I discovered a piece of software called Scribus. It is an open source desktop publishing program, and it allows one to work in CMYK space. I made a document with a black square made from 100%K and a "400% black" square, made from 100% of C,M,Y and K. Scribus allows you to use ICC colour profiles and display what your printer will actually print. I downloaded the 4550's colour profile from HP, plugged it into Scribus and sure enough the black square was lighter than the 400% black square.
I printed to postscript and looked at the print preview. The preview allows one to look at the different colour components, and sure enough one square had just K, and one had CMYK. So I was very excited, and printed it out.
Disappointment. Both squares printed the same, K only. So I hunted around for ages. I looked at the postscript documentation, and found the property "Overprint", which is set by "[boolean] setoverprint". According to the postscript specification if overprint is false a printed colour will delete all other colours in the same place. If it's true, the printed colour will be printed on top of any other colours present. I was very excited to find that scribus disables overprint. So I edited the postscript file to read "true setoverprint" instead of "false setoverprint".
More disappointment, still no overprint. The postscript spec does say that some devices don't support overprint, so perhaps I have one of those. No 4550 documentation mentions it.
So I could attempt technical support, but that will obviously be quite painful.
Instead I had a look at how the transfer belt is cleaned between prints. It has a charged cleaning roller that is pushed against the belt by a cam. The cam is connected to the main drive train by an electromagnetic clutch CL3. I found CL3 and put an ammeter in series with its connections. As the image is printed to the belt no current flows. The clutch is activated (150mA flows) as the transfer to the paper starts, so that the cleaning roller catches any toner that doesn't get transferred. Another clutch, CL4, is turned on at the same time. It lifts the secondary transfer roller to squash the paper against the belt.
So I need to incapacitate CL3 (to prevent cleaning) and CL4 (to prevent printing onto the secondary transfer roller), and print a page without the board in place. Then I will enable CL3 and 4 and print a second time, this time onto the board. I can repeat the first step N times to get N layers of toner, but hopefully 2 will do.
However at the moment the printer must be rebooted between prints as every print results in a "paper jam" as no paper passes through the fuser unit. Rebooting the printer makes it recalibrate - printing test patterns onto the transfer belt. So before I can disable the clutches I must fool the fuser unit paper sensor to allow consecutive prints. This is another annoying timed affair.
As Steve pointed out this morning, I can't really survive much longer without a microcontroller on there!
Submitted by jeff on Sat, 03/24/2007 - 17:24. categories [ ]