HP 19C big print font size

[tangdfx]

Recently, I acquired a HP19C from ebay.

Initailly it can not print and the other calculator function is OK.

Then I open it and clean the corrision and try again.

This time , the printhead run forever and can't stop.

But I can print without problem except endless movement of the printhead.

One day after, I get the help from Tony and suspect that the home position can not acquired is the root cause.

Finally fix the endless movement issue. But I get another issue is that the HP19C can print and stop at the home position. But the printed character is bigger than normal.

With this situation is OK for me but for longer number (example 3.1415926 ),there will be not enough space to print out.

[Dreato]

I believe there is a pot associated with print motor speed. Could be this part is dirty or corroded. There's 2 on the circuit board, it's the one on the left. Please check a helpful post from bernhard at Panamatik. I ws unable to open the link to the attachements. maybe you'll have better luck!

Others in this forum know more than I do about these machines.

Regards,
Dave

[tangdfx]

Yes. I know the left pot can adjust the motor speed.
Though I don’t understand why the motor speed can impact the font size.
But I try to adjust this pot and my hp19c.It only print at the fast speed of the motor
If I slower the speed of motor, It can’t print correctly.
I don’t know why.

[teenix]

The printer characters are output to the print heads at the same rate. Therefore if the motor speed changes, the print text will shrink or expand.

cheers

Tony

[tangdfx]

Thank you tony.

Today I find that this HP19 draw unnormal current.
I think there must be something wrong with the P SU board.
Maybe some where has short circuit.

So I want to learn from bernhard to rebuild the PSU with new component.
And use the fixed resistance 25k and 100k to replace the 50k and 200k Potentiometer pot. I also doubt that the transistor is failed.

[tangdfx]

Good news.

After I change the potentiometer. The print is fully function now.

The root cause is the potentiometer has been corroded.