Oldies but Goodies. Well, sort of. Yaesu put a decent effort into developing these accessory devices for their FRG-7700 (The FROG) receiver
back in the late 1980s. The hardware quality is typical of the period for high-end receivers and the components are all first rate. The choke
coils in their active antenna and tuner are good high Q devices and the gear driven air variable capacitors are excellent ... but their circuit
design, of that period, needed some help in being brought up-to-date with current technology and current development knowledge. The
finished units are very, very effective. I can knock out a 3 KW station 2 miles across the river only 10 KHz away from my target station, WLW in
Cincinatti, - and I'm about 700 miles away in Southern Connecticut!
I have been aware, for some time, that these two Yaesu products are
nicely built products with good quality components. However, I was never
really happy with the circuitry as designed. Here is the  FRT-7700
receive-only tuner. I really didn't change much inside this unit: I added
SO-239 connectors on the back for input and output and rewired the "ANT"
switch so that the "Out" position grounds the antenna terminal. Also, the
"OUT" switch now either enables or inhibits the Attenuator. These mods
only added some user convenience and flexability. Very Sharp tuning -
thanks to the high Q coils provided.
Here's the FRA-7700 which was originally Yaesu's attempt at an active
antenna. Inside is a "parallel" resonant preselector (not very effective with
a very low "Q") but a nice quiet transistor preamplifier. This little bugger
took all afternoon to modify. As above, I added two SO-239s on the rear
apron. The "PREAMP" switch turns on the 9-volt feed and switches the
preamp in and out of the RF path. The "OUT" switch now switches the
preselector in and out of the RF path (it was always "in-circuit" in the old
configuration). I cut new lands into the printed circuit board and
reconfigured the preselector from parallel to series resonant. WOW, real
sharp filtering now !
Some shots of the completed listening post in the family room.
Everything has to be very neat and orderly to pass the "wife test."
Note the sexy blue LED pilot light in the preselector.
YAESU  modifications
                                     FRA-7700 & FRT-7700
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Here's a Japan Radio
NRD-515 with my Type-5
Preselector in a side-by-side
comparison with the
"Yaesu-Twins." The modified
Yaesu stuff really stood up
well in tests against my
homebrewed Type-3.

Although some bench testing
was performed, I finalized the
designs with "real world" use.

Most of the tests were in the
LF, Lowfer band where I
compared the reception
qualities of NDB's, and in the
MF, AM broadcast band, and
up through the 160-meter
Ham band to about 2 MHz.

The greatest effectiveness
seems to be from about 100
KHz to well above 10 MHz. But
even at 15 to 20 MHz, some
selectivity advantage was
noted.

Of course, this all depends on
the necessity and utility of the
preselector. Obviously, if its
use isn't needed, then there
isn't much advantage to using
it.

One exception to the above
might be in the reduction of
"out of band noise." Under
high noise conditions, that
utility can be of great
advantage.
A closer view of the test.
The devices under test. The
testing was almost completely
subjective, based only on my
ears and the 515's S-meter -
some bench tests were
performed, but actual usage and
your ears are a good final judge.
Go to the schematics