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I doubt that you can find a replacement print head. However, you might want to check that the flex connector cable that comes off the print head is inserted into the socket on the edge of the printer control circuit board. I've found this to be the problem on one 19C that I have. You can also test the continuity of each heater element on the print head from the end of the cable to see if you have an open. If you do, the problem might just be a broken trace on the flex connector cable.
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Another possibility is that the driver output for the affected row has failed. The good news is that it is possible to build a substitute circuit using a transistor.
If you obtain the Museum CD-ROM set or the CD-ROM that contains the HP-97 service manual, it'll provide some useful advice on diagnosing the printer and the driver circuit. Much of this advice is applicable to the HP-19C.
Viktor
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First make sure it is the printhead!. I've never worked on a 19C, but if it's like all the other
HP thermal printers you will find that the printhead is connected to one of the circuit boards using a flexible PCB ribbon cable.
Open the machine up and unplug -- CAREFULLY -- that ribbon from the circuit board. You will find 1 connection per dot row and 1 common connection. It's normally at one end of the connector
and normally a wider track on the ribbon. Measure the resistance between each of the other contacts and the common one with an ohmmeter.
A good heater element is < 10 Ohms or so, dead ones are much higher.
If the printhead is good, first make sure the ribbon is connected properly to the connector on the circuit board and then
trace back to the driver circuitry. Often HP used standard transistor arrays to drive the printhead elements so it is possible to repair this circuitry.
If one of yor printhead elements is open, then you've got problems. I've never managed to repair an HP thermal printhead, and I've tried
a couple of times. One problem is that the metalisation layer on the printhead is aluminium, so you can't solder to it. You can't
even repair broken contacts between the ribbon and the printhead and expect it to 'hold'.
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Thanks for all the suggestions so far. I took it apart and measured the resistance at the print head flex PC connector. From the wider connection to 6 of the 7 remaining measured about 11-13 Ohms. But from #6 was over 2k Ohms. That corresponds to the row that doesn't print.
So I'm afraid it is the print head, not the connector or transistor driver. Bummer.