Opening HP DeskJet 500 Font Cartridges

[Martin Hepperle]

I have a DeskJet 550 ink jet which works fine after repairing the power supply (electrolytic capacitors).
It came with a font cartridge and still has one empty cartridge slot.

I would like to have a peek inside the cartridge, but don't know how to open it without destruction.
It should be easy to create a cartridge with my own fonts, but for this I need to know the wiring of the address and data lines.

The DeskJet cartridge seems to be glued or welded shut.
(I also have a Pacific cartridge for LaserJets, which can be opened easily as it is only pressed together).

Does anyone know how to open such a cartridge without destroying it?
I am not sure whether there is more documentation for these DeskJet 5xx printers than the Service Manual (which has no schematics)

Martin

 

[NeXT]

My guess is they are internally snapped together than glued. My LaserJet font cartridges are like that and are from around the same era. Are you able to wedge something like a guitar pick down the split in the case?

 

[Martin Hepperle]

Yes, I tried, but could only bend the covers so far that I feared they might break or deform permanenetly.
So I thought there might be a trick of some sort or some hidden latches.

 

[hunterjwizzard]

How rare/expensive are these things? Could you potentially buy a second one to destructively open and use that knowledge to do what you want with the first one?

 

[Martin Hepperle]

Yes, this might be an option, but then - it is not too important.

Today nobody will use such an old printer, so I guess that these cartridges have probably mostly been thrown away together with the printers.
Nevertheless, these early DeskJets were a technical milestone after the ThinkJets as they were the first inkjet printers with an excellent, laser-like print quality at their resolution of 300x300 dpi on plain, non-coated office paper.

There was also an Epson and a IBM Proprinter emulation cartridge to make the printer compatible with older software. These probably contain a ROM with code for the emulation which is then executed by the main processor.

Similar cartridges exist for the LaserJets, including HPGL and Postscript emulation.

 

[hunterjwizzard]

"Using" and "Exploring" are two different things, and I feel like old printers definitely deserve to be explored, preserved, and made functional wherever possible. I think it'd actually be pretty great if someone out there could perfect methods of refurbishing old cartridges/ribbons/etc so more classic printers could be restored.

 

[Martin Hepperle]

In the meantime I found a way to open the cartridge. There is no glue or welding, but four strong hooks which requires some force to open.
I desoldered the ROM and read its contents. I also drew a schematics of the PCB. The larger DIP-32 footprint is connected ion parallel to the DIP-28 but adds one address line, increasing ROM/RAM capacity from 64 kB to 128 kB.

Unfortunately, the structure of the directory and data is not known and differs from the structure used for the softfonts.

HP published a "DeskJet Developers Manual", but no copy seems to have been found/scanned so far.
This document may contain more information as some 3rd parties sold additional multi-font cartridges.

 

[Martin Hepperle]

The attached archive contains updated data - unfortunately, I thought that the ROM would be 64 kB, but it is 128 kB. So I re-read it as a 27C1000 with additional VCC wiring to capture the full content. The schematic and text has also been adapted accordingly.

 

[ArtiomWin]

What kind of font cartridge is? I've a similar cartridge for DeskJet 500 Series with Cyrillic fonts, and would like to read its EEPROM some day too.

 

[Martin Hepperle]

This seems to be a selection of fonts for business applications, using the encoding for Windows.

A similar cartridge exists for the LaserJets (picture lifted from a current EBay auction):

 

[ArtiomWin]

For LaserJets we used this one: