HP-97 troubleshooting

[Paul Berger (Canada)]

(04-07-2020 07:44 PM)Kees Bouw Wrote:  Thank you all who replied. A short status update:
I connected the calculator to my lab power supply at 2,5 V just to see what the current would be. The current was 0,5 A which is about twice as much as it would normally use at 5 V.
So there was no bad contact, but instead someone was using more than their fair share of current! There was no magic smoke or even the slightest smell, just like when I had connected it to the battery. I suspected the printer because that uses a lot of current if it is in use. So I disconnected the printer and also the printer pcb, knowing the calculator would not work without it. Normal current of about 130 mA at 5 V. Then I connected the printer pcb back but without the actual printer, still normal current and a working calculator!
So what could it be? Easy to suspect the printer motor or the thermal head, but if either is bad would that be an issue if it is not activated from the pcb? Maybe the pcb is bad and it continuously sends current through all 7 thermal points? I will have to do more measuring, there are some hints in the service manual.

Kees.

Was there paper in the printer? If there was and current was continuously flowing through the print head there should be a mark on the paper, also since the print head is not designed for 100% duty cycle I suspect it would not last long. Another possibility is the home position switch is not working. When the 97 is first turned on if it detects the print head is not homed it will turn on the motor in reverse to bring it home and if the home switch never closes it will drive forever, mind you if your printer still has the nylon idler that will soon take care of itself by stripping teeth off the gear. This switch is a reed switch and is activated by a magnet on the print head carrier.

Paul.