Adjusting R8 to Increase HP-97 Printer Intensity

[teenix]

It is possible to get contact bounce, where the wiper briefly moves away from the surface of the resistance track inside the pot, but I would think this unlikely unless the pot is tapped quite hard.

I included another wiring diagram which illustrates what Dave meant by including wiring on the remaining end terminal, and an encapsulated pot with the 3 terminals which should be able to be wired in exactly the same way. Check with the supplier or with a multimeter if you suspect the pins may be a different order.

If you imagine looking down from the top of the round pot with the wiring as shown, at the full CW position, the wiper will be in direct contact with the left terminal and move (or wipe) across the resistance track towards the right terminal as you turn CCW and increase the resistance between the wired terminals. As you do this, the resistance between the wiper and the other end terminal will decrease. You may have to check with a meter or data sheet what happens with the smaller encapsulated types as some have top adjusters, or side adjusters.

A linear pot will be the best choice here as you will get the same resistance change for a given movement of the wiper. Logarithmic types give a large change in resistance per unit move at one end of the pot and progressively get smaller as you get to the other end. This is usually what you get in a volume control.

Just my personal preference, I would put up with a faint printout as long as it is readable, especially if it is paper age related.

I know the printer components degrade over time and they may need tweaking to try and normalize the printer operation, but these as well as the print head are heading for 50 years of age now and increasing the power sent to the head may inflict additional stress causing failure (especially if the resistance is already at the maximum specified by HP). Sadly some components like print heads are generally not replaceable anymore.

cheers

Tony