In my experience, the Icom IC-746/756/PRO-series are quite susceptible to RF overload and electrostatic discharge (ESD). A common failure is the HRX/RFRX-muting circuit (Q25 and its associated components on the CTRL Unit). Sometimes, components on the RF Unit may also be affected. In the IC-746 series, this circuit mutes the HRX-line during transmission. In the IC-756 series, it mutes the RFRX-line.
Important Note: Although I have first-hand knowledge of the damaged circuitry and the necessary repairs, I only have second-hand knowledge as to the cause of damage, which was acquired by interviewing customers about their station setups and operating practices.
Possible causes of failure are:
- Strong RF field, such as from a nearby transmitter/amplifier with antenna in close proximity.
- Low-grade or defective coax-switch to select between the transceiver and another transmitter and/or amplifier.
- Lightning surge, precipitation static, or static buildup on the antenna.
- “Hot chassis” due to inadequate RF ground.
- Running full power during antenna tuner adjustment. Reduce power to 10 watts, then increase power after tuning. And avoid using the “PTT Start” or “PTT Tune” feature.
When the HRX/RFRX-muting circuit fails, it can affect receiver sensitivity. But many times, the only noticeable issue is the transmitter will tend to oscillate intermittently 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 the following components on the CTRL Unit:
IC-746: Q25, D22
IC-746PRO: Q25, D21, D22, D24
IC-756: Q25, D22
IC-756PRO: Q25, D22
IC-756PROII: Q25, D22, D23
IC-756PROIII: Q25, D22, D23
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: The following two 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 view):
IC-756PRO CTRL Unit (bottom view):
The owner of this IC-756PRO had two stations, selectable with a coax-switch. The other station had a 1-KW amp. His IC-756PRO 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).
NOTE: The previous two 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: I can add an additional surge-absorber across the HRX/RFRX-line to help protect against ESD. I won’t claim it to be a “cure all”, but it certainly can’t hurt. I use a DSP201M or GS35-201M.
Some of the IC-746/756’s have a 1K resistor across the HRX/RFRX-line, but some don’t. So I add one, if needed. This may also help protect against ESD.
Update: I can also add 2 PIN diodes, as Icom did in the late-production IC-746PRO. This may help protect against RF-overload from a nearby transmitter. I use MA4P7102F diodes. Scroll down to see photos.
IC-756PRO CTRL Unit with surge-absorber modification:
IC-756PRO CTRL Unit with modification:
Surge-absorber with PIN diodes:
IC-756PROII CTRL Unit with added surge-absorber and PIN diodes:
NOTE: If you don’t have the skills or equipment to work with static-sensitive surface-mount components, please leave it to an experienced technician.
Mike Nadeau – N1EQ
Posted December 27, 2008
Updated May 15, 2019