transponder questions

My transponder can't be seen by ATC anymore and I'm
trying to narrow down the cause and plan for the
repair. If it can be done simply by removing the
radio and taking it somewhere to be bench tested,
that'd be much easier for me. It's some vintage
of a KT76.

A couple of months ago, the altitude encoder was
inspected and found to be mostly fine. The inspector
did warn me about the lifespan of something but I
don't remember if it was the radio or encoder.
It seemed to run fine after this inspection but
I think a couple towers mentioned it was weak.

Recently, after a 13 minute flight, it still wasn't
visible to ATC so I don't think it's a warm-up problem.

Another symptom is that the light is constantly
flickering (although it goes solid when I press
ident.)

If the encoder failed, would the transponder be
completely silent ?

Are there any simple tests I can perform myself ?
Maybe a SWR meter on the antenna ? A certain
voltage coming from the encoder ?

If it is the radio, I'd like solid state this
time around.

Are there any transponders that have a built-in
encoder ? That way, I figure I can take care of
two birds with one stone. If not, I guess I should
narrow it down before making a purchase.

Is a transponder something that the owner can
take care of ? I noticed that Aircraft Spruce will
ship the low-end models but not the higher end
ones, saying that they have to install them.
Nevertheless, I have to believe that there has to
be a sign-off at some point. Additionally, the
encoder static system is supposed to be inspected
each time... I forget the wording, something like,
"any time there is a modification that may
affect its operation."

Thanks for any insight.