ICOM IC-746 & IC-756 Series: Damage Due To RF-Overload

The following transceivers can be susceptible to damage from a strong RF field (or RF-overload):

• IC-746
• IC-746PRO
• IC-756
• IC-756PRO
• IC-756PROII
• IC-756PROIII

In the 746-series, the HRX-muting circuit can be damaged.

In the 756-series, the RFRX-muting circuit can be damaged.

The damage is usually due to a strong RF field from a nearby transmitter, or possibly from the transceiver itself in some cases. Here are possible causes:

• Having the transceiver connected to an antenna which is in close proximity of another transmitter's antenna.
• Using a cheap or defective coax-switch to select between the transceiver and another transmitter and/or amplifier.
• Persistently trying to tune a non-resonant antenna.
• Persistently transmitting during high-SWR conditions.

If the HRX/RFRX-muting circuit fails, the transmitter will often oscillate intermittently. This is most noticeable on the higher HF bands (6, 10, or 12 meters). This will not affect the 2-meter band on the 746-series. The symptoms can be one or more of the following:

• Unusually high current demand from the power supply.
• Fluctuating RF output.
• Erratic antenna-tuner operation.

To get an idea if the HRX/RFRX-muting circuit is at fault, turn the receive attenuator (ATT) on. If the problem goes away when the ATT is on, the HRX/RFRX-muting circuit is probably at fault. This test works because the attenuator interrupts the path of oscillation.

Suspect one or more of the following components on the CTRL Unit:

• IC-746 ... D22, Q25
• IC-746PRO ... D21, D22, D24, Q25
• IC-756 ... D22, Q25
• IC-756PRO ... D22, Q25
• IC-756PROII ... D22, D23, Q25
• IC-756PROIII ... D22, D23, Q25

Note: To check the DC voltages at Q25, transmit on SSB with the mic-gain at minimum. The base voltage should be 0-volts RX, and 0.7-volts TX. The collector voltage should be around 13.5-volts RX, and near 0-volts TX.

If left unchecked, the situation may deteriorate until the damaged SMD components overheat and become charred, possibly damaging the circuit-board. The pictures below show a worst-case scenario.

NOTE: These photos depict an extreme (and rare) example. Most of the time, the damage isn't so drastic, only requiring replacement of the affected SMD components.

IC-756PRO CTRL Unit (top)

IC-756PRO CTRL Unit (bottom)

The owner of this IC-756PRO had two stations, selectable with a coax-switch The other station had a 1-KW amp. His PRO was turned off. While he was transmitting from his other station, his wife alerted him to smoke coming from his IC-756PRO (in another room). In this case, it wasn't practical to repair the RFRX-muting circuit. The entire CTRL Unit was replaced.

NOTE: These photos depict an extreme (and rare) example. Most of the time, the damage isn't so drastic, only requiring replacement of the affected SMD components.

NOTE: If you don't have the skills or equipment to work with static-sensitive surface-mount components, please leave it to a qualified technician.

Mike Nadeau - N1EQ
www.n1eq.com

Last updated: December 28, 2007