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HP Forum Archive 20

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Re: Printing quality on HP-97
Message #1 Posted by Christophe Dubreuil on 24 Jan 2011, 5:43 p.m.

Here is the scan , but in reality the difference is more importnat than on the scan : The printing before i opened the HP-97 is a lot darker than after.

Hope somebody can help me.

Thanks

Christophe Dubreuil

      
Re: Printing quality on HP-97
Message #2 Posted by Ignazio Cara (Italy) on 25 Jan 2011, 3:35 a.m.,
in response to message #1 by Christophe Dubreuil

Hi Christophe, if the paper is the same and you haven't take apart the printer or swap the printer PCB with another one, it's very strange. If my eyes are still good, you are loosing one resistor of the thermal printer head, the third from the bottom. You should measure each resistor from the edge of the flat cable that is connected to the printer's PCB. The value of each resistor should be about 11 ohms. If you have one spare printer PCB, try to swap it on the printer. Let me know after you have done this series of checks.

Regards

Ignazio

            
Re: Printing quality on HP-97
Message #3 Posted by Christophe Dubreuil on 25 Jan 2011, 3:48 a.m.,
in response to message #2 by Ignazio Cara (Italy)

It is the same paper ... I havent touched the printer part of th HP-97 (or unwillingly). I agree that one line is missing (but not entirely , there are very small dots where there shoul be normal ones) , i'll check the resistor , but as i said all the printing is lighter also.

Regards

Christophe

                  
Re: Printing quality on HP-97
Message #4 Posted by Katie Wasserman on 25 Jan 2011, 12:30 p.m.,
in response to message #3 by Christophe Dubreuil

Quote:
but as i said all the printing is lighter also.

I don't know why this changed but there's and easy and documented fix for this in the service manual.

Quote:
2-31. The remaining comparators in U4, along with the resistor network in U5, produce the variable duty-cycle signal STB, which is nominally 10 kHz. The STD signal will change its duty-cycle to keep its rms value constant and thus print intensity constant. By changing the value of R8, the nominal duty-cycle of STD can be changed to adjust print intensity.

There's no way to tell what the current value of R8 is without looking. Each HP-97 was shipped from the factory with a selected value, they list values from 34.8K to 196K in the service manual, but they've all 1% resistors.

-Katie

                        
Re: Printing quality on HP-97
Message #5 Posted by Christophe Dubreuil on 31 Jan 2011, 3:14 a.m.,
in response to message #4 by Katie Wasserman

I finally went thru the service manual ... i checked the various things i could (resistances and voltages) and all seems OK , i guess it may be one of the transistors that is weak (explaining meybe the light line).

I should get an oscilloscope ro further investigate.

But as i pointed out ... it happened after i opened it for fixing the magnetic card reader ... it was perfectly working before ...i should have inadvertably modified something.

I am a bit reluctant to change R8 and modify it. I guess i will use it as it is, and if it get worst i then will proceed further.

Thanks to the community.

Christophe Dubreuil

                              
Re: Printing quality on HP-97
Message #6 Posted by Ignazio Cara (Italy) on 31 Jan 2011, 10:25 a.m.,
in response to message #5 by Christophe Dubreuil

Hello Christophe, I live in Italy, and I can send you a quad transistor 1858-0044, for free. Let me know your address if you are interested.

Regards

Ignazio


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