Designjet 500 constant print head alignment

"Recently we changed the belt on a hp 500 designjet and although the belt was installed correctly using as a guide your video, the plotter keeps trying align the print heads in a loop cycle. we've tried everything. we read on a forum that the carriage has two bushes that if installed upside down could cause this but we didn't see any bushes on the carriage. any thoughts? Thanks in advance."

The print heads were removed for the belt replacement and probably dried up.  Try removing the print heads blast them at short range with Windex 5-10 times and then press them onto a folded up paper towel to pull the Windex and ink out of the nozzles. If this procedure doesn't help, try the recovery kit that allows you to mechanically force ink back into your printheads:

HP Designjet 500-800 Printhead Recovery Kit

If that does not bring the printer back to life, replace the print heads. 

Bob