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Icom IC-R20



Icom IC-R20

The Icom IC-R20 is a wideband receiver with a frequency coverage from 150 kHz to over 3300 MHz.

The Icom R20 has two independent receiver units with different frequency ranges. Both units use a TA31136. The unfiltered audio can be found on pin 9 and on check points CP1 and CP11.

This is not an easy modification. Especially disassembling the different circuit boards is a challenging task. It should only be performed if you have sufficient experience with this kind of work.

IC-R20

Disassembly

To open the radio, remove the 4 screws in the back and 2 in the battery compartment. The two halves can now be split.

Next, detach the loudspeaker connector from the circuit board socket. The two halves are now completely separated.

The top PCB with the display is the 'Logic Board'. It needs to be removed. Unscrew 3 screws [1] and clear 10 pins from solder [2]. This is best done with a professional desolder station, but it can also be done with solder wick.

Disassembly step 2
Disassembly step 1



Next, the two dials L and R are taken off. The washers can be untightened with fine tip pliers. After that, the top circuit board can be taken out.

Disassembly step 3

The two TA31136 receiver ICs are now visible. If you wish, you can make a connection to pin 9 on one of the ICs. I have chosen to take out the RF Unit: on the bottom, check points CP1 and CP11 can be found, so that it's not necessary to solder directly to the ICs.

The circuit board now visible (left picture) is the recording board (REC unit). This board needs to be removed as well. Unscrew 4 screws [3] and desolder the resistor lead [4].

After removing this board, the radio circuit board (RF Unit) shown below becomes visible.

Demontage stap 4

Desolder 3 points [5](antenna socket and resistor and remove 3 screws [6].

Location

Discriminator tap

The Icom IC-R20 has two receiver units: A-Band from 150 kHz to 470 MHz and B-Band from 118 to 175 MHz and 330 to 3305 MHz. Also check the table below.

Due to its frequency range, I have chosen the B-Band receiver. The test point is indicated 'CP11'. If you want to use the A-Band receiver, use test point 'CP1'.

Connect a piece of insulated wire to the check point of choice: CP1 for A-Band of CP11 for B-Band. Here, CP11 is used.

Lead the wire to the top side of the circuit board. Replace the RF Unit in the case, tghten the screws [6] and restore the three solder connections [5].

Also replace the REC unit. Tighten the screws [3] and restore the solder connection [4].

Discriminator output
Resistor R57

There is very little space in the radio for an additional chassis socket. However, the external speaker connection can be modified to carry the unfiltered baseband signal. This is at the expense of the ability to control the scanner through the CI-V protocol.

The center ring of the external speaker socket is connected to the CI-V circuit via resistor R57. When this resistor is removed, the centre ring is isolated.

R57 is a tiny SMD resistor. It is located close to the power socket and is black. Next to it, C37 can be found. This capacitor is grey.

Carefully remove the resistor. A hot air SMD rework station with a fine nozzle is ideal, but it can also be done with a fine tip soldering iron. A magnifying lamp or headband magnifier helps to focus on this microsurgical operation.

Remove R57
External loudspeaker socket

The centre contact of the external loudspeaker socket has now been cleared. A 10 k, 1/8 W resistor is connected directly to the socket. Connect the other lead of the resistor with the wire coming from CP1 or CP11.

In the picture below, the resistor has been insulated with shrink tubing.

Next, replace the Logic Board. Tighten the screws [1], solder the 10 pins [2] and replace the washers and dials L and R. Connect the loudspeaker connector to the circuit board socket. Mission completed: the receiver can be closed.

Overview
A-Band and B-Band frequencies

In this modification, I chose the B-Band receiver. To activate this receiver unit, the radio should be in Dual Watch mode for frequencies between 118-175 ans 330-470, with the required frequency in the bottom row.

Above 470 MHz, the B-Band unit is activated when a single frequency has been selected.

If you choose the A-Band unit (check point CP1), this receiver is both activated in single frequency as dual watch mode. The range for this unit is 150 kHz - 470 MHz.



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