HP-19C Repair Odysee

[PANAMATIK]

My first attempt to understand and control the printer Motor.

In the photo below you see an original HP-19C mainboard and printer board, opened for oscilloscope measurements. To get access with the probes I had to put the keyboard aside and connect via flat ribbon cable. It is a fully working HP-19C hardware, however without printhead and flex cable (see post #1 to learn why the printhead is missing), but the motor moves correctly.



In the above image to the right you see a fully working HP-19C, which I hopefully don't have to open for this research. In the background you see another printer board with defective motor control. With this board the motor moves twice as fast as intended and prints the digits much too wide. The number PI is printed partially reverse on the way back, which you see below.





The oscilloscope screenshot shows the FWD (Forward) Signal and the REV (Reverse) signal of the PIK chip with correct motor control. The forward speed is regulated and needs about 1 s for printing, the reverse signal causes the motor to move full speed to get to the HOME position in about 400 ms. Notice that the HP-19C motor shaft always moves in one direction, the REV signal is just used to accelerate. As the PIK chip was originally designed for the HP-97 hardware, which moves the motor bidirektional and uses the REV signal normally, the HP-19C hardware uses the REV signal just for going full speed. Another crazy design detail.

Bernhard