• Please review our updated Terms and Rules here

Opening HP DeskJet 500 Font Cartridges

Martin Hepperle

Experienced Member
Nov 10, 2014
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)

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?
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.
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?
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.
"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.
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.


  • DeskJet-Font-Cartridge(2).jpg
    1.5 MB · Views: 11
  • DeskJet Font Cartridge Schematics.png
    DeskJet Font Cartridge Schematics.png
    63.9 KB · Views: 11
  • HP22707L_Cartridge.zip
    17.2 KB · Views: 1
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.
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.


  • PXL_20240620_115604958[1].jpg
    2.6 MB · Views: 7
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):


  • ODS-Business-Cartridge-LaserJet.jpg
    157.6 KB · Views: 2