ICOM IC-746PRO TX Problems

Note: This info applies mainly to early-production units that do not have the factory upgrades.

I don’t know the exact serial number when the upgrades began. But from what I’ve seen, serial numbers from 02023xx and up have all the updates.

So far, serial number 02021xx is the highest I’ve seen that did not have the factory upgrades. And all 5-digit serial numbers I’ve seen did not originally have the upgrades.

Later on, Icom re-designed the PA board. And from what I’ve seen, serial numbers from 0207xxx have the new board.

Chip IC151:

Chip IC151 on the RF Unit can fail due to electro-static discharge (ESD).

Icom released a service bulletin (#920), which recommends adding two PIN diodes to the HRX-line. This can help prevent ESD-related failure.

Late-production units have a couple of diodes soldered on top of IC151 (see the picture below). I’ve drawn a diagram of the factory modification. The SMD diode is an MA77. The other diode is 1SS53. This upgrade is to prevent damage from ESD.



In the latest-production units, Icom revised the RF Unit board to accommodate three extra SMD diodes in the IC151 circuit (for additional ESD-protection). Its basically the same as the circuit described above, except it uses the smaller version of the MA77 diode (MA2S077). Two of these MA2S077 diodes are in series, in place of the large amber diode in the previous circuit. The other MA2S077 is used in place of the MA77 in the previous circuit. A picture and schematic are shown here:



PA Pre-Driver Circuit:

The pre-driver FET (Q1 on PA Unit) will sometimes become intermittent or fail completely. This is a 2SK2973, and it runs fairly warm. When I replace the FET, I solder a small heat-sink to its source tab. A 2AG-style fuse-clip fits perfectly by cutting off one of its legs, then soldering the remaining leg to the FET tab. See the pictures below:






Some of the later IC-746PRO’s were manufactured with the wrong value resistor in the predriver circuit. R3 on the PA board should be 10 ohms, but some boards had a 4.7 ohm resistor instead. This can cause failure of the predriver FET (Q1).

PA Driver Circuit:

In early-production units, the two driver FET’s (2SK2975) run quite hot, because the factory set the idling-current (bias) too high. The excessive heat can eventually cause their solder connections to become intermittent, which causes fluctuations of the transmitter RF output. This affects all bands and all modes, and it is most noticeable by observing the ALC meter.

If the FET’s are still good, they can be re-soldered. But re-soldering the FET’s is no easy task. It requires proper tools and skills. The trick is to melt the solder on all three of an FET’s strips at same time, while holding the FET firmly against the mounting plate, so it doesn’t slide out of place. If you’ve re-soldered without the FET becoming loose, you didn’t get it hot enough, and the joints will fail again. To get an idea of how the FET’s are soldered, here are some photos:




Personally, I prefer not to remove the DRV Board. Instead, I remove the entire PA Unit. This may be more labor-intensive, but it eliminates the risk of damaging the traces of the DRV Board, as it requires considerable heat to unsolder its mounting pins. Removing the PA board only requires unsoldering the VHF SO-239 connector and two of the Varistor Boards.

Note: After the FET’s are re-soldered or replaced, the the PA idling current adjustment (R11) should be set to 1.5-amps (instead of 2.5-amps, as stated in the service manual).

PA Driver Idling Current:

Some folks have set the driver idling current lower than the latest factory-recommended setting of 1.5 amps. I don’t recommend doing so, as this can increase IMD (inter-modulation distortion). Here is spectrum analysis of an IC-746PRO with idling current set to 1.5 amps, compared to 0.5 amps:


PA Unit Factory Upgrades:

Here are the changes made to the late-production PA Units:

  1. The two mounting screws on the DRV Board have been replaced with longer ones that have larger washers.
  2. Inductor L2 has been changed from 100uH to 10uH.
  3. Inductor L6 has been changed from a ferrite-bead type to a toroidal type.
  4. Resistors R41 and R42 have been changed to 6.8-ohm.
  5. Resistor R11 on the DRV board has been removed.
  6. The PA idling current adjustment (R11) is set to 1.5-amps (instead of 2.5-amps, as stated in the service manual).

HRX-Muting Circuit:

Another common problem is failure of the HRX-muting circuit. This is usually caused by RF overload, high SWR, or ESD. If this circuit fails, the transmitter will tend to oscillate intermittently. This is most noticeable on the higher HF bands (6, 10, or 12 meters). 2-meters will not be not affected. The symptoms are: unusually high current demand from the power supply, fluctuating RF output, and/or erratic antenna-tuner operation.

To get an idea if the HRX-muting circuit is at fault, turn the receive attenuator (ATT) on. If the problem goes away when the ATT is on, the HRX-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:

Q25 (2SC4213)
D21, D22 (MMBV3700LT1)
D24 (MA77)

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 13.8-volts RX, and 0-volts TX.

CTRL Unit Factory Upgrades:

In the latest-production units, Icom has made changes to the CTRL Unit. Here are the changes made to the latest CTRL Units:

  1. Two PIN diodes were added back-to-back across the HRX-line, from the junction of L27/R160/RL4 to ground.
  2. Resistor R154 has been changed to 1K.

Here is a photo of the added 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.

DISCLAIMER: Attempting these modifications on your own equipment is at your own risk. If you have any doubts, don’t try it.

Mike Nadeau – N1EQ

Last revised: February 15, 2018
Revised: March 16, 2009
Originally posted: December 17, 2007