Icom R71A - IF Filter Select Modifications
Gregory K Doerschler
MODIFICATIONS TO THE ICOM R71A IF FILTER SELECTION CIRCUIT
by Greg Doerschler
(g...@wpi.edu)
This article may be distributed or reposted freely provided that it
remains completely intact.
INTRODUCTION:
Simple modification to the IF filter selection circuit of the ICOM R71A
general coverage receiver can provide additional bandwidth selection
options in receivers which have been equipped with the optional FL-44A
high grade 2.4 KHz crystal filter (or equivalent). In the AM mode, the
modification enables a 2.8 KHz bandwidth to be selected in addition to the
6 KHz and 2.4 KHz bandwidth settings which the "FL-44A"-equipped R71A
already offers. In the SSB, CW and RTTY modes, the modification can be
used to enable the PBT knob to function as either a passband tuning
control or an IF-shift control.
The potential benefit of this modification depends to a large extent on
how the receiver is used. R71A owners considering the modification should
take some time to decide whether it is worth the effort before proceeding.
Much of this article discusses the rationale behind the modification and
explains its actual effect. The procedure itself - described at the end
of the article - is not very complex, but it does involve some delicate
soldering and the ability to read schematic diagrams and identify
components on the PC boards.
I can attest that my modified R71A performs as indicated, but I assume no
responsibility for the outcome of anyone's modification efforts. Make
sure you understand and are comfortable with what you are doing, and
proceed at your own risk. While not absolutely critical in this case, it
is generally smart to have a service manual handy when modifying
equipment.
BACKGROUND:
Intermediate Frequency (IF) bandwidth filtering in the R71A is
accomplished by filters in the 9 MHz 2nd IF stage and in the 455 KHz 3rd
IF stage. The stock R71A comes with the following standard filters:
2nd IF: FL-70 (2.8 KHz) 3rd IF: CFW455IT (6 KHz)
FL-30 (2.3 KHz) CFJ455K5 (2.8 KHz)
While the 2nd IF filters are of reasonable quality, the 3rd IF filters are
inexpensive ceramic filters. ICOM offers the FL-44A 2.4 KHz high grade
crystal filter as an optional replacement for the 2.8 KHz CFJ455K5. Other
filter manufacturers have comparable replacements available too. One of
two optional (additional) CW filters may also be installed in the 2nd IF;
the FL-32 (500 Hz) or the FL-63 (250 Hz).
With only its stock filters, the R71A utilizes the following filter
combination in its various modes:
2nd IF 3rd IF
AM wide...............(none)................ 6 KHz (CFW455IT)
AM normal............ 2.8 KHz (FL-70)....... 2.8 KHz (CFJ455K5)
SSB/CW/RTTY wide..... 2.8 KHz (FL-70)....... 6 KHz (CFW455IT)
SSB/CW/RTTY normal... 2.3 KHz (FL-30)....... 2.8 KHz (CFJ455K5)
At first glance, one might think that it would have been more logical to
use both 2.8 KHz filters in the SSB "wide" position (similar to the AM
"normal" position), rather than a 2.8 KHz filter followed by a 6 KHz
filter. (Since the narrower of the filters in the IF chain really
determines the overall bandwidth, the 6 KHz filter is actually doing
nothing.) However, if things had been set up this way, replacement of the
2.8 KHz 3rd IF filter with the optional FL-44A 2.4 KHz high grade crystal
filter would have restricted the SSB "wide" bandwidth to 2.4 KHz. The SSB
"wide" and "normal" bandwidths would then have been virtually identical.
Observe that the installation of the FL-44A does in fact reduce the AM
"normal" bandwidth from 2.8 KHz to 2.4 KHz. Here's how the filter chart
would look for an R71A with both the optional FL-44A and the optional
FL-32 CW narrow filter installed:
2nd IF 3rd IF
AM wide.................. (none)................ 6 KHz (CFW455IT)
AM normal................ 2.8 KHz (FL-70)....... 2.4 KHz (FL-44A)
SSB/CW/RTTY wide......... 2.8 KHz (FL-70)....... 6 KHz (CFW455IT)
SSB/CW/RTTY normal....... 2.3 KHz (FL-30)....... 2.4 KHz (FL-44A)
Narrow-all modes......... 500 Hz (FL-32)....... 2.4 KHz (FL-44A)
Having the razor sharp 2.4 KHz bandwidth of the FL-44A accessible in the
AM mode can sometimes be great for critical DXing, but AM audio at that
bandwidth does lack sufficient fidelity for comfortable listening.
With some minor rewiring of the filter selection hardware, the 2.8 KHz AM
bandwidth can be restored in "FL-44A"-equipped R71A receivers. This is
done by making the AM "normal" setting select the same 2.8 KHz (FL-70) / 6
KHz (CFW455IT) filter pair which is used in the SSB "wide" setting. For
experimental purposes, I had temporarily configured my R71 to select this
filter pair in the AM "wide" setting in order to make direct receive
fidelity and skirt selectivity comparisons against the 2.4 KHz AM "normal"
setting.
My findings revealed that with the receiver tuned directly to a broadcast
station, both bandwidths produced very similar sounding audio without much
fidelity. But by tuning the R71A's main tuning dial 1.5 KHz away from the
AM carrier frequency, the 2.8 KHz filter passed the carrier and enough of
one of its sidebands to provide halfway decent fidelity. (It was
noticeably less than the 6 KHz bandwidth, but still quite usable.) The
same trick was also tried with the narrower, 2.4 KHz passband by tuning
the main dial 1 KHz off frequency; but even then the fidelity was still
too restricted for comfortable listening.
Since the 2.8 KHz bandwidth seemed to isolate noisy AM signals just as
well as the 2.4 KHz setting in most of my tests, I had initially
considered replacing the 2.4 KHz setting with 2.8 KHz in order to take
advantage of the additional fidelity. However, I did encounter some
relatively rare instances when trying to tune in a weak AM station
adjacent to a strong local broadcaster where the 2.8 KHz/6 KHz filter pair
allowed some odd residual noise from the strong adjacent signal to become
embedded in the desired signal. The noise was not there when using the
2.8 KHz/2.4 KHz filter pair. This revelation led me instead to devise a
way of making both bandwidths available in the AM mode (in addition to the
6 KHz bandwidth).
TECHNICAL DISCUSSION:
For the following technical discussion of the IF filter selection circuit
of the R71, you may wish to refer to your schematic diagram or service
manual. The filter select IC is IC3 on the main PC board. It has three
input pins (pins 9, 10, and 11); the logic state of which are determined
by the operating mode (AM, FM, SSB/CW/RTTY) and by the position of the
"wide" and "narrow" filter switches on the front panel. The configuration
of these settings determines which IC output pin goes high. (Output pins
are pins 1, 2, 4, 5, 12, 14 and 15.) Diodes connected from the output pins
to the various filters determine which filters are activated by each
specific mode-bandwidth combination. There are two output pins (pins 1
and 4) which are only activated in the FM mode. These outputs are unused
(not connected to anything) since the optional FM board has its own IF
filter which is automatically selected in the FM mode.
The following table from the service manual shows the input-output
relationship of IC-3:
INPUT PIN
MODE 9 10 11 OUTPUT PIN
SSB/CW/RTTY normal L L H 14
SSB/CW/RTTY wide H L H 5**
SSB/CW/RTTY narrow* L H H 12
AM normal L H L 15
AM wide H H L 2
AM narrow* L H H 12
FM normal H L L 1
FM wide H L L 1
FM narrow* H H H 4
* assuming optional filter switch (S1 on main PC board) is ON.
** this output was incorrectly listed as pin 3 in the service manual.
There are two wires which send logic data from the filter switches on the
front panel to the IC input pins. One goes high when the "wide" button is
depressed and the other goes high when the "narrow" button is depressed.
The switches are deliberately interconnected in such a way that both wires
are low when the switches are simultaneously depressed (just like if
neither switch were depressed). By installing two solder bridges on the
PC board where the switches are mounted, this interconnection can be
defeated so that both wires will go high when the switches are
simultaneously depressed. Doing this causes one of the unused "FM" output
pins on the filter select IC (pin 4) to be activated in any mode.
Installing diodes from this pin to the IF filters essentially provides an
additional filter selection option.
I decided that my AM normal "both switches out" configuration would be the
2.8 KHz setting (2.8 KHz FL-70 and 6 KHz CFW455IT), and that the "both
switches in" configuration would be the narrower setting employing the 2.4
KHz FL-44A in the 3rd IF. But rather than use the 2.8 KHz (FL-70) filter
in the 2nd IF for this narrower setting, it made more sense to use the 2.3
KHz (FL-30) instead since the FL-30's bandwidth more closely matched that
of the FL-44A.
There was another advantage to setting the options up this way. The "both
switches in" bandwidth selection is the same in all reception modes.
Since the FL-30/FL-44A combination was already the "normal" (both switches
out) SSB/CW/RTTY selection, I had the opportunity to change that
selection without losing access to the FL-30/FL-44A filter combination in
the SSB/CW/RTTY modes.
By making the SSB/CW/RTTY "both switches out" setting use only the FL-44A
in the 3rd IF (bypassing the 2nd IF filters altogether), the passband
tuning control would act like an IF-shift control when this setting was
selected. I found this option to be useful when tuning in AM broadcast
stations in the SSB mode using ECSS reception. By adjusting the IF shift
function so that the passband was centered directly over the AM carrier,
the upper and lower sideband components could be heard beating against
each other. The main tuning dial could then be adjusted until a "zero
beat" was heard; indicating that the signal was properly tuned.
Note that since bypassing the 2nd IF filters does reduce the dynamic range
of the receiver, use of the IF-shift function in the presence of very
strong signals on nearby frequencies may impair reception (similar to what
happens in the AM wide position).
With the above changes done, here's what my filter selection
configuration would look like:
2nd IF 3rd IF
AM, wide button in............. (none)................ 6 KHz (CFW455IT)
AM, both buttons out........... 2.8 KHz (FL-70)....... 6 KHz (CFW455IT)
SSB/CW/RTTY, wide button in.... 2.8 KHz (FL-70)....... 6 KHz (CFW455IT)
SSB/CW/RTTY, both buttons out.. (none)................ 2.4 KHz (FL-44A)
ALL MODES, both buttons in..... 2.3 KHz (FL-30)....... 2.4 KHz (FL-44A)
ALL MODES, narrow button in.... 500 Hz (FL-32)....... 2.4 KHz (FL-44A)**
** This bandwidth is of no use in the AM mode, but there is no way to
change it without affecting the other modes as well.
The only negative consequence of these changes is that operation of the
filter select switches is a bit more complex, but I found it became second
nature very quickly.
PROCEDURE:
There are many possible ways that the filter selection circuit could
potentially be modified. I will detail three options:
(1) Implement the modification exactly as I have described above.
(2) Implement the modification exactly as I have described above,
except use the "narrow" filter switch rather than the "both
switches in" configuration to access the extra bandwidth
setting. This may only be done if no optional CW narrow filter
has been installed.
(3) Replace the 2.4 KHz AM "normal" bandwidth with 2.8 KHz. Make no
other changes to the filter select circuit.
These three options are listed in decreasing order of complexity. Options
1 and 2 produce identical results, except that different filter switch
positions are utilized. Options 2 and 3 only require the top cover of the
R71A to be removed. Option 1 requires that both the top and bottom covers
be removed so that the front chassis can be partially removed to access
the filter switch PC board. For option 1, you will need two small general
purpose switching diodes. No additional components are required for
options 2 and 3. A short length of thin flexible insulated wire will be
needed to make a few small jumper connections.
PREPARATION:
Read the procedures and make sure you have all the necessary part and
tools on hand. I suggest taking all appropriate anti-static precautions
whenever working on radio gear. Be sure power is disconnected from the
receiver. Use only a low wattage, fine point soldering iron. It is
important to use as little heat as necessary for all solder connections
on the top side of the main PC board to avoid melting the solder on the
underside.
[Remember, these modifications will only be of potential benefit if the
optional FL-44A crystal filter (or equivalent) has been installed. Some
dealers also offer 4 KHz filters as replacements for the 6 KHz CFW455IT
ceramic filter in the 3rd IF. Installation of these replacement filters
should not have any effect on the results of the modifications.]
**************************************************************************
OPTION 1 - CHANGES ARE IMPLEMENTED EXACTLY AS DISCUSSED IN THE PRECEDING
TEXT:
A. Remove the top and bottom covers. Place the receiver (top side facing
up) on a stack of magazines or large books at least 2" thick. Only the
metal chassis should rest on the stack, not the front panel.
B. You will see two flathead screws on either side of the receiver which
hold the front chassis assembly to the main chassis. Remove only the top
screw on each side and loosen the lower ones slightly. Using the lower
two screws as a hinge, carefully fold the front assembly down so that the
face of the radio rests on the desk. (A soft cloth may be used to prevent
scratches.)
C. Right beneath the back of the S-meter, you will see a PC board to
which the wide and narrow filter switches connect. Locate the 12 solder
points in a straight row associated with the two switches. There is a
slight gap between the switches which contains two additional solder
points just above and below those in the line. (The board layout diagram
supplied with the service manual is quite helpful in locating the
appropriate solder points.) You need to create solder bridges across the
outermost set of normally open/normally closed switch contacts for each of
the two switches. The appropriate pins are shown below:
wide switch narrow switch bottom of radio
^
. | XX=solder
. .XX. . . . . . . .XX. . | bridge
.
D. Inspect your work. Carefully return the front assembly to its proper
position and secure the four flathead mounting screws. Re-install the
bottom cover.
E. Turn ON the filter switch (S1) on the main PC board. This switch
will already be on if an optional CW narrow filter has been installed.
S1 is a small slide switch not far from the center of the board.
F. You may wish to verify that everything is working OK so far. If so,
be sure that all tools, etc. are out of harm's way and reconnect power,
antenna and an external speaker to the receiver. The bandwidth switches
should work the same as before, except that (1) there will be no receive
with both switches pushed in and (2) there will be no receive with the
"narrow" switch pushed in unless an optional CW filter is installed.
Disconnect power again once the tests are complete.
G. Locate D37 on the main PC board right next to IC3 near the center of
the board. Cut the top (cathode) lead of the diode, leaving enough wire
on both sides of the cut to make connections to. Bend the wire lead on
the diode up. Solder a jumper wire from the cathode lead of D37 to the
cathode (top) lead of D39 (located on the opposite side of IC3).
H. PERFORM THIS STEP ONLY IF YOU WISH TO MAKE THE "IF-SHIFT"
MODIFICATION FOR THE SSB/CW/RTTY "BOTH SWITCHES OUT" POSITION, AS
DISCUSSED IN THE PREVIOUS TEXT. IF YOU ONLY WANT THE EXTRA AM BANDWIDTH,
SKIP TO STEP "I." Cut the top (cathode) lead of D44, leaving enough wire
on both sides of the cut to make connections to. Bend the wire lead on
the diode up and solder a jumper wire from it to the top (cathode) lead of
D42 (located near filter switch S1).
I. A switching diode must be connected from pin 4 of IC3 to the top
(cathode) lead of D36 (located right next to D37). A second diode must be
connected from pin 4 of IC3 to the lead sticking out of the PC board which
was clipped from D44. If D44 was not clipped in step H, the diode just
connects to top (cathode) lead of D44. To make the IC3 connection, tin
both pin 4 and the lead which will connect to it; then place the two in
contact with each other and heat up the connection.
I performed this step by first soldering the anode of one of the switching
diodes to the anode of the second; right where the lead enters its base.
I then cut the anode lead of the first diode to a fairly short length and
soldered it to pin 4 so that the two diodes were held in a vertical
position just a tad above the IC. I then clipped the two cathode leads
very short and ran insulated jumper wires from one cathode to D36 and from
the other cathode to the D44 clipped lead coming out of the PC board.
However you decide to do it, make sure the IC3 connection does not short
out any other pins and make sure the switching diode leads cannot come in
contact with any other components.
J. Inspect your work and verify all steps were done properly. Make sure
all tools are out of harm's way and test the modification. If everything
works OK, reassemble the receiver. Note that with this modification, the
narrow button must be in the "out" position for the FM unit (if installed)
to function properly.
***************************************************************************
OPTION 2 - CHANGES ARE IMPLEMENTED EXACTLY AS DISCUSSED IN THE PRECEDING
TEXT EXCEPT THE "NARROW" FILTER SWITCH IS USED INSTEAD OF THE "BOTH
SWITCHES IN" CONFIGURATION TO ACCESS THE ADDITIONAL BANDWIDTH. THIS
OPTION MAY ONLY BE USED IF NO CW NARROW FILTER HAS BEEN INSTALLED IN
THE RECEIVER.
A. Remove the top cover of the receiver.
B. Turn ON the filter switch (S1) on the main PC board. S1 is a small
slide switch not far from the center of the board.
C. Locate D37 on the main PC board right next to IC3 near the center of
the board. Cut the top (cathode) lead of the diode, leaving enough wire
on both sides of the cut to make connections to. Bend the wire lead on
the diode up. Solder a jumper wire from the cathode lead of D37 to the
cathode (top) lead of D39 (located on the opposite side of IC3).
D. PERFORM THIS STEP ONLY IF YOU WISH TO MAKE THE "IF-SHIFT"
MODIFICATION FOR THE SSB/CW/RTTY "BOTH SWITCHES OUT" POSITION, AS
DISCUSSED IN THE PREVIOUS TEXT. IF YOU ONLY WANT THE EXTRA AM BANDWIDTH,
SKIP TO STEP "E." Cut the top (cathode) lead of D44, leaving enough wire
on both sides of the cut to make connections to. Bend the wire lead on
the diode up and solder a jumper wire from it to the top (cathode) lead of
D42 (located near filter switch S1).
E. Cut the top (cathode) lead of D46 (located a little toward the front
of the main PC board). Make sure the two clipped leads are not
touching.
F. Locate D45 on the main PC board (near IC3). Cut the top (cathode) lead
of the diode, leaving enough wire on both sides of the cut to make
connections to. Bend the wire lead on the diode up. Solder a jumper wire
from the cathode lead of D45 to the lead sticking out of the PC board
which was clipped from D44. If step D was not performed, then solder the
jumper to the cathode (top) lead of D44.
G. Inspect your work and verify all steps were done properly. Make sure
all tools are out of harm's way. Reconnect power, antenna and an external
speaker, and test the modification. You should now have the following
bandwidth settings:
AM wide................ 6 KHz (CFW455IT) [no change]
AM normal.............. 2.8 KHz (FL-70/CFW455IT)
SSB/CW/RTTY wide....... 2.8 KHz (FL-70/CFW455IT) [no change]
SSB/CW/RTTY normal..... 2.4 KHz (FL-44A) [IF-shift, if step D was
or performed]
SSB/CW/RTTY normal..... 2.3 KHz (FL-30/FL-44A) [no change, if step D
was not performed]
Narrow, all modes...... 2.3 KHz (FL-30/FL-44A)
If everything works OK. reassemble the receiver.
****************************************************************************
OPTION 3 - REPLACE THE 2.4 KHz AM "NORMAL" BANDWIDTH WITH 2.8 KHz. MAKE
NO OTHER CHANGES TO THE FILTER SELECT CIRCUIT.
A. Remove the top cover of the receiver.
B. Locate D37 on the main PC board right next to IC3 near the center of
the board. Cut the top (cathode) lead of the diode, leaving enough wire
on both sides of the cut to make connections to. Bend the wire lead on
the diode up. Solder a jumper wire from the cathode lead of D37 to the
cathode (top) lead of D39 (located on the opposite side of IC3).
G. Inspect your work and verify it was done properly. Make sure
all tools are out of harm's way. Reconnect power, antenna and an
external speaker and test the modification. Your AM "normal" bandwidth
will now be 2.8 KHz (FL-70/CFW455IT) instead of 2.4 KHz (FL-70/FL-44A)
If everything works OK, reassemble the receiver.