"I'm repairing HP plotter 7550 and there are some chips that I suspect to be broken. They are related to serial communications ( thats what is broken). The chips are labeled as 1820-3321 and 1820-3322. I suspected that they could be 1488 and 1489 but changing them did not fix the problem"
(From Paul Grohe (firstname.lastname@example.org).)
Been there, dunnit...
The serial interface on the 7550's is...well...weird!
Without the proper cable, the interface will seem "dead" or unresponsive.
First, you need to use the MALE 25 pin connector, not the female one (which you may automatically assume).
It requires a specially wired serial cable (you can build it yourself). A standard modem cable with a F-F adapter will not work. There are a few handshake lines that need to be wired. Note that you will also need to get a FEMALE plug for the plotter end of your serial cable.
There are also some menu setups required.
HP has a diagram of the needed cable and menu setups on their FTP site. Get these two documents:
Everything you need, short of RTFM, is in these files.
(These no longer exist. If anyone knows what happened to them or has the info, please send me mail via the Sci.Electronics.Repair FAQ Email Links Page. --- Sam.)
There are some other plotter docs at:
The 7550 suffers from paper pickup rollers that dry/wear out and have trouble picking up the paper from the tray. If you look at the rollers, there is a flat portion. The edge of the "flat" part wears down and becomes rounded, causing it to no longer pick up paper. Try to keep the paper tray as full as possible. Overfilling the paper tray will cause it to pick up multiple sheets, and that can cause a real mess!!
Check the paper feed slot and pinch rollers for shredded paper.
Also, check and make sure the screw that holds the pen carousel together is tight. If this screw becomes loose, it will cause pen loading problems.
It's a "helluva" plotter when it works!