a mechanical SSB filter on the FRG-7
The Yaesu FRG-7 is a general coverage MW/SW receiver
that uses the Wadley Loop system for stabilizing the frequency tuning.
The receiver has a good sound on AM mode, that reminds me the tube
receivers sound. However, on sideband mode, it is pretty much useless.
The IF ceramic filter that is used, does not have enough selectivity to
reject the opposite sideband. No matter if the front panel mode
selector switch states USB/CW and LSB, these just shift the BFO,
nothing more. The receiver is a DSB set not SSB. A cheap way you can
accomplish single signal sideband reception with the FRG-7 is described
in this link. Wereas it works, it increases the audio bandwidth of the signals to the high pitch.
A better approach is to install an additional mechanical filter to
the receiver. This of course requires expensive 455KHz mechanical
filters, but if you have one in hand or if you are willing to pay for
the improvement in performance, then this is the recommended option.
But you can't just desolder the ceramic filter of the receiver and
solder a mechanical filter in place. On AM mode, you need wider
bandwidth, but on SSB mode you need narrower. So both filters must be
in place and a selection must be done in each mode. Thankfully, this
modification is pretty easy on the FRG-7 and it does not require any
modification of the external appearance of the radio.
The schematic of the FRG-7 is
shown above. Everything with red color, are part of the modification.
The modification is pretty straight forward. You have to desolder the
original ceramic filter from the FRG-7 PCB and install it on a separate
PCB along with the new 455KHz mechanical filter. To select between the
two filters, a 9-12v DPDT relay can be used and it must be connected as
shown in the schematic. The power for the relay coil is derived from
one section of the mode switch (S3d). On USB or LSB modes, the BFO is
energized and this power is also used to energize the relay, which in
turn switches to the narrow mechanical filter on these modes.
A good place for the new
PCB that accomodates the filters, is just below the main tuning dial of
the receiver. There is a hole there and three screws, which can be used
to also hold this PCB in place. I needed to replace these screws in
mine with longer ones, because I used spacers to prevent the PCB from
touching the chassis. But this is optional.
Two small pieces of coaxial cables are
used to connect the new PCB to the pads of the ceramic filter, that has
been now removed from the original PCB of the receiver. Ground these
cables on both ends.
The power cables for the relay coil
(shown with red and black in the picture above), are passed below the
PCB to the chassis opening and through a hole to the bottom of the
original PCB of the receiver. The ground wire is soldered to the filter
ground point and the red wire is soldered to the mode selector switch
S3d. S3d is the outer wafer onto the switch. Use a multimeter to find
the contact of the switch that has VCC when the mode is switched to USB
or LSB. This is the point where you want to connect the red wire.
After installing everything, you should perform an alignment of the
TC404 and the T406 in the BFO section as described in the manual. This
requires a frequency counter, but I did my alignment by simply
adjusting the two controls by ear, until I got roughly the same pitch
on LSB and SSB audio bandpass. These controls interact, so you have to
do a bit of back and forth in both of them. It is very easy.
After installing the modification and aligning the receiver, the result
is pretty obvious. No more DSB reception, SSB signals are received just
once in the dial and their bandwidth is limited as it should on SSB.
The mechanical filter I had, was a bit narrow (2.1KHz) so I can also
hear a bit os "seashell" sound on SSB, but SSB voice signals are
perfectly understood. It is interesting that the audio volume between
the ceramic filter and the mechanical filter was just about the same,
which indicates that there is no additional loss in the newly installed
filter. Another interesting thing is that there was no need for any
impedance matching using active devices or transformers on the
mechanical filter. It worked just by directly connecting it. Neither
it's loss, not it's response seems to be affected by any possible
Note that Collins produced both symmetrical and asymmetrical mechanical
filters (yes they used two filters, one for USB and one for SSB in some
of their gear). My filter is a symmetrical one (same roll-off response
curve on both sides of the filter passband). If you use an asymmetrical
filter, expect a bit different pitch when switching from LSB to USB and
vice versa. Not a huge problem, but just a note.
By performing this simple modification, you will end up with an FRG-7
receiver that is trully selective, allowing for real SSB reception.
Most importantly you do not ruin the appearance of your prescious
FRG-7, but just improving it's performance. This modification would
probably be appreciated much when deciding to sell your FRG-7 to
UPDATE 21 Nov 2022
I removed the Collins mechanical filter, which was too narrow, and I
replaced it with a Murata CFJ455K5 SSB ceramic filter This made things
much more compact. Because of this I made a tiny new PCB with the two
ceramic filters onto it and I placed this PCB vertical, right at the
pads where the original AM filter was soldered onto the FRG-7. Things
are definitely looking better now!
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