Bizhub c220 weak back print

[ammorex112]

hello guys! last week we had a problem with a bizhub c220 machine. suddenly it started to print weak black. the cmy color prints were fine. the technician replaced K toner, black drum, black developing unit, transfer belt, hvb board, prcb board, laser unit. nothing worked. we had to replace the entire machine ( it was under maintenance contract), so we brought it back to our service. surprise, after transport now is working fine. we cannot duplicate the initial issue, no matter what we tried. i remember encountering this issue years ago, but i don't remember what was the cause ( on a c450 machine). i suspect an e bad electrical contact somewhere, but i don't know where to look. i must mention that the machine is clean in the back of k developing unit , drum unit contacts are clean on the machine. any ideas? pictures attached.