RST Engineering wrote:
"Marco Leon" wrote in message
oups.com...

Another point of info, when ATC claims that they don't see me, I do
notice that the Reply Light is not flashing. When it does, ATC sees me
a minute or two later after their radar sweeps. So, if it *was* the
encoder and not the transponder, I should still see the Reply Light
flashing, correct??


The encoder working or not working has absolutely nothing to do with the
reply light.



If so, it sounds like I need to replace the antenna first before
thinking about the transponder.


Why don't you consider running some tests before shotgunning parts that may
or may not be bad?

Jim


The reply light should come on whenever the transponder decodes a valid
interrogation and generates a reply. The reply can be suppressed by an
interrogation coming too soon after a previous one, by the box being put
in standby, or with an externally applied suppress signal. If the
encoder ready is hooked up to that suppress signal, it would prevent a
reply...but it isn't supposed to be connected that way. That input is
meant for other equipment to assert if a transponder reply would mess up
the equipment function. It is typically used for DME to suppress a
transponder transmission while the DME is waiting for a reply.

More likely, your transponder may be out of calibration. Older
transponders (I'm familiar with the guts of the Narco line) like the
Narco AT150 and earlier use resistor-capacitor combinations to set the
delays for the interogation decode. If the values drift or the trimming
pots move slightly, your transponder may no longer reliably decode
interrogations. The delay is also somewhat temperature sensitive to the
extent that temperature changes the values of the components slightly.
It could be your transponder needs to be recalibrated.