The Hamtronics manual says you HAVE to use a
duplexer with at least 95 db of isolation (try to find one)
and mine is 77db. I really can't imagine there is not
enough isolation, because I know where there are other
machines running with old duplexers with much less
than 77db of isolation

Rick-

I would guess your duplexer was intended for a commercial repeater that had
greater separation than 600 KHz. You are kidding yourself if you think you can
make it work. Those other repeaters with old duplexers either have desense or
have some kind of super filters in the transmitter and receiver chains.

About the only thing that might help you (after you get that double-shielded
co-ax) would be adding a couple extra cavities. Although that approach will
probably have a higher insertion loss, it may be possible to tune the resulting
set for sufficient isolation.

If you can't fix the cavities, then all you can do is reduce transmit power out
of the repeater. If you reduce from one hundred to one watt, you pick up the
equivalent of 20 dB of isolation.

(A four-cavity duplexer with 95 dB isolation may be possible, but the cavities
will be big ones.)

73, Fred, K4DII