Adjusting R8 to Increase HP-97 Printer Intensity

[James Linder]

What are the possible cures for the printers in the desktops printing too
Lightly? I find that only a few of the dozen I own print dark enough. Thanks

[teenix]

(02-15-2021 03:11 PM)James Linder Wrote:  What are the possible cures for the printers in the desktops printing too
Lightly? I find that only a few of the dozen I own print dark enough. Thanks

Poor paper quality - age??.

Printer platen not holding the paper close enough to the print head.

Poor battery quality may affect this if it cannot supply the current required for the print heads.

Some sort of additional resistance in the print head ribbon cable connector or the connectors between the print circuit board and the main CPU board - corrosion??.

R8 (from the 97 service manual) can be adjusted to change the print intensity.

Over a range of calculators doing the same print problem, may suggest the battery.

cheers

Tony

[isanchez]

(02-15-2021 10:41 PM)teenix Wrote:  

(02-15-2021 03:11 PM)James Linder Wrote:  What are the possible cures for the printers in the desktops printing too
Lightly? I find that only a few of the dozen I own print dark enough. Thanks

Poor paper quality - age??.

Printer platen not holding the paper close enough to the print head.

Poor battery quality may affect this if it cannot supply the current required for the print heads.

Some sort of additional resistance in the print head ribbon cable connector or the connectors between the print circuit board and the main CPU board - corrosion??.

R8 (from the 97 service manual) can be adjusted to change the print intensity.

Over a range of calculators doing the same print problem, may suggest the battery.

cheers

Tony

...and probably in that order ;-)
Regards

Ignacio

[James Linder]

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.
[/b]
I restore the card readers and printer page advance on these calculators and resell
them, however I would like to also make the print quality as perfect as possible for
my customers.

I'm equipped to replace chips but I only do so by installing sockets and trial and error
process to determine which chips are good and which ones are faulty; I'm not an
electronics person but a chemical engineer with a passion for the hobby. I restore
HP calculators to help finance the cost of building my personal collection and because
it is a great retirement past time.
[b]

[AndiGer]

Look into the service manual:
See the flow chart in figure 4-18 and note 1 on R8: for darker print increase resistance.
Table 4-7 gives the suggested values and specs of R8.
Figure 4-19 shows you the position of R8 on the printer PCB

[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

[James Linder]

I ordered the PCB mount potentiometers. I see they have 3 connectors.
I see 1st or left terminal is to be grounded, middle #2 is input and right # 3 is output to PCB. In the pictures showing R8 which end is the output from R8 that is Input to #2 of the potentiometer? Also, where would the most convenient ground be located for terminal #1 of the potentiometer?I’m just a beginner on circuits and want to make certain that I do not screw up the installation. Do you think increasing resistance is guaranteed to increase print intensity? I bought some 50k ohm potentiometers in addition to the 20K’s just in case I need to add more resistance than 20K to achieve the desired results. Thank you very much for your help!!

[teenix]

(02-17-2021 04:33 AM)James Linder Wrote:  I ordered the PCB mount potentiometers. I see they have 3 connectors.
I see 1st or left terminal is to be grounded, middle #2 is input and right # 3 is output to PCB. In the pictures showing R8 which end is the output from R8 that is Input to #2 of the potentiometer? Also, where would the most convenient ground be located for terminal #1 of the potentiometer?I’m just a beginner on circuits and want to make certain that I do not screw up the installation. Do you think increasing resistance is guaranteed to increase print intensity? I bought some 50k ohm potentiometers in addition to the 20K’s just in case I need to add more resistance than 20K to achieve the desired results. Thank you very much for your help!!

Connection diagram attached.

The pot will use 2 connections. One from any end terminal, and the other the center terminal.
Unsolder one end of R8 and connect it to the pot end terminal via a short length of wire.
Solder a wire from the pot center terminal to the PCB hole where the R8 connection was unsoldered.

When you get the correct contrast on the printer by adjusting the pot, unsolder the pot wires, measure the resistance of the pot between where the wires were. Measure R8 resistance and add the two values to get the required resistor value.

As resistors come in certain ranges of values, you may have to choose the closest one.

A 1/4 or 1/2 watt type will do.

cheers

Tony

[James Linder]

I ordered the PCB mount potentiometers. I see they have 3 connectors.
Is the third connector just for a ground wire? I assume if yes, is this 3rd
connector the middle prong? Do I Just connect the middle prong to the black
wire going to the battery? Are the other 2 connectors bidirectional (does not matter
Which one is connected to R8 and which end is connected to the PCB connector?

[teenix]

(02-17-2021 06:46 AM)James Linder Wrote:  I ordered the PCB mount potentiometers. I see they have 3 connectors.
Is the third connector just for a ground wire? I assume if yes, is this 3rd
connector the middle prong? Do I Just connect the middle prong to the black
wire going to the battery? Are the other 2 connectors bidirectional (does not matter
Which one is connected to R8 and which end is connected to the PCB connector?

Same setup as in the previous diagram. Usually the end connectors are the extremes of the pot resistance, and the center one is the wiper which slides across the resistance and gives the change as you rotate the shaft.

Definitely, no ground or battery wires connected, just to the disconnected R8 end and the PCB pad as mentioned previously.

If in doubt, and you have a multimeter, you can set it to ohms mode and measure the resistance between any end pin and the center one. You should see a change when you rotate the shaft.

cheers

Tony

[Hans Brueggemann]

HP-97 never had good print quality when compared to HP82143A or HP82162A, which is mostly related to the paper type.
the HP82045A paper that is specified to be used in the HP-97 doesn't even come close to the much more sensitive and "crisper" HP82175A for the HP82143A.

[James Linder]

Thank you gentlemen for your input.

I did try different paper rolls that had never been used and it does make a huge difference. The R8 resistor on the PCB of a spare printer module I have is 196,000 ohms so it is the most resistance that Hp installed in the 97. I ordered 20K and 50K potentiometers to install in series and will probably go with the larger one since it will permit me to increase resistance a maximum of 25% which is as High as I dare go for fear of burning up the print head.

Once the parts arrive I will post the results here!

[Dave Frederickson]

(02-17-2021 06:46 AM)James Linder Wrote:  I ordered the PCB mount potentiometers. I see they have 3 connectors.
Is the third connector just for a ground wire? I assume if yes, is this 3rd
connector the middle prong? Do I Just connect the middle prong to the black
wire going to the battery? Are the other 2 connectors bidirectional (does not matter
Which one is connected to R8 and which end is connected to the PCB connector?

Which end of the pot you connect depends on whether you want the print to get darker when you turn the pot CW or CCW. Also, it's common to see one end of the pot connected to the wiper. This ensures that if the wiper experiences contact bounce that the max resistance will be that of the pot.* If it were not connected the pot would show an open, for a very short period of time, but depending on what the pot adjusts the result could be bad.

Dave

*EDIT: On second thought, it's not contact bounce as the wiper has multiple contacts. The issue is corrosion which can cause momentary open circuit conditions. This is exhibited commonly as a scratchy volume control or brightness control. Consequently the makers of DeoxIT have something called Fader Cleaner that worked wonders on my 82912A's brightness and contrast controls.

[James Linder]

It’s being used to increase the resistance on the current to the print head so that it will run hotter and according to Hewlett-Packard this will increase print intensity (darker print on the chemically treated paper).

Why would you assume I have any idea what the wiper is? Please explain being very specific what this is. I’m told if the pot adjustment knob is facing up or toward me, then the leftmost terminal receives the output FROM R8 and the rightmost terminal is the output to the PCB where R8 was disconnected, and the rotating the pot knob clockwise will increase the resistance and make the print head run hotter. The demo video I looked at shows a light becoming brighter as the pot is rotated clockwise which I assume is causing voltage to increase. Why would just installing in series cause any additional bounce? Why would adding a little more resistance cause this bounce?

[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

[[kby]]

(02-17-2021 06:11 PM)Dave Frederickson Wrote:  *EDIT: On second thought, it's not contact bounce as the wiper has multiple contacts. The issue is corrosion which can cause momentary open circuit conditions. This is exhibited commonly as a scratchy volume control or brightness control. Consequently the makers of DeoxIT have something called Fader Cleaner that worked wonders on my 82912A's brightness and contrast controls.

Oddly enough, Caig doesn’t recommend fader lube for conventional pots and I’ve had issues with it. The doc says it’s only for plastic-based controls, not metal & carbon. Their actual recommendation is metered squirt or two of D100; for my old audio equipment i ‘very found that usually works better in conjunction with a cleaner. The F5 fader lube seemed to make things worse (although with no long-term damage).
-kby

[My turn to edit: it looks like the product literature for the fader series now includes carbon pots, but it’s not updated in the bulletin which stills says d100 usually works best:
https://acct113328.app.netsuite.com/core...a&_xt=.pdf
]

[Dave Frederickson]

(02-18-2021 10:03 PM)[kby] Wrote:  

(02-17-2021 06:11 PM)Dave Frederickson Wrote:  *EDIT: On second thought, it's not contact bounce as the wiper has multiple contacts. The issue is corrosion which can cause momentary open circuit conditions. This is exhibited commonly as a scratchy volume control or brightness control. Consequently the makers of DeoxIT have something called Fader Cleaner that worked wonders on my 82912A's brightness and contrast controls.

Oddly enough, Caig doesn’t recommend fader lube for conventional pots and I’ve had issues with it. The doc says it’s only for plastic-based controls, not metal & carbon. Their actual recommendation is meter squirt or two of D100; for my old audio equipment i ‘very found that usually works better in conjunction with a cleaner. The F5 fader lube seemed to make things worse (although with no long-term damage).
-kby

[My turn to edit: it looks like the product literature for the fader series now includes carbon pots, but it’s not updated in the bulletin which stills says d100 usually works best:
https://acct113328.app.netsuite.com/core...a&_xt=.pdf
]

Thanks for the tip.

[James Linder]

After trying different things I now just replace R8 with a new 200K ohm resistor in every HP desktop I restore and this always results in a high quality, dark print quality. It’s wasted effort to play around with other resistance values when just installing the maximum resistance HP lists in their service manual is a sure thing!

[Hans Brueggemann]

the printer's thermal compensation is pretty smart.
however, it doesn't hurt to check as to whether that resistor value is also good enough for a "long run print", say, a 200+ steps program print-out in MAN mode. along the print, the entire printhead gets more and more heated up, so it may happen that the final lines of your print-out are getting too dark. but then again, this is highly dependant on the type of thermal paper you are using.

[teenix]

(04-01-2021 02:15 PM)James Linder Wrote:  After trying different things I now just replace R8 with a new 200K ohm resistor in every HP desktop I restore and this always results in a high quality, dark print quality. It’s wasted effort to play around with other resistance values when just installing the maximum resistance HP lists in their service manual is a sure thing!

I would be checking the print head PWM signal at this setting just to check the mark space ratio. If HP installed a much lower resistance in a particular unit , then there would have been a reason.

It also might be a different component that aged causing the faded print, but this would be hard to test as it will be encapsulated on the printer's analog board. Myself, in this case, I would put up with a slightly faded printout. I guess the analogy is I wouldn't drive a tired and slow old car flat out all the time, it wouldn't last ;-)

cheers

Tony