Re: QRP with IC735


From: Jim Cummings (
Date: Fri Sep 29 1995 - 15:31:49 EDT

>Hi gang,
>Some of you may be interested in the following which shows how to run an
>IC735/731 for QRP operation.
>---------------------------- Cut here ----------------------------
> Use of IC735 H/F Transceiver for QRP operation
>Others may be interested in this simple project to use IC735 or
>IC731 H/F transceivers for QRP work as it took a fair amount of
>investigation to find out what to do and then to make it work.
>These rigs normally operate with power output in the range 10 - 100
>watts which is controlled by means of a slider on the front panel .
>There is also an internal switch which halves the power output. I had
>heard of these rigs being used for QRP and after enquiries to ICOM
>Australia and other sources, who could offer no help beyond what was
>in the manual, was finally advised by ICOM Support in the U.S., via
>the Compuserve network what to do.
>They suggested using the external ALC connection on the ACC(2) socket
>at the rear of the set where a voltage from -4 to 0 would control
>output from zero to the power level set on the front panel. ACC(2) is
>a 7 pin socket used to connect to an automatic ATU (the 8 pin ACC(1)
>socket could also be used as it contains similar connections but note
>that the pins are different).
>My first attempt at this using only a potentiometer across an external
>battery gave either full power or nothing. A later attempt got it
>right. The voltage controlling the power is very sensitive and on my
>IC731 only a small change of around 0.1 volt or so in the 1.5 volt
>region changes the output from zero to the set amount.
>The circuit is shown below. 3 volts seems OK to supply the voltage (as
>polarity is reversed, the internal supply can't readily be used), and
>the 12k resistors allow the 5k linear pot to cover only a small
>voltage range. You may need to experiment with the resistor values to
>ensure that the pot. covers the control range on your rig. This
>circuit allows smooth control of power from 0 to 10 watts, with the
>front panel setting at minimum power. The output power can be read
>from either the P/O range on the rig's meter or from an external
>SWL/PWR meter in the antenna lead.
>The unit may be built into a small PCB box and left connected to the
>rear socket, switching on as needed. You may like to include an LED
>plus resistor across the 3v after the switch to remind you to turn
>off (I forgot once when QRO and wondered why no power was getting
> IC735 ACC(2)
> -------------------- Grnd Pin 2 +
> |
> 12k v 12k
> -----/\/\/\----/\/\/\----/\/\/\------- ALC Pin 5 -
> | 5k linear |
> | |
> ----- 3v |
> --- 2 x AA cells |
> ----- |
> --- on/off |
> | / |
> --------/ --------------------------
>------------------------- Cut here ------------------------------
>Simon Buxton VK2EII Sydney, Australia
>E-mail : Compuserve :100352,1612

The following is from the QRP column of June 1990 73 Magazine.
The column was written by M. Bryce, WB8VGE on the adjustments of
the Icom IC 735. I tried them and they work.

     First, a modification to convert the popular ICOM IC-735 to
QRP operation. These modifications will let you operate the IC-
735 at QRP power levels down to one watt or less. DON'T ATTEMPT
someone who feels secure working inside a modern transceiver.

     Start by removing the bottom cover. With the radio turned
upside down, find the slide switch at the rear left center. This
is the switch that selects either 50 of 100 watts as the maximum
output available from the transmitter. Push the switch to the
rear of the radio. This is the 50 watt position.

      Next, find the pot labeled (sic) R-267 at the right rear of
the radio. Turn the pot to full clockwise position. This is the
minimum output setting. You should now have about four watts
output with the front panel control set at a minimum, and about
50 watts out at maximum. R-268, locate just to the of R-267,
sets the maximum power output when the front panel control is
advanced to the maximum position.

     Find the pot labeled (sic) R-361 at the front left of the
radio. If you want lower power output, adjust this pot for the
lower power setting desired. This adjustment is quite touchy at
settings below two watts out but, if you are careful, you can get
outputs down to approximately one watt. You can maintain this
power level through a reasonable amount of vibration, such as
while operating portable or mobile. Be aware that this
adjustment will probably change the power output obtainable when
the front panel control is advanced to the maximum position. For
example, with the R-361 adjusted to 1.9 watts, the maximum that
you get is 30 watts.

     Replace the bottom cover. You should now have a power
control for the IC-735 while operating QRP.

     The newer IC-725 can be adjusted down to QRP levels via the
front-mounted drive control. You won't need the get inside the
radio to modify it for QRP use.

End of instructions for the 735.

If I may add, I would suggest that you find the mod which swaps
the SQUELCH control with the RF DRIVE control. In the stock IC-
735, the SQUELCH is an outer ring pot control surrounding the
VOLUME control, while the RF DRIVE control is one of those
sliding pots in the front. The modification is quite simple and
swapps the lines. As a result, the SQUELCH will adjust the power
out, and do so more smoothly. As I can recall, the mod was
described in a Hints and Kinks collection of QST. If you can't
find it. let me know and I will see if I can look it up for you.
              Jim Cummings
      73 and live better digitally
  because remember, today is the first
     day of the rest of your life.

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