I often get calls from Designjet owners with the 700 and 600 series printers. Their display windows with Vacuum Fluorescent Display (VCD) is dimming or totally burned out. I've now sold out all the extra front panels that I had in my workshop and I too am looking for the replacement windows. If you've found a place to buy a replacement and tried it and it works -- most likely in China! -- Please share.
Front of VFD from HP Designjet 750C
Back of VFD from HP Designjet 750C
This seller on eBay claims to have a VFD that will fit the Designjet 750C and the Designjet 450C. He's wrong about the Designjet 450C as that printer doesn't have a display window!! However it may be true about the 750C, but I don't have a 700 series printer to test it on right now. The seller has more than 10 available and I'm hoping someone will try it and see if it fits and works. The five little pins on the back look like they will fit and our VFD needs 2 x 20 characters and this one has that...but I'm not smart enough about this type of electronics to know if it will work. If you try this, please let me know!