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HP-19C repaired
05-20-2017, 04:05 PM (This post was last modified: 05-20-2017 04:11 PM by PANAMATIK.)
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HP-19C repaired
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]


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Messages In This Thread
HP-19C repaired - PANAMATIK - 05-20-2017 04:05 PM
RE: HP-19C repaired - Matt Agajanian - 05-20-2017, 09:09 PM
RE: HP-19C repaired - Paul Dale - 05-20-2017, 11:21 PM
RE: HP-19C repaired - Dr. Mabuse - 05-21-2017, 02:34 PM
RE: HP-19C repaired - PANAMATIK - 05-21-2017, 03:30 PM
RE: HP-19C repaired - AndiGer - 05-21-2017, 07:18 PM
RE: HP-19C repaired - Geoff Quickfall - 05-21-2017, 11:19 PM
RE: HP-19C repaired - PANAMATIK - 05-23-2017, 06:00 PM
RE: HP-19C repaired - Dr. Mabuse - 05-23-2017, 06:57 PM
RE: HP-19C repaired - PANAMATIK - 05-24-2017, 02:03 AM
RE: HP-19C repaired - Geoff Quickfall - 05-24-2017, 04:08 AM
RE: HP-19C repaired - PANAMATIK - 05-24-2017, 10:25 PM
RE: HP-19C repaired - PANAMATIK - 06-02-2017, 05:42 PM
RE: HP-19C repaired - BobVA - 06-02-2017, 06:04 PM
RE: HP-19C repaired - PANAMATIK - 06-02-2017, 06:42 PM
RE: HP-19C repaired - Dr. Mabuse - 06-02-2017, 06:17 PM
RE: HP-19C repaired - rprosperi - 06-02-2017, 08:25 PM
RE: HP-19C repaired - PANAMATIK - 06-02-2017, 09:07 PM
RE: HP-19C repaired - rprosperi - 06-03-2017, 12:25 AM
RE: HP-19C repaired - PANAMATIK - 06-03-2017, 11:28 AM
RE: HP-19C repaired - david sanz - 06-07-2017, 12:05 PM

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