Modification of the R-71A to 37 MHz
TF Marcotte, N5OFF
n5off%w5ddl.aara.org@usl.edu
 

This article will cover the use of the Willco Electronics Company's
ICM-1024 No-Fail memory board for Icom products as a means of
achieving a 37 MHz upper frequency limit for the Icom R-71A receiver.
This modification is done without tampering with the RF circuits of
the radio.

Caveat: Don't attempt this modification if you are uncomfortable with
soldering small electronic devices.  With that said, lets continue.

Some Icom products, the R-71A and the IC-751 to name two, have a
volatile random access memory (RAM) board which stores the radio's
operating parameters (i.e.  band limits) as well as 32 user programmed
memories.  The RAM is kept alive by a small lithium battery.  In most
cases, the battery should last a long time, ten years or more.
Battery health may be assessed at any time by comparing its present
voltage to an approximate 2.7 volt warning level.  If by some chance
this battery should fail before you've had the opportunity to solder
in a new one, then the board (not the whole radio) must must be
returned to Icom for replacement of the battery, and for reprogramming
of the RAM chip.

Willco Electronics markets a product called the ICM-1024 No-Fail
memory board.  This product has two key features:

1). The board offers the user 32 x 32, or 1024 memories.  

2). The board has the radio's operating parameters stored on a
read-only memory (ROM) chip.  In other words, there is no danger of
the radio becoming inoperative due to battery failure.

The Willco board does in fact have a RAM chip and battery like the
Icom, but this chip holds only the memories, not the operating
parameters.  If the Willco battery fails, then one would only lose the
memories (albeit 1024 of them!).

It should be stressed that careful replacement of the battery on
either of the boards when the voltage drops to 2.7 volts will prevent
a memory loss of any kind.

Installation of the Willco board is relatively straightforward, and
well documented in the instructions.  Mechanically, the board (not a
kit) presses right down into the original Icom connectors.  To enable
the 1024 memory capability, one must make one logic unit connection
and eight connections to two chips in the radio's matrix unit.  The
instructions don't mention that you can get the no-fail capability and
32 memory function by not wiring the board in at all, just press it
down to the original board's connectors. This may be a good
alternative to solder-shy users.

Being that the Willco board holds the operating parameters, the
company took the opportunity to extend the primary default upper band
limit to 31.0 MHz (the lower limit is moved as well, from 100 KHz to
10 Khz).  The default limit was not programmed higher than this
because the Icom R-71A (and the IC-751 as well) shut down at that
frequency.  That is, the Icom control chip that tells the radio to
turn on the proper VCO (one of four) and bandpass filter (one of five)
for the top band doesn't address frequencies higher than 31.0 MHz with
its factory wiring. When the frequency exceeds 31.0 MHz, all VCO's and
bandpass filters are switched off.

The Willco boards of post-December 1993 manufacture have an alternate
upper frequency limit of 55 MHz programmed into a separate R-71A
program location.  This location can be accessed by jumpers.  The
jumper configurations for the various radios that the Willco board
supports are:

JP01  JP02  JP04  JP08 Icom Radio Note

0     0 0 0 R-71A  31 MHz upper limit
1     0 0 1 R-71A  55 MHz upper limit
1     0 0 0 IC-745A       
0     1 0 0 IC-751A       
1     1 0 0 IC-271A  
0     0 1 0 IC-471A  
1     0 1 0 IC-251A  

With the jumpers installed properly, the radio will now program up to
55 MHz, but will only operate up to 31 MHz.

There is a solution to this problem, however.  The R-71A's control
chip has an unused pin than turns on at frequencies greater than 31
MHz.  The solution is to wire this pin into the top VCO and bandpass
filter circuit to keep then turned on above 31 MHz.

This is done by first cutting the trace from the control chip pin's
original path for the top band, and wiring in two diodes. One diode
connects the original path for the old top band, the other utilizes
the formerly unused pin on the control chip to keep the top VCO and
bandpass filter turned on above 31 MHz.

These are the steps necessary to perform this modification:

1). remove the top and bottom covers of the radio.  Make sure that
your radio power is off, and that the radio frame and soldering iron
are grounded.

2). remove the option unit installation plate from the bottom section
of the radio and gently fold it frontwards.  This is the board that
holds the remote control board, the speech synthesizer, and the
computer interface unit.  Your radio may have some or none of these
options installed.

3). The large exposed board will be the logic unit.  On this board,
locate IC-14.  This is the IC that controls the bandpass filters and
VCO's for the various bands.  Pin 17 (refer to your schematic) of this
IC goes high between 22 MHz and 31 MHz.  Remove the screws from the
logic board, and gently turn it over.

4). On the underside of the logic board, locate the circuit trace from
IC-14, pin 17 (goes to C28 and R41) and sever this trace near the
chip.

5). Install a small diode from pin 17 to the near side of R41 (this
duplicates the original path of the trace you just cut) with the
polarity such that current can flow from the chip to the resistor.  I
used 1N914 diodes, Radio Shack number 276-1122.

6). Install a second diode with like polarity as in step 5 from pin 18
(pin 18 is not shown on the schematic, but you can locate it as the
unused pin next to 17) to the same point as in step 5, the near side
of R41.  This new circuit path will allow the chip to switch on the
top VCO and bandpass filter above 31 MHz.

Now, with this accomplished, you should verify that the radio receives
at 31.100 MHz.  This will confirm that the wiring change was done
properly.  You will soon find, however, that the radio will only
receive up to about 32 MHz now, because the VCO will go out of lock at
approximately this frequency.

Icom originally employed this top VCO for an 8 MHz range, between 22
MHz and 30 MHz.  The VCO actually has a lock range of about 15 MHz,
however.  This generous margin may be exploited by adjusting C107 in
the PLL unit of the radio (the trimmer of four closest to the front
panel in this unit).  Carefully adjust this trimmer until a maximum
locked frequency limit is reached.  This should be about 37 MHz or
higher.  All the while, verify that you still have VCO lock at 22 MHz.
In my radio, a top frequency of 37.95 MHz was reached.

That does it.  The radio will now receive up to 37 MHz, and you are
ready for some VHF-Low banding.

Since no modifications were done to RF circuitry, I was originally
concerned about the performance of the radio above the original design
limits.  More ambitious readers may be able to achieve higher limits
and better performance by tackling the RF and VCO circuits, but I
elected not to.

Sensitivity fiqures for the modified radio are shown below, in terms
of a dB reduction relative to an S9 signal at 24 MHz (optimum).

Frequency    dB sensitivity reduction
   MHz         relative to 24.0 MHz 

22                     -2.5
23                     -2
24                      0
25                    -0.9
26                    -2.3
27                    -3.4
28                    -3.8
29                    -3.6
30                    -2.9
31                    -2.6
32                    -4.5
33                    -7.6
34                   -11.6
35                   -15.8
36                   -20.5
37                   -25.8
37.95                  -30.3

As you can see from the fiqures, the radio performs relatively well
above 30 MHz, up to about 33 MHz, with no RF modifications.

Another advantage of the Willco board is its shear memory capacity.  I
have used several of its 32 banks of 32 memories to store all possible
commercial and military FM frequencies.  This way, in the scan mode, I
can monitor openings from all over the country.  Foreign languages are
not uncommon when the band is really hot.

In conclusion, I have found that modification of the R-71A opens up a
whole new way of 10 meter band spotting for me.  The radio has a "big
sound" on FM that scanners simply don't have.


Willco Electronics can be reached at P.O. Box 788, New Lenox, IL
60451. I believe the board cost about $125.

Acknowledgement

Special thanks to Mark Salyzn, VE6MGS, for technical support in
performing similar modifications to the Icom IC-751.
-- 
-- James Dugal, N5KNX  Internet: jpd@usl.edu
Associate Director  Ham packet: n5knx @k5arh (land), UO-22 (sat.)
Computing Center  US Mail: PO Box 42770  Lafayette, LA  70504
University of Southwestern LA. Tel. 318-231-6417 U.S.A.




