19C - printer - 12Ohm OK - voltage pulses OK - but not printing

[RolfSchlagenhaft]

Hi,
glad that I found this forum. Maybe there is somebody who has an idea how to proceed with my problem. I did a search before and didn't find a similar problem described or solved. So bear with me, that my first post is a question.
Situation: I have a 19C. Calculator, display, power supply, printer head motor and paper transport work fine. Yet, no symbols show up on the paper.
What I did so far:

1. I disconnected the printer head's flex wire from its connector on the "mother board". Then I measured the resistance of the printer head's dots to check whether they are OK. All dots have a resistance 12.0-12.4 Ohm. So the flex wire and the printer head look OK to me.
2. The I assembled everything again and measured whether there is actually voltage on the lines to the dots when issuing a print command. And in fact, I observe some voltage showing up shortly on the lines to the dots. As I have only a simple voltmeter and no oscilloscope, I cannot judge whether the "dot pulses" (voltage and width) are correct.

So I'm a little stuck now. There is voltage to the dots. The dots have the right resistance and still nothing is on paper.
Apropos paper: I'm using two kinds of paper: One from HP-97 which I cut to the correct width for the 19C, which would print in blue. And a third party thermal paper with correct width, which would print in black.
I have no idea what next analysis step I shall take. I don't want to replace the dot driver transistor arrays just based on speculation that they could be the root cause.
Any hint would be much appreciated.
Rolf

[RolfSchlagenhaft]

Additional info:
The mechanics of the print head look OK to me. The paper is properly pressed against the moving print head. So, if the dots create heat, I'd expect something on paper.
Rolf

[Dave Shaffer]

(07-03-2020 12:47 PM)RolfSchlagenhaft Wrote:  Additional info:
The mechanics of the print head look OK to me. The paper is properly pressed against the moving print head. So, if the dots create heat, I'd expect something on paper.
Rolf

Have you applied some heat to your paper(s) by some other means? Is it possible you've got old/non-functioning paper?

Can you measure the (average) current to a dot with your voltmeter?

[vassilisprevelakis]

(07-03-2020 12:35 PM)RolfSchlagenhaft Wrote:  Situation:I have a 19C. Calculator, display, power supply, printer head motor and paper transport work fine. Yet, no symbols show up on the paper.

have a look at this document.
Taschenrechnersammlung.pdf.

The second part (in English) contains lots of info on fixing HP calcs in general and HP-19C in particular.

Best

Vassilis
http://www.series80.org

[BobVA]

(07-03-2020 10:18 PM)vassilisprevelakis Wrote:  The second part (in English) contains lots of info on fixing HP calcs in general and HP-19C in particular.

Yes, seconded! Bernhard has a great deal of information there. (The 19C he repaired had a similar printer fault that turned out to be the potentiometer that controls the print darkness. You might want to check that first.)

[RolfSchlagenhaft]

(07-03-2020 09:08 PM)Dave Shaffer Wrote:  

(07-03-2020 12:47 PM)RolfSchlagenhaft Wrote:  Additional info:
The mechanics of the print head look OK to me. The paper is properly pressed against the moving print head. So, if the dots create heat, I'd expect something on paper.
Rolf

Have you applied some heat to your paper(s) by some other means? Is it possible you've got old/non-functioning paper?
Can you measure the (average) current to a dot with your voltmeter?

Thanks fo the proposals!
The paper works. I confirmed it by burning it and also by writing with a soldering iron on it. I reacts in same way as I was used to it in past.
I need to check the manual of my multimeter whether it allows averaging. Good idea, thanks!