgrade kit is ready.

Incredible accomplishment.

[Geoff Quickfall]

I go away to study for a week and come back to this.

Bernhard, another trip to Mainz is required :-)

Incredible!

Geoff

[PANAMATIK]

(07-16-2017 12:47 AM)rprosperi Wrote:  Hopefully, the date function will no longer allow 51 days in July, but then again it will probably be past July 51st before the upgrade kit is ready.

Ha, this was a bad coincidence. When I took the photo of the time display saturday afternoon it could be read as date. I will display a 4-digit "2017" in the final version and date display can be chosen between european and american format. But you could be right that the Low Power kit will be ready around july 51st

Bernhard

[PANAMATIK]

(07-16-2017 12:35 AM)Paul Dale Wrote:  What character are you using for 'X'? There are more than a few of these in the command names. There is one 'k' and no 'm', 'v' or 'w' (m and w are easy with two characters but not present, k is kind of possible but not necessary here since CLST works instead of CLSTK)

The only critical characters are the arithmetics +, *, div and the letters W,X. W is special and X is the same as back slash. And you will learn very fast to distinguish that U is capital and v is small letter, like M and n. Small letter k and capital W are also not too bad, at least they are unique. + and * are a little bit similar and division is the normal slash, but there is always to accept a compromise. I could have used PLUS MULT and DIV instead, but then STO + 0 would be too long. % is written as PERC (Percent).



The 7-Segment alphabet.



Some Mnemonics examples.

Bernhard

[Paul Dale]

Thanks for the quick response and it looks great.

For arithmetic STO, PL MU DI would fit. For the standard operations the longer names PLUS, MULT, DIV work.


J might improve with an extra segment and if G loses one it is distinguishable from 6:

Code:

 J    G
      _
  |  |
|_|  |_|

A closer approximation to X is possible if H is changed:

Code:

 H    X

|_   |_|
| |  | |

Two digits are good for M and W (not that either is required on the 25):

Code:

 M         W
 _   _
| | | |   |     |
|     |   |_| |_|

The arrows in X<>Y could be either of:

Code:

 _    _
|      |
 _|  |_

c doesn't look quite right but these aren't much better.


Pauli

[rprosperi]

(07-16-2017 07:57 AM)PANAMATIK Wrote:  Ha, this was a bad coincidence. When I took the photo of the time display saturday afternoon it could be read as date. I will display a 4-digit "2017" in the final version and date display can be chosen between european and american format. But you could be right that the Low Power kit will be ready around july 51th

LOL, the joke is on me, I thought those 2 screens were different date formats; Looking closer now, I can see the "." is a time separator and "-" for dates.

OK, July 51st it is, I'll have an HP-25 ready for conversion.

Are both HP-25 and -25C suitable for conversion?