Icom 3210 for Packet

This is a detailed description on how to add a tnc connection to an icom 3210 2m/70cm transceiver to allow packet operation on either band. This modification details connections specific to the db9 connector on kantronics tnc’s but could easily be adapted to other tnc types.

The original modification (31-09-2001) worked but had excessive retries. The injection point for the tnc transmit audio has been removed from a common point in the tx audio chain and now feeds each vco directly via a resistive divider. The remainder of the original modification is untouched.

The physical modification is to add a custom made cable to the radio with a short (150mm) tail ending in a db9 male connector which plugs into the tnc (in the case of the prototype a kpc3plus). All required tnc connections are brought out via this cable.

The custom cable consists of two 3mm coax cables (one red and one blue) and four small diameter stranded wires (black, white, brown and yellow) in a 90% braid and a heatshrink sleeve. A custom wiring loom is added internally to the ic3210.

The physical connection points are detailed in ic3210w.jpg and are summarised as follows:

transmit audio:

is connected to both vcos’ via a resistive divider. Two 3mm coax cables are wired to the solder side of the main pcb which is the only easy access point to the tx audio part of both vco chains.

The main board is removed by unplugging all connectors (remembering where they go!), removing the vco shield and hex standoffs, and then lifting the main pcb straight up so the inter-board connector (front edge of the pcb) unplugs.

This pcb was most likely designed by someone who would never have to service it as reassembly is merely difficult, but not impossible…….

A red coax (centre only) is soldered to the c89 end of r126 (27k) for vhf and a yellow coax (centre only) is soldered to the c90 end of r125 (6k8) for uhf. Insulate both coaxes so that the braid of both can’t touch anything else. These coaxes are routed over the back of the pcb and past the uhf vco to the approximate centre of the main board. The main pcb can now be carefully replaced, not forgetting that nice inter-board connector. Don’t replace the vco shield just yet, but do replace the hex standoffs.

At the approximate centre of the board is a hex standoff. Both coax shields are earthed at this point only (with a solder lug under one of the vco shield mounting screws) and a resistive divider is also added. The divider consists of a 120k 1/4w resistor wired in series with the red coax (vhf) and a 56k 1/4w resistor wired in series with the yellow coax (uhf).

Both resistors and their connections to the coax cables are insulated with heatshrink. The other end of the resistors are commoned and soldered to the centre only of another red coax and that joint is also insulated with heatshrink.


a brown wire is soldered directly to the rear of the front panel on/off switch so that power is only applied to the wire when the ic3210 is turned on.


the black wire is fitted under the head of the main pcb mounting screw near c33.

The physical layout of the divider and cables is shown in ic3210t.jpg.

receive audio:

the centre only of the blue coax is wired via a 10k 1/4w series resistor directly to pin 9 of IC1 on the RX unit. The resistor and its connection to the coax cable is enclosed in heatshrink.

external carrier detect:

The white wire is connected to the cathode of a 1n914 signal diode which is connected to a bc557 transistor. The bc557 base is wired via a 10k 1/w series resistor to the junction of d21/r88 and most easily accessed at the exposed pigtail on d21. The bc557 is mounted flat on ic2. Only the 1n914 diode is enclosed in heatshrink. When mute opens the white wire is grounded via the 1n914 diode and the bc557 transistor.


the yellow wire is soldered directly to pin 5 on the rear of the front panel mic connector.

The blue coax cable and white wire is routed between the cans of L1/X1 and from there through the gap between the rx board and the front panel board. The physical layout of the components and cable is shown in ic3210b.jpg.

The rear panel external speaker socket is removed and wired out so that the internal speaker is still active. All connections are then insulated. The custom cable then exits via the sockets’ mounting hole which may have to be enlarged slightly to accommodate it. A cable tie serves as a strain relief.

The db9 male connector is wired as follows:

pin 1: red coax centre

pin 2: white wire

pin 3: yellow wire

pin 5: blue coax centre

pin 6: black wire, red coax shield, blue coax shield

pin 7: brown wire

the shield of the custom cable is wired to the metal shell of the db9

The modified ic3210 is used with a kantronics kpc3plus configured as follows:

external power on radio port (db9) pin 7

cd software

txdelay 30 (300ms)

xmitlvl 310

On test vhf deviation is 3.4kHz for the space tone and 3.6 kHz for the mark tone, uhf deviation is 3.2kHz for the space tone and 3.4kHz for the mark tone. Retries are no longer evident.

Warren VK3XSW 06-01-2003

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