Adjusting R8 to Increase HP-97 Printer Intensity

[teenix]

(02-16-2021 01:51 PM)James Linder Wrote:  Can anyone help me find R8 on the printer PCB of the HP-97 desktop calculator.

The service manual says that if I adjust this resistor, which I presume means if I
replace it with a lower resistance resistor, then it will increase print intensity and
make the print darker.

Also, if you should know the specific resistor I should install (its ohms rating) and
where I can buy such a resistor (part number, etc) this would be very much
appreciated.

R8 shown in attached image according to service manual.

There is quite a span of resistance values shown in the manual, but I would be careful in increasing the resistor value too much as this may cause the pulse width of the driver circuit to reach maximum and the print heads may overheat and damage them.

Maybe un-solder one end of the resistor and solder (say a 20K) linear potentiometer in series with it. That way you could increase the resistance gradually and see the effect.

A fully charged battery will help during the testing :-)

cheers

Tony