A FOKKER F28; AFTER CAPT'S HSI INDICATOR REPLACEMENT; WAS DISPATCHED WITHOUT REQUIRED DME TEST; DUE TO NO TEST AVAILABLE AND ZZZ TEST XMITTER NOT OPERATING.

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ON NOV/XB/98; THE ABOVE ACFT WAS FERRIED FROM YYY TO ZZZ DUE TO A #1 HSI FAILURE; WHICH SUBSEQUENTLY CAUSED A DME FAILURE. I WAS INFORMED BY ACR MAINT CTL THAT THE ACFT WAS INBOUND WITH #1 HSI DISCONNECTED AND THAT '#1 HSI NEEDS TO BE REPLACED.' AFTER THE ACFT ARRIVED; MY LEAD MECH AND MYSELF INSTALLED A NEW #1 HSI. THE OUTBOARD AIR CREW ARRIVED AT THE ACFT AS WE STARTED OUR CHKOUT (AS OUTLINED IN A STEP-BY-STEP ON A MAINT TASK CARD). THIS WAS A CHKOUT TO RETURN THE ACFT TO CAT II STATUS. THERE WERE 2 PROBS WITH THE CHKOUT: 1) THE 'TO/FROM' POINTER WAS NOT RESPONDING; AND 2) THE DME WAS INOP. FIRST I AND THEN THE OUTBOUND CAPT (HE WAS OBSERVING) CHKED THE CIRCUIT BREAKER PANEL -- NONE WERE FOUND POPPED. THEN I HAD HIM CHK HIS RELEASE AND HE RESPONDED 'NO WONDER; ZZZ DME IS DOWN.' WE THEN FOCUSED ON THE 'TO/FROM' POINTER. AFTER ABOUT 10 MINS AND PWRING DOWN THE ACFT; THE 'TO/FROM' POINTER RESPONDED NORMALLY. AFTER TALKING WITH MY LEAD; I ASSUMED THAT SINCE NO CIRCUIT BREAKERS HAD POPPED (AS HAD HAPPENED IN YYY) ON THE DME SYS WHEN THE ACFT HAD FLOWN IN OR DURING THE CHKOUT; THE SYS WAS PROBABLY OK; BUT SINCE I HAD NO WAY TO CONFIRM IT (ZZZ DME DOWN) I COULDN'T RETURN THE ACFT TO CAT II STATUS. I ELECTED TO SIGN OFF THE ACFT; BUT KEPT IT ON CAT I STATUS. AFTER DEP AND AT SOME TIME DURING CRUISE; THE CAPT REALIZED THE DME SYS WAS INDEED INOP. (HE STATED TO ME SEVERAL DAYS AFTER THAT HE HAD NO PROB WITH THE PLANNED APCH; BUT THE WX WAS WORSENING AND THAT THE DEP FROM YYY WAS GOING TO BE THE PROB.) AFTER DISCUSSING THE SIT WITH ACR DISPATCH AND MAINT CTL BY RADIO; THEY ELECTED TO DIVERT INTO XXX TO MAKE FURTHER REPAIRS. WE DO NOT NORMALLY HAVE DME TEST EQUIP AT ZZZ. WHEN IN THE PAST WE HAVE NEEDED ONE; WE HAVE HAD TO HAVE THE TEST BOX SENT FROM OUR XYX MAINT BASE FOR A SPECIFIC PROB (USUALLY ON AN EXPEDITE BASIS) AND THEN RETURNED. WE DO NOT HAVE ONE ON HAND AT ZZZ MAINT. THE TIME PRESSURE I MENTIONED MADE ME HESITATE TO CALL AROUND TO DIFFERENT MAINT HANGARS AT ZZZ TO BORROW ONE WHEN I THOUGHT THE SYS WAS OK.