HP-19C Printer

[teenix]

The printer seems to have been pulled apart in the past as the spring that holds the pin into the worm shaft has gone AWOL. Not much can be done except find a replacement.

Edit: Fixed.

I found an old miniature push button switch that was used for resetting my old mainly dead 486 PC. The switch came apart easily and the spring inside was the right diameter but too long. I cut it to size, installed it, and now the print head mechanics work :-)

Now to try to figure out why the printer doesn't work.

Edit2:

The LLD pin on the Cathode Driver is normally HI when it detects a good battery voltage. If the battery voltage goes below a certain threshold, this pin goes LO and usually lights a LED in the display. In the 19C this pin controls a flag that is tested in the Microcode and a low battery starts the decimal point led flashing.

When trying to print, the HP-19C microcode tests for this Logic LO condition for the duration of a timeout. If it is still LO after the timeout then an Error is displayed and printing aborts.

Also, before printing starts, the print head home switch is tested and if it is not home, the microcode will stay in a permanent loop until it is. If the printer is faulty and doesn't home, then the user will just see a blank display until the power is switched off then back on.

The 19C here seems to exhibit symptoms like the first scenario, as every time a print is started "Error" is displayed, however, the oscilloscope never sees a LO level appear on the LLD pin. From the code point of view, I cannot see any other reason for an error to be displayed. Also, the LLD flag input is independent from the other multiplexed flag circuitry for the other ACT flag input.

The motor drive signal from the PICK chip will never occur under these conditions, and indeed it doesn't.

The only thing I can think of is that the ACT LLD flag input is faulty.

It could be possible that the power supply capacitors have begun to fail so maybe changing them might offer a fix.

cheers

Tony