[Racal] RA6778C Microprocessor Noise

[Dallas Lankford]

My 6778C had some uP noise at the lower end of the MW band.  Even if I =
were not a MW listener, I would still want to eliminate it.  I =
discovered that disconnecting the RF amp and using the 6778B barefoot =
caused the uP noise to go away.  Examination of the RF amp module =
revealed some curiosities.  First, I noted that the shield was held in =
place with nylon screws.  Next, with the cover removed, I observed that =
the RF amp itself (the PC board) was isolated from ground (the mounting =
posts which held the amp were mounted to the metal base with nylon =
washers and nylon screws).  Someone had gone to a lot of trouble to =
isolate the RF amp from ground.  But they hadn't gone far enough.  From =
the RF amp the signal path to the rear panel is through miniature coax, =
a BNC (m), and a bulkhead  BNC (f) to BNC (f).  Disconnecting the BNC =
(m) inside the 6778C and connecting it via a BNC (f) BNC (f) adapter to =
the BNC (m) connector of the antenna eliminated the uP noise.

What had not occurred to someone to do was to replace the traditional =
(grounded) bulkhead connector with an Amphenol 31-4803, Mouser =
523-31-4803.  When this was done, the microprocessor noise at the lower =
end of the MW band was eliminated.

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