I just purchased 1721 mkII on eBay for $900 to replace my 'dying' 1720. So I
would say that $700 is a little high.

The 1720 is '80s technology. They may have been selling a few in the early
'90s but I purchased a 1721 in 1994 for about $2,000 for my last boat.

More important than the age, is the number of hours the unit has been in
transmit mode. Unfortunately the 1720 doesn't have a 'hour counter' like the
1721 does.

If the unit is still installed on a boat, I recommend that you test it in
all ranges and especially the longer distances. As the magnatron gets
weaker, only the closer ranges will work.

There is a procedure in the manual (page 48) on how to measure the magnatron
current. You only need a digital volt meter since this procedure measures
the current as a voltage (??Yes that is what the manual says.) Anyway the
procedure does require you to remove the radome cover and remove the drive
belt so you can measure at a test point on the modulator circuit board.

Don't purchase a unit with a weak magnatron because a replacement magnatron
will cost around $450.

My opinion and experience, FWIW.

Steve
s/v Good Intentions