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