HP-97 Print Intensity Mystery


I had been working on two HP-97s, swapping parts in an attempt to obtain one complete and fully functional unit. At one point I tried swapping the two printer assemblies, when I noticed some odd behavior. Prior to the swap, the print intensity on both printers was normal, however, afterwards the print intensity became very light and thin on one printer but very dark and thick on the other. I swapped paper rolls, but there was no change. So, I read the HP-97 service manual, and found out that there is a resistor R8 that controls print intensity, which is located on the printer PCA. This PCA is connected to the printer by a cable and by connector pins to the main logic PCA. The thing is, when I swapped the printer assemblies, the printer PCAs remained with their respective printers and were not interchanged. So, since the intensity resistance was not changed in the process, why did the intensity change? As soon as I returned the printers to their original locations, the print intensities returned to normal. I am trying to understand the inner workings of these calculators and would like to know if anyone on this forum has encountered this behavior and determined the cause.


You kept the printer PCA together with the printer but the PIK which drives the printer did not swap since it is on the main logic PCA.

I'll bet if you keep the PIK's with their original printer assemblies, intensity will remain the same.


Thanks for the info. I see where the pinouts from the PIK connect to the printer PCA. There must be some variability in PIK outputs which require each printer to be adjusted for its PIK that accounts for the range of resistance values. I thought that it had something to do with variability in the thermal print heads. So, would swapping the resistors accomplish the same thing, or is there more to it than that? Changing a resistor is a lot easier than an IC.


I've had good (but not dramatic) results with resistor changes on the 97 printer board. It's certainly easy enough to try.


The type of the paper (very old paper) :-)

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