HP-92 printer problem (not the gear)

[dramsey]

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?

[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

[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...

[AndiGer]

Did you try to disconnect the 3 wires of the print feed button from the PCB?
Maybe a short or similar error with the button?

[teenix]

It would seem some printer related component has failed after a bit of use. However, the problem appeared as the paper was being fed into the printer. Does the hum stop if the paper is removed?

It may have a switch or wiring problem but this appeared to work properly while trying to feed the paper. While not 100% accurate, slowly pressing and releasing the advance switch should give a definite click sound. The HP-97 service manual has procedures for testing the wiring and switches.

Do any of the IC's get warm after being on for awhile, especially the PICK - large black horizontally mounted chip under the printer board. If it is warm it might suggest a faulty IC. Even if not warm, it is possible that the PICK or a ROM has failed.

If the printer motor is the humming culprit, perhaps disconnecting it from the printer board might help, although it will probably not be the actual problem. You cannot leave the printer board assembly disconnected because there are timing components on it and the calculator will not work without it.

Capacitors often fail in old equipment, so it may be one in the power supply, although the calculator itself seems to be functional, or the one on the back of the print motor.

cheers

Tony

[ignazio]

Hi,
have you check the reed switch, disconnecting it from the printer board and check it with a ohmeter? Maybe it's the culprit.
My little help.

[Jonas Sandstedt]

Hello,

I ran into similar (but possibly not the same) problems a couple of years ago, see

https://www.hpmuseum.org/forum/thread-1613.html

Even if your problems are different, some valuable tips may be found here as well.

For me, finally it seemed as one of the quad transistor IC seemed faulty and I replaced the printer sub PCB which worked well. Later, I learned that these IC could be substituted with MPQ3904, but I have not investigated this further. If you have a working HP-97 you might try to switch sub-PCB. You need a flat tool to such as plastic or metal sheet that is cut into shape to remove the printhead flat cable, different tips are discussed in the forum. I use the protective metal sheet of an old 3.5 inch floppy disc that is cut into shape – a tip from Ignazio. It must be said, however, that working with electronics, in particular old electronics, is risky and may result in unexpected failures.

Finally I would again like to mention that a new HP-97 service manual is available at
https://literature.hpcalc.org/

Best regards
Jonas

[ignazio]

Hi,
have you check the reed switch, disconnecting it from the printer board and check it with a ohmeter? Maybe it's the culprit.
My little help.

[AndiGer]

(02-11-2022 09:46 AM)Jonas Sandstedt Wrote:  ... You need a flat tool to such as plastic or metal sheet that is cut into shape to remove the printhead flat cable, different tips are discussed in the forum. I use the protective metal sheet of an old 3.5 inch floppy disc that is cut into shape – a tip from Ignazio. ...

See https://www.hpmuseum.org/forum/thread-17437.html
Tony's printer ribbon removal tool :-)

[teenix]

Jonas' idea seems an excellent candidate for the problem, and if so would be relatively easy to repair.

There are also 2 discrete PNP transistors that drive the print motor which might have failed too.

The MPQ3904 is available from Mouser and seems cheaper than other places where some are selling it for upwards of $20 each

I'm not sure if this is an exact equivalent of the original as the transistors are rated only up to 200mA continuous. But these were also used to drive the print heads which could draw up to 500mA. These are both driven with PWM but the duty cycle could approach 100%. The motor braking transistor Q8 is enclosed with the print head drivers in quad package U2.

The original part number was 1858-0044 but I cannot find an equivalent in the HP cross reference tables.

Part No. 1853-0393 for the discrete PNP transistors is also missing, however 2N3906 are PNP equivalents of the ones used in the MPQ3904 and are also available cheap from Mouser.

I don't know if motor inductive effects will damage these devices over time.

cheers

Tony