A P180 CAPTAIN AND FIRST OFFICER EHSI'S AND EADI'S BECAME UNUSABLE IN FLIGHT AFTER HAVING DEPARTED IN RAIN SHOWERS. MAINTENANCE DISCOVERED MOISTURE IN THE RADOME AVIONICS BAY.

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DESCRIPTION OF EVENT: AFTER FLYING THROUGH RAIN SHOWERS OUT OF ZZZ AND AGAIN ON DSCNT INTO ZZZ1; WE NOTICED THE COPLT'S EHSI DISPLAY ACTING STRANGE. THEN THE MFD NAV DATA BECAME UNREADABLE EVEN THOUGH THE RADAR DISPLAY WAS STILL WORKING. THEN I LOST BOTH ELECTRONIC DISPLAYS ON THE PLT'S SIDE WITH BOTH INDICATING DPU FAILURE. THIS WAS DURING DSCNT TO 5000 FT. AFTER ALL THIS; I HAD MY COPLT ASK ATC FOR A HDG TO KEEP US VMC SINCE WE HAD JUST BROKE OUT OF THE CLOUDS. THIS WAS APPROVED AND SINCE WE STAYED IN VMC I TOLD ATC I DID NOT NEED TO DECLARE AN EMER; WHEN THEY ASKED; AS LONG AS I STAYED IN VMC. I DID TELL THEM THAT I WAS HAVING INSTRUMENTATION PROBS. WE WERE THEN CLRED DIRECT TO THE FIELD. AFTER RPTING FIELD IN SIGHT WE WERE CLRED FOR THE VISUAL APCH INTO ZZZ1. WHEN TURNING BASE FOR RWY 18; WE CANCELED IFR. WHILE WE WERE IN VMC; I TRIED THE DRIVE XFER SWITCH. WHAT I GOT ON THE EADI WAS A PICTURE THAT LOOKED LIKE 'DOUBLE VISION.' THIS INCLUDED THE COMPOSITE MODE FOR EADI AND THE PLT'S SIDE EHSI. SINCE WE WERE IN VMC; I SWITCHED EVERYTHING BACK TO NORMAL SWITCH POSITIONS AND LANDED. WHEN I CALLED MY CHIEF PLT HE TOLD ME THIS WAS A RECURRING PROB IN THIS AIRPLANE THAT WAS CAUSED BY MOISTURE GETTING IN THE AVIONICS BAY IN THE NOSE OF THE AIRPLANE. A MECH CAME OUT THE NEXT DAY; REMOVED THE NOSE RADOME; AND FOUND EXACTLY THAT. AFTER REMOVING THE DPU; THEN DRYING OUT THE AVIONICS RACK; CLEANING CONTACTS; EVERYTHING WAS PUT BACK TOGETHER. AN EFIS CHK WAS RUN AND IT CHKED OUT OK. AFTER CONSULTING WITH MAINT CTL; THE AIRPLANE WAS SIGNED OFF BY THE MECH AS OK TO FLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT HIS FRACTIONAL CARRIER HAS A LARGE P180 FLEET AND THIS PARTICULAR AIRCRAFT IS CURRENTLY THE ONLY ONE THAT HE IS AWARE OF WITH THIS RADOME MOISTURE PROBLEM. THIS PARTICULAR AIRCRAFT HAS A HISTORY OF RADOME SEAL LEAKAGE ALLOWING MOISTURE ONTO THE AVIONICS AND HE BELIEVES A SERVICE BULLETIN HAS BEEN ISSUED ADDRESSING THE RADOME SEAL.