Icom IC-756PROIII Quick start guide

XRef-PLL
Icom IC-756ProIII Installation Notes (Draft)
1. Overview
Most modern Amateur transceivers now generate their operating frequency from a single
internal Reference Oscillator. In many cases, this Reference Oscillator is voltage-controlled,
allowing front panel adjustment of the Reference frequency.
The XRef-PLL locks the radio’s Reference Oscillator to an external 10 MHz reference signal
from a GPS reference or other high-accuracy source. It intended to be installed internally within
the radio. When no 10 MHz reference is connected, the XRef-PLL completely disconnects from
the radio, allowing it to operate as from the factory.
In the case of the Icom IC-756ProIII, the Reference frequency is 32.0 MHz.
2. Technical Specifications
Connections to the board are shown below:
Connection
Description Specifications
Test Do Not Connect
Lock PLL Lock Indicator +3.3V 20mA max
Vp Do Not Connect
Gnd Power Ground
Vin Supply for XRef 5 to 16 V @ 20 mA
Ref 10 MHz Reference Input 0 to +15 dBm
(0.5V to 3.6V p-p)
RF Reference frequency from Radio. Radio dependent
Ctl Control output to Reference VCXO 0 to +3.3V
Test
Lock
Vp
Gnd
Vin
Ref
Gnd
RF
Gnd
Ctl
A Word of Caution: The installation requires a small amount of soldering to very small and
fragile SMD components. If you do not have experience with SMD techniques, please seek
the assistance of someone who does. We are happy to provide any advice needed, but we
cannot take responsibility for any damage to your radio.

3. Circuit Modifications
No modifications are required to the standard circuit. The XRef is simply connected to several
points on the PLL board. An extract from the circuit diagram in Figure 1 shows where
connections are to be made.
Figure 1. – Icom IC-756ProIII Reference Oscillator Circuit
 The Blue arrow indicates where the output of the Reference Oscillator is sampled.
 The Green arrows indicate where the control voltage is fed back to the Reference Oscillator.
 The Red arrow indicates where +8V is taken to power the XRef.
4. Installation
For this installation, you will need about 300mm of very thin coax - say RG178B or RG316 and
a socket to suit. As well, a small signal NPN transistor is needed (e.g. BC547/8/9). You will also
need some short lengths of very fine gauge hookup wire and a small piece of double-sided
foam adhesive tape.
Remove the bottom cover of the radio.
A photo of the PLL board, taken from the Service Manual, is shown in Figure 2. Locate the
Reference Oscillator area in the top right corner of the PLL board.

Figure 2. – PLL Board Location
The connection points are shown in Figure 3. The green connections are on the left edge of the
PLL board.
Figure 3. – PLL Board Reference Oscillator Connections

There are two possible points (two green arrows) for controlling the Reference Oscillator. The
point with the dark green arrow should be more sensitive. Connect the NPN transistor with
Collector to whichever of these points you choose, and Emitter to ground.
Using thick double-sided foam adhesive tape, mount the XRef board either on top of a shielded
box or in some other suitable location near the Reference Oscillator.
Mount the board with the Ref/RF/Ctl connections closest to the Reference Oscillator area. The
XRef connection pads are double-sided, so check that the underside pads aren’t shorting to the
shielded box.
The next step involves soldering three short lengths of fine hookup wire to each of the
connection points circled in Figure 3. With a very fine tip soldering iron, it is possible to do this
with the board in place. Be very careful not to overheat the components as they can be easily
damaged. I normally apply a small blob of “normal” (i.e. not lead-free) solder to the connection
point first, and then attach the already-tinned wire.
Attach the wires to the XRef as follows:
 Blue connection to RF pin.
 The base of the NPN transistor to Ctl pin.
 Red connection to Vin pin.
Also connect a wire from the XRef Gnd pin to a nearby earth connection, like the tin shield.
Finally, we need to bring 10 MHz in to the XRef. Mount a suitable connector on the back panel
of the radio and run some thin coax to the XRef.
Optionally, an LED can be connected from the XRef Lock pin to ground to indicate when the
radio is locked to 10 MHz. The LED could be mounted next to the connector on the back panel.
Before putting everything back together, carry out some basic tests.
 With 10 MHz NOT connected, turn the radio on, select SSB mode and a higher band, and
tune to a beacon or carrier.
 Adjust the Ref Freq Adjust control up and down and you should hear the beacon tone
change indicating the Reference Frequency is changing.
 With Ref Freq Adjust set to say 75% of its travel, connect 10 MHz and you should hear the
beacon tone jump slightly. If there is no jump, try again with a setting of 25%. The radio
should now be frequency locked.
Finally, re-assemble the radio in the reverse order and enjoy your new frequency accuracy and
stability!
5. Operation
The board continually tests for the presence of the external reference. If 10 MHz is removed,
the XRef-PLL will automatically disconnect from the radio, leaving the radio to operate as
standard.
If you notice a warble on the signal indicating a marginal PLL lock, try altering the Ref Adjust
setting so that there is very little change in the radio’s frequency with 10 MHz connected or not.
The 10 MHz source can be a sine or square wave, but it is important that it is clean. The board
is, in effect, adjusting the radio’s Reference in step with the incoming 10 MHz, including
whatever imperfections there may be. The old adage garbage-in, garbage-out applies here.
6. Support
If you have any difficulties, you can contact David Smith VK3HZ by email : [e-mail]