HP 10 printer issue

[tangdfx]

Hi, All

Recently I acquired a good cosmetically HP 10.

[attachment=10462]


It can print but there is an issue when print digit zero and also the decimal point will be double print.

You can see from the picture the "0" is enlarged but meanwhile other digit is normal.

I already search the forum and I find another case is similar with mine.

Here is the link: https://archived.hpcalc.org/museumforum/...ight=HP-10

I already try a lot of approaches such as I change the 1826-0287 with a new LM339 comparator.

I also change the the old transistor with BC338.

Now I don't know where to go.

I am suspect the printhead timing circuit has something wrong.

[attachment=10463]

Otherwise maybe the CPU has defected which integrate the PIK in one die.

[attachment=10464]

[teenix]

Is it possible to notice a change in print head speed when this happens? ie a slower motor speed.

The motor speed is controlled by U2a and a 1V reference output from the Vref/Oscillator component package fed back into the PICK circuitry internal to the IC package. The linked post mentions a poor power supply causes the problem and this may upset the reference voltage causing printer motor speed variations.

Have you measured the power supply rails when printing fails, maybe there is a voltage drop. This may cause the PICK chip motor drive circuitry to change the motor drive pulse widths.

If you have a storage oscilloscope you should be able to capture the pixels being printed on any print row to see if they are changing pulse width and thereby stretching the printed pixels. If they stay the same width then the printer speed is the only other thing that can cause a stretched digit. Maybe try row 1, IC pin 25 output.

Why just with 0's, who knows, somewhat strange.

cheers

Tony

[tangdfx]

Updated.

I use the lipo battery to provide the power.

And the print is works normally.
[attachment=10484]

So tony your guess is right. The stretching is caused by weak power supply.

But these is still something wrong with the PSU.

Enen with the lipo battery which regulate voltage always 5V. The calculator show low power indication. I know the VR2 to adjust but it does not work.

So the defect area is locked to the PSU circuit.

I currently don't know which part is defected.

But I can change them one by one.

[teenix]

Good news, at least you know the printer works properly. It seems the original repair post was on the right track.

Maybe the battery monitoring circuitry around transistors Q2 and Q3 is faulty. I'd check the transistors, diode and the 2K resistor. The trimpot may be faulty also.

cheers

Tony

[tangdfx]

Hi, All

Good news. The problem has been solved.

The root cause is that I use a NPN transistor to replace the Q2 which should be a PNP transistor.

Now I have a perfect working HP 10.

After this HP10 repairing, I gained a lot of experience and help from others.

Thank you everyone.