After searching months for an HP 19C, eBay surprinsingly produced 3 of them and I managed to get one. It works great but the sixth row of dots on the printer doesn't print. So I suspect one of the themal elements of the print head is dead. Any suggestions for repairing this?
Or anybody know where I can get a replacement print head?
Thanks.

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.

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

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'.

I had someone else ask me this question. I took a look at the print head and flex circuit of a 143a (while installed) and they look almost the same as what is in the 19C.

The print head looks the same size and the flex has the same number and placement of the conductors.

However the flex circuit looked like it had a different angle to the connector. I don't have a disassembled 19C and don't know if this is the same flex but with a bend in it.

Anyone else look to see how compatible the 143a print head is with the 19C?

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.

When you say it's 'over 2K' do you mean that it's a little over 2K, or that 2K is the maximum reading on your meter (so the
resistance might, in fact, be infinite)? If the resistance is
infinite (total open circuit), you might inspect the flexible cable for
cracks before totally giving up on this printhead. But while cracks in the flexible cable are the easier to repair than
other printhead problems, they're not that easy to repair.

When I said "over 2K" I meant that 2K was the maximum reading on the ohmmeter. I didn't want to change the range, since this particular ohmmeter uses a higher voltage to take readings on higher ranges. It's a good bet the resistance is infinite.


I did inspect teh flex cable looking for cracks, and tried to measure the resistance closer to the print head by piercing the outer layer. But that came up over 2K as well.

Sounds like the printhead is dead :-( :-(. Good luck in trying to find a replacement (the best source would seem to
be another 19C which has a logic fault). Anyone know if any of the other HP thermal printheads (from the Topcat or 82143/82162 printers) can be
made to fit?

I really doubt the 82143a or 82162a print heads would work. I know they don't work on the HP-97's. Around 1980, HP changed some things with the print heads when they develop[ed the HP41C printers. The resistance is different and if my memory is correct, the resistence was lower? It might be that they wanted the printers to run faster so they needed a hotter print head. Anybody know if this is correct?

I will measure the resistance tonight of the HP-82143a print heads that I have and take a look at my HP-19C to see if the print heads are even physically near the same. If so, I can send Kenneth one to try.

I know there are two types of heads. One with a resistance of around 80 ohms, the other with around 10-20 ohms (if memory serves correctly). I also know the pinouts of them are the same and the same as the 19C. If you can find one with the matching resistance (I don't remember if the 82143A or '97 matches the 19C) I'm pretty sore you can make it work. I think the flat cable on the 19C is shorter, so you may have to put a "Z" fold in it to make it fit. Make the fold close to the connector and wrap it with some good tape. I also have a '19C with a missing dot and am planning on trying this when I have the chance. If you get it to work please let us know.

I checked the resistance of the HP-82143A print head and it is 87 ohms. It sounds like a HP-97 print head may work to fix the HP-19C. The resistance of that print head is similar if not exactly the same as the 19C. They are also of the same vintage calc so it makes sense. The HP-82143A came along a bit later.

> I think the flat cable on the 19C is shorter, so you
> may have to put a "Z" fold in it to make it fit. Make
> the fold close to the connector and wrap it with some
> good tape.

I did a gross measurement and it looks like there is enough extra to make the fold, if required. Physical size of the head looks identical.