HP-92 printer problem (not the gear)

[dramsey]

(02-10-2022 02:02 AM)teenix Wrote:  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.

The battery is charged, and an HP-97 with this battery installed works and prints perfectly.

(02-10-2022 02:02 AM)teenix Wrote:  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.

This does enable the keys to work, however, the hum remains.

(02-10-2022 02:02 AM)teenix Wrote:  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?

No. The print head does not move under any circumstances that I can find. Pressing the paper advance button does nothing. The hum is constant.

(02-10-2022 02:02 AM)teenix Wrote:  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)

Sadly this has no effect.

(02-10-2022 02:02 AM)teenix Wrote:  An oscilloscope might be the only way to properly diagnose this type of problem.

Yeah, which is beyond my hardware-repair pay grade, I'm afraid...