HP-92 printer problem (not the gear)

[teenix]

(02-10-2022 01:12 AM)dramsey Wrote:  I recently acquired a beautiful HP-92. It's cosmetically and functionally perfect, but didn't have any paper in the printer. While trying to install paper, the printer seemed to fail-- that is, the printhead was moving back and forth as I held the paper feed button down, but then it stopped and emitted only a low hum.

Removing the bottom of the printer, the gears appear fine. I can manually rotate them to position the head anywhere I want, but powering the calculator on simply produces the same low hum from the printer area (it's not obvious what's humming). While the LED initially displays "0.00", pressing any key at this point causes the display to go blank and stay blank.

I've never seen anything like this before and don't know where to start. Any ideas?

Seeing 0.00 on the display proves that most of the system is working.

Do you have a well charged battery that is not relying on a charger? Low battery might rob the system of usable power when trying to print and the system oscillates between off and on causing a hum.

If the printer is trying to print, the LED display turns off so that extra power is available to the print heads. If the printer is faulty, perhaps the software is trying to drive the printer and failing to get any feedback, and the LEDs stay off.

Try switching to MAN mode for the printer, then turn the calculator on. Maybe the keys will work.

While the rear case is open:

With the print head homed, does it still hum?

if you advance the printer head 1 inch or so by turning the larger printer drive gear with the power off, does it try to home when the power is switched on?

The hum might be from the motor and could be caused by faulty PWM outputs from the PICK chip which is responsible for driving the transistors for the motor drive.

It could be a poor connection on the small printer circuit board contacts. Try easing it upwards to disconnect it from the CPU board and then re-connect it. That might be enough to clean up the contacts. (be careful not to force the contacts and try not to stress the ribbon cable)

An oscilloscope might be the only way to properly diagnose this type of problem.

cheers

Tony