Help - Agilent HP 6627A

[Marco_Mar]

Hi everybody, my name is Marco and I am a newcomer in this forum.
I need help.
I recently bought an used Agilent 6627A PSU.
It has four independent outputs.
OUT1, OUT2, OUT3 work fine.
OUT4 is always in "UNR" state, even whitout load.
I note that the OUT1, 2, 3, when the PSU starts, show about 0 V; OUT4 instead shows -0,3 V.

Yesterday I opened the cabinet and I cleaned the inside from dust.

Any suggestion?

Thank you in advance