HP-19C print adjustment

[mareksaw]

Hi All,

I have recently acquired a working HP-19C with "untested" printer. It turned out that the printer is generally working but no print appears on the paper. So I opened the calc, there was some bad corrosion on the PCB which I cleaned and I tried to adjust print intensity potentiometer. After this it started printing. Wow!

But unfortunately the print was very unstable. Just after turning the calc on print was too wide. After a minute or two it was getting back to normal width but then after a few minutes it started to get smaller and smaller. Taking the batteries out and in usually solved the problem - for a few minutes. Sometimes even turning off and on helped. I suspected some more PCB problems regarded to corrosion.

So I opened the calc again, found some more uncleaned corrosion and I had to bypass one PCB trace which was almost broken by corrosion. I also did more cleaning on battery terminals. After this the print quality is good, the print is more or less stable and even after the calc is switched on for some minutes the print keeps its quality.

But now print is too wide - I can not get all characters to be visible on print. Surely I was trying to adjust it with the motor speed potentiometer but I am now in its max position and I get what you see below:



Is there any other way to get the print to fill the paper width correctly? Or maybe I have still problems with some signals due to damaged PCB traces? Does the print width depend on battery voltage?

Regard

Marek

[isanchez]

Hi,

I am very interested in solving this issue because I have an HP10 suffering the same problem. Even more, the characters printed are enormous....at least, yours look proportioned.

Thanks and kind regards

Ignacio

[teenix]

There could also be another poor connection somewhere that also causes a change to occur, maybe hidden under something. After being used, the wiper on the trimpot may also have poor contact with the carbon under it. Some Deoxit cleaner may fix this.


The motor speed is determined by comparing the trimpot voltage and a fixed voltage reference that comes from the brownish rectangular package underneath the power supply board.

Inside the sealed package, there is a zener diode providing a fixed voltage and string of resistors where the motor speed reference voltage is tapped from. This is about 1V. If this voltage varies, the print motor speed will also change just like adjusting the trimpot.

It may be that the aging zener is a bit touchy, maybe due to temperature changes and is causing a drift in voltage and hence motor speed.

You can probably test this by placing the calculator in a sealed plastic bag in the refrigerator for a spell and then using a hair dryer to warm the lower end of the calculator. (Not too much obviously or you may damage the plastic casing.

Check the parts near and around the brown package and check for more corrosion.

The PIK chip also has a say in speed control but is more digital in nature, but it is still dependent on the 1V reference and trimpot position. There is a LM339 quad OP Amp IC near the trimpot and one Op Amp is used for the speed control and it may be failing also or have poor connections.

Unfortunately, due to the construction of the 19C, it is difficult to test the working circuit.

cheers

Tony