Applicable Reference:
WDM 34-51-41
Discrepancy:
VHF Nav control panel displays “FAIL” in both windows.
Possible Fix:
Repair bonding of ground terminal blocks GD503-DC/GD515-DC at P5 overhead structure.
Tip:
This is a intermittant problem, you might be able to duplicate the defect by pushing the frequency transfer switch on
the VHF nav control panel a lot while monitoring for a Fail message.
Check the resistance at the VHF Nav control panel plug J2 pin 15 for a proper ground. The faulty aircraft had a
resistance value in the kilo ohm range.
During the time it was failed a fault is seen as both the MMR and the VOR/Marker Beacon receiver of “Tuning port B
faults”, in BITE.
Reason/Explanation:
The ground is used for Frequency transfer at the VHF control panel. It is wired through the Instument switching
module on the P5 panel this is why the ground is above the P5 panel.
With high resistance at the ground a very intermittant “Fail” is seen on both VHF Nav control panels. It is often times
cleared on the ground or with a power up reset, but will continue to return.
Thanks again to Tom S. for his input.
