HP-19C Repair Odysee

[PANAMATIK]

(05-09-2021 05:56 PM)teenix Wrote:  Too true :-)

I'm pretty sure the HP-97 has full motor speed for reversing the carriage.

The PIK chip can gate the PWM signal from its VM pin so that the motor is controlled by the PWM or steady voltage for full speed.

The FWD output must also have circuitry to detect when the OPS switch is activated to stop the printer when paper has run out even though this is also done in 97 hardware. This is tested when PWM is present and when not, although I don't know if the 19C has this.

Hi Tony,

You are right. The HP-97 indeed has a complete H-Bridge for controlling the motor in both directions. For short print lines the HP-97 skips blank characters and returns earlier, this is much faster than the HP-19C, which has to go always the full distance to the left and right (see HP-97 service manual schematics). It seems the PIK generates a PWM signal only on the FWD signal, not REV signal, because it always goes full speed backwards and doesn't print bidirectional.

Just gating the FWD PWM signal, instead of adjusting the duty cycle, would be an easy way to control the speed. The FWD PWM duty cycle of my defective HP-19C at full speed is about 75% on my scope.

The HP-19C has no OOPS, (OOPS! Out Of Paper Switch).

I'm currently programming the time-shifted output of the print characters, which let them appear in "italic style", which has the intention to switch the current of the printhead pixels not all at the same time. The PIK of HP-19C and HP-97 has 62 printable characters, 26 upper case letters, 1 lower case letter e (for showing e^x), 10 numbers and 25 special characters + - * etc. (the HP-97 PIK may have different special characters) . I read the exact print matrix with a magnifier glass from the paper printout and programmed it already in my newPIK.

Because the HP-19C doesn't use all 26 letters in the ROM code, I couldn't assign the 6-bit codes for letters J,K,Q,U,V,W. Perhaps analyzing the HP-97 ROM will reveal it.

The timing for the printhead output is relaxed, because the pattern must be changed about every ms while printing, this is easy for a todays micro. A print line has a maximum of 22 characters, each has 5 print columns and 1 blank column between the characters, time delayed output by 7 rows, printed in 1 s, needs to output a new print pattern every 1000/22/6/7 = 1,08 ms.

Bernhard