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This weekend I restored my HP-19C calculator. It was waiting a long time for this repair. When acquired it showed "Error" in the display for one second then got dark and started the printer motor erratically. When measuring current consumption it draw more than 500 mA, limited only by my lab power supply.

The repair had to be done in several steps. First I had to take the calculator apart. The HP-19C is the most complicated HP-calculator from that time for disassembly and assembly in my opinion. Second I had to draw a schematic of the complete calculator, as obviously the power supply or more was defective.

Third, I had to locate the problem and find spare parts.

For creating the schematic I had to remove all components from the PCB, because it is very difficult to measure the connections in a multilayer board with components assembled. Then I had to give names to all components.

[Image: uc?export=view&id=0Bwx8KUfOUL_RU2ROOWpvalRuV2M] [Image: uc?export=view&id=0Bwx8KUfOUL_RTmVENmQ0VUFnMkE] [Image: uc?export=view&id=0Bwx8KUfOUL_RTmdKZHlhTzYzNk0]

Then I reassembled the parts until the power supply generated 6.4V and -12V again. Two transistors were defective, I replaced them by BC338. Astonishingly all capacitors resistors and diodes were still OK, I replaced only the resistors by new ones. The diodes are normal silicon diodes 1N4002 and 1N4150 and one 6V zener and could have been replaced easily too.

[Image: uc?export=view&id=0Bwx8KUfOUL_RWlVvaWdyREk1X2M]

[Image: uc?export=view&id=0Bwx8KUfOUL_RQU5lR2NLTk42WHM] HP-19C schematic

After removing the printer from the circuit and simulating the ON/OFF switch, MAN/TRACE/NORM and the home switch, I could start the display for the first time, and the HP-19C could do calculations. When reattaching the printer, the motor was running normally but I didn't get a printout, at least the overcurrent condition no longer occured.

There is valuable information about the printer circuit in the HP-97 service manual, which is nearly identical in the HP-19C. One of the potentiometers responsible for printer intensity was corroded and one trace was completely interrupted by corrosion somewhere else. After having repaired the corrosion I got a normal print.

Very interestingly nearly all components for repairing the printer part of the HP-19C are available until today. The ICs are MPQ3904 quad transistor array and LM339 quad comparator. Only the strange brownish yellow 10-pin circuit is not available, it is a passive R.C.D. (Resistor Capacitor Diode) network, which normally doesn't get damaged. You can measure the functionality of your printhead just with a Resistor meter. Each of the 7 elements should have about 12 Ohm resistance, if you measure these values you can be rather sure that your printhead will be working. By applying just 2-3 Volt to the printer motor you can check whether the motor is running. It should draw about 150 mA. The home switch has gold plated contact areas and does work even after 40 years. If not, the motor will run indefinitely after startup.

Now I have to admit, that I made the repair with my second HP-19C and I removed all components from my first HP-19C, which is beyond repair, because it was totally damaged by under water storage for years in a sunken ship. But it would be possible to reassemble all parts and replace them by new parts as mentioned above.

And miraculously I bought a third HP-19C only a few days ago, which didn't need any repair at all, because it just needed new batteries and it is originally working like a charm. But now I cannot distinguish the original and the repaired HP-19C from outside. Here is an image of the nice ensemble.

[Image: uc?export=view&id=0Bwx8KUfOUL_RWERWRFlybThvUDA]

Congratulations and what a nice family photo!
Great work and a good description of the repair.

Great job and thanks for the nice pictures !

Best regards,

Thomas alias Dr. Mabuse
Thanks for the compliments!

When comparing the HP-19C printer with the HP-97 there is of course the smaller HP-19C paper size, but there are some other differences.

The printer mechanic of the HP-19C is made for an undirectional motor, which always runs in the same direction. If the motor is running continuously forward, the printhead is running back and forth, reversed automatically by a rather tricky mechanic. Therefore the motor electronics is designed more simple and cannot reverse the motor direction.

The HP-97 printer instead has a more sophisticated motor electronics, a so called H-bridge, which can reverse the motor direction any time. The mechanic would block on either side if the motor would run continuously in the same direction.

Although both printers are printing unidirectional, the HP-97 is faster, because when short print lines occur, the motor reverses earlier to get to its home position, while the HP-19C has to go the full distance to the end of the line and back.

Another HP-97 small improvement is the paper out switch, which produces an "Error" if the printer is out of paper. The HP-19C doesn't have a paper out detection.

Yes Bernhard,
That is always funny to see the connectors on the PCB titled OOPS - out of paper switch
A wondererwachsen if ever there was one!
(05-21-2017 11:19 PM)Geoff Quickfall Wrote: [ -> ]A wondererwachsen if ever there was one!

No idea what this means! Perhaps something positive?

(05-23-2017 06:00 PM)PANAMATIK Wrote: [ -> ]
(05-21-2017 11:19 PM)Geoff Quickfall Wrote: [ -> ]A wondererwachsen if ever there was one!

No idea what this means! Perhaps something positive?


Maybe "wondererwachsen" means "Wundererwachen" !?
Whaddaya think !?


Thomas alias Dr. Mabuse
Does anybody have a source of the HP-19C schematics from Tony Duell? Or the complete schematics set?

The opposite age to a wunderkind!

A wonder adult!

Bernhard, I will be in London three days before I see you. Will see if Tony can bring them to the HPPC meet and transfer them to me on a thumb drive.

I can give that to you the next day or so.
(05-24-2017 04:08 AM)Geoff Quickfall Wrote: [ -> ]Bernhard, I will be in London three days before I see you. Will see if Tony can bring them to the HPPC meet and transfer them to me on a thumb drive.

I can give that to you the next day or so.

Great plan! Looking forward to meet you.
HP-19C not yet repaired!

After having two successfully running HP-19Cs I still have my hopelessly corroded third calculator (actually this third calculator was the first I bought). I do not want yet give up to repair this underwater HP-19C. When I removed all components from the power supply/printer board last week, I did not really had in mind to reassemble them, but now I did. I ordered new components and completed the work today.

[Image: uc?export=view&id=0Bwx8KUfOUL_RdmlybWpGanVqX2s]

HP-19C power supply/printer board, all components removed

[Image: uc?export=view&id=0Bwx8KUfOUL_RVDhFOTFTQzUwVWM]

With new components assembled. All labeled components and resistors are new, they can be bought from todays electronic providers, even the flat ribbon connector.

The functional test is not yet performed. I will insert this board into one of my working HP-19C and see if it prints? Give me thumbs up please! Smile

Viel Erfolg!
[Image: djr0q7p5hsya2zm7u.png]

Kind regards,

Thomas alias Blofeld
(06-02-2017 06:04 PM)BobVA Wrote: [ -> ]Viel Erfolg!

Unbelievable! It works and prints! I am flatted!

Now I have to check the poor processor board.

Left original, right already removed bad components. I'm not sure whether also the big PIK chip is defective. If so, it will become quite difficult.

[Image: uc?export=view&id=0Bwx8KUfOUL_ReGoyLTVFR3pkLUE] [Image: uc?export=view&id=0Bwx8KUfOUL_ReElDYjJOLXR2WlE]

and make a replacement for the display.

[Image: uc?export=view&id=0Bwx8KUfOUL_RVG9FVDZaYXZhVmM]

This kind of corrosion was seen nearly everywhere inside this HP-19C. It must have taken a bath in salt water.

Seeing old machines rescued like this is so cool Bernhard, thanks for sharing this with us.

Best of luck with the processor board! It looks like it will take all your skills to pull that one off... but give it a shot. Then you can really brag!
(06-02-2017 08:25 PM)rprosperi Wrote: [ -> ]Best of luck with the processor board! It looks like it will take all your skills to pull that one off... but give it a shot. Then you can really brag!

Damn, I recognize I'm doing this already. Sad

(06-02-2017 09:07 PM)PANAMATIK Wrote: [ -> ]Damn, I recognize I'm doing this already. Sad

Not at all! Sharing these rescue stories with folks that love to see them is wonderful, we all feel good reading these 'saved from needless death' stories and you should be justifiably proud of making such rescues.

I'm saying that if you can salvage that processor board and get it working, after spending much time underwater, it will be a near-miracle, and then bragging would be just right!
HP-19C Display is running!

The HP-19C mainboard gave its first light!

Thanks Bob for encouraging my work.

This morning I started to work on the HP-19C processor board. My first surprise, when I measured the 21 pins of the 13-digit LED display, was that it is pin compatible to the 20-pin HP-25 Woodstock display, except for missing the rightmost digit. I could easily place one of my HP-25 LED modules directly into the HP-19C. And during test stage, I can easily do without the rightmost digit.

[Image: uc?export=view&id=0Bwx8KUfOUL_RUDdEYzI4SmJGVlU]

The first light!

What you see in the image above seems to be an upside down "HP-25E" Logo, but this is not possible electrically, and indeed it is not. I inserted an HP-25 ACT, which shows the normal Logo "HP-25E" on a Woodstock calculator. But the different HP-19C display driver displayes a small "d" instead of "P", which coincidentically is the letter "P" turned by 180 degrees. The letters "2" and "5" are symmetrical and identical when turned and only the letter "E" gives evidence that it is not an upside down text, but a reversed text. "E" would convert to "3" if turned. Thus the HP-19C display hardware simply is running in reverse direction compared to the HP-25.

It will be easy to change the newACT for displaying the digits in reverse order.

Next step will be to get any key press. This will prove, whether the PIK chip is still OK. Chance is given and better than I thought yesterday, because both display drivers chips are still OK and could be used already.

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