and FRT-7700 (bottom of page)
FRA-7700: From Active Antenna to Preselector
FRT-7700: From Decent Tuner to Added flexibility and Convenience
FRG-7700: From Inconvenient BCB and SW Switchover to Smooth, One-Port Transition
Don't let my scratched out notes scare you. Take some time to see what I did here. First, note the little block diagram at the top with the 'filter' and the 'amp'. That's
the concept; filter first, then amplify or attenuate as needed.
Then look at the 2 DPDT push button switches which have been completely disconnected and rewired as shown. Follow the insertion points 'A' thru 'E' and add
the connectors of your choice. There's some 'land cutting' on the PC Bd. and some jumpers to add, but following the notes, one-by-one, should get you through
I've modified 4 of these things so far and they are really great assets to a complete listening post. The first one took a full Saturday afternoon because I had to
trace things out and design as I went along. The subsequent ones took about 3 hours.
The tuner mods. Note that the "Antenna" switch is now an "ANT or GND" switch and the "Out" switch is now an "Attenuator Bypass" switch. Study the schematic. There
really isn't too much to do on this one. I suggest that you print this page for study purposes. Also, right-click the schematics and save them to a folder. Later, you can make
blow-ups and print those to study as well.
Perform these modifications at your own risk. Changing, modifying, or altering any product, in any way, may invalidate any warranties, and/or cause refusal of repair.
These data are presented here for informational and archival purposes only and do not guarantee any specific performance or operational standards.
The author cannot be responsible for others' craftsmanship or workmanship.
Never work on "LIVE" circuitry.
Always disconnect/unplug all equipment before opening the case or attempting to work on internal circuitry.
Single Antenna Port for the FRG-7700
Condition: The FRG-7700 comes with two antenna ports. One
is a low impedance input for a shortwave (HF) dipole,
transformer (balun/unun) fed random wire, or similar antenna.
The other is for a high impedance AM (MW) broadcast
antenna. This is fine if you use the FRT-7700 tuner or
FRA-7700 active antenna, but if you don't have, or use, those
devices, or if you have completed the modifications on the
above two devices, then the two ports are an inconvenience.
Analysis: Feeding the SW port only, causes a 36 dB loss when
you tune the BC band. Feeding just the BC input gives you
virtually no signal for the SW bands. I tried several design
approaches in an attempt to "unite" these two inputs. On the
test bench, several measurements were made in each
condition, on each band, with every permutation possible. The
final verdict was boiled down to a simple option of mixing the
two input filter sections with, either (A) a fixed resistor (50 to
200 Ohms)* or (B) a variable resistor (20 K Ohm potentiometer).
The higher value of separation offered by the pot allows the
front end to give max. gain and port isolation. The 100:1 ratio
(difference between the pot and fixed resistor) only buys you a
few dB, and a 2K to 5K pot will suffice, with a 1 to 2 dB loss.
When listening between SW and AM Broadcast, just adjust the
pot for max S-meter readings. Using the fixed resistor will spoil
the sensitivity by anywhere from 2 to 4 dB. If that's a big deal to
you, then use the pot method, not the fixed resistor. Since I
already have electro-OCD, I decided to drill the chassis and go
for the full flexibility by using a "mixing pot."
*(Use a carbon composition or carbon film type, not a
Above is a view of the 20 K Ohm pot and the two wires which connect it to the two antenna ports. Note that the SO-239 connector is already connected to the SW circuitry.
Operation: The advantage of the pot over the fixed resistor is that you may adjust it for either maximum signal on BC or for minimum loading and noise on SW. The differences are
minescule and barely noticeable under most conditions, but they are visible on the test bench and can be heard under some extreme listening conditions.
Turning the pot to zero ohms, which shorts the two inputs together, provides maximum signal transfer for BC, but may add a small amount of (mismatch) noise while tuning SW.
Conversely, increasing the pot's resistance to, say, 10 or 15 K Ohms, isolates the SW filter section of the RF amplifier stage, and provides the cleanest signal, but, as mentioned above,
will cause a significant signal attenuation on AM (MW), B'cast.
The fixed resistor mod is a good compromise, and the less-than-fanatic user will probably never notice any deficiency, nor desire anything more in performance.
You can easily try the resistor route first, by simply inserting it into both ports. Listen to all the bands with the resistor in, with the ports shorted, and with both ports opened. This is easily
accomplished with a couple of clip leads connected to the two ports and quickly trying all three conditions. It is my feeling that, your antenna, a particular station's signal strength, and/or
your location, will dictate whether you make the simple mod or the variable one.
Remember all the usual safety disclaimer stuff: Unplug the radio before opening it, or working on it, and don't do any mods if you are not comfortable working on such equipment, etc, etc
... use common sense, and remember to unplug your soldering iron when you are done (especially if you have a cat)!
|Unify the Two (rather confusing) Antenna Ports
On the Yaesu FRG-7700
One Simple Resistor Will Fix it !
Ooops, I forgot: At the
point marked (C),
break the line and
insert a .01 disk
ceramic cap (.1 if you
use the LF band).
When using the
preamp with the
switched in, this
You may also want to
hang 2-pair of
protection diodes off
J1, and a 10K resistor
across that as well for
NOTE: For best
performance, use the FRT
ahead of the FRA.
Although not necessary,
the combination will amaze
you, for grabbing the real
Below are the mods performed to the Yaesu Active Antenna (FRA-7700) and its companion Tuner (FRT-7700).
See the bottom of the page for associated mods to the two antenna ports to the FRG-7700 Receiver.