All posts by n1eq

ICOM IC-746/756/PRO Series: Damaged Dual-Concentric Pots

Note: I don’t sell parts. My inventory is for repairing my customer’s radios only.

Frankly, I shouldn’t have to publish this article. But unfortunately, some folks, as well as some pack-and-ship stores, fail to properly pack these radios for shipment.

When these rigs are not packed properly, their dual-concentric potentiometers (pots) are quite vulnerable to damage.

If the radio is not secure in its packaging and/or there isn’t enough protection of its front-panel, the weight of the radio bearing down on the front-panel controls can cause these pots to crack.

The following front-panel controls can be damaged:

IC-746: AF/RF/SQL and NR/APF
IC-746PRO: AF/RF/SQL and NR/NOTCH
IC-756/PRO/II/III: AF/RF/SQL and BAL/NR

These pots are no longer available. But I’ve acquired some suitable replacements, which are electrically identical. But they must be wired in. And the inner knob must be replaced with a different style.

See the pictures below for details.

A damaged pot shown on the left, and an undamaged pot on the right:

An original pot shown above a replacement type:

An original pot shown above a replacement type, with knobs:

Mike Nadeau – N1EQ
www.n1eq.com

September 25, 2019

ICOM IC-746/756/PRO Series CCFL-to-LED Backlight Conversion

Note: I’m not marketing any sort of kit or “upgrade.”
My priority is doing repairs.
I offer this option for repairing faulty backlights.
My service is for customers/end-users only, not other shops.
The entire radio must be sent in for service, not just the front-panel.

April 2018: I’m now able to convert backlight circuits from CCFL to LED in the following radios:

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

If the original circuit is in bad shape, or the CCFL tube is very dim, I can replace the CCFL tube with a 12-volt LED strip. (Scroll down to see pictures.)

  • I add a simple circuit (shown below) that allows the rig’s dimmer function to work as intended.
  • No inverter or regulator circuits are used. So there are no RFI issues, and there are less components to fail.
  • A regulator circuit is not required. The original CCFL driver circuit was not regulated, nor does the LED circuit need be. (If you experience flickering while transmitting, you likely have marginal supply voltage, or voltage loss through your DC power cable.)
  • The circuit is so small, it can be built inside the original CCFL circuit’s shielded enclosure. But I find it best to mount the components on the back-side of the circuit board, where they can be easily accessed, without having to remove the board.
  • The LED strip is a cool-white type, so the display looks as close to original as possible.
  • I use the Flexfire UltraBright Slim Series LED strip, which is only 3.5mm wide, and fits perfectly in the IC-756PRO series displays.
  • In the IC-746/PRO displays, there’s a wider opening, so I add a spacer.
  • The IC-746/PRO series requires 3 sections of this LED strip. And the IC-756PRO series requires 7 sections.

(Update: I now use BD437TG instead of 2N6121.)

IC-756PROII with LED conversion at 100% brightness:

IC-746 with LED conversion at 100% brightness:

Comparison of an original Pro2 CCFL tube (bottom) and a new LED strip (top): 

An LED strip all lit up:

ICOM IC-746PRO Frequency Calibration

This radio is capable of very low-frequency RX-audio response. And this allows one to get very accurate results when zero-beating.

It is best to use the highest frequency WWV signal you can hear (usually 15MHz).

Frequency calibration is a user adjustment on the rear panel. (See page 94 of the manual.)

Here’s an accurate way to do it:

Tune-in to 15MHz WWV (15.000.000).
Switch to either USB or LSB.
Make sure PBT and all noise reduction is turned off.
Set the bandwidth to 3.6KHz, with “sharp” filtering.
Set the “RX bass” to maximum (in the menu).
You should hear the audio cut in and out. This is normal.
Now adjust “CAL” (on the rear panel) until the audio cuts out at the slowest rate possible.
After calibration, be sure to change “RX bass” back to it’s normal setting.

Here is a demo video I made :

https://youtu.be/WnYP4JS6zlQ

April 28, 2014

ICOM IC-746/756/PRO Series: Failure of Remote/CI-V Port

In my experience, the Icom IC-746/756/PRO-series are quite susceptible to electrostatic discharge (ESD). And a common failure is the “Remote” port (or CI-V port).

Usually, one or two surface-mount components are affected: a transistor and/or an RF-bead/choke (or jumper).

Here are the components most affected in each model:

IC-746: Q1702 and/or W1261 on Main Unit
IC-746PRO: Q1941 and/or EP760 on Main Unit
IC-756: Q802 on Main Unit
IC-756PRO: Q3772 and/or W701 on Main Unit
IC-756PROII: Q3772 and/or W701 on Main Unit
IC-756PROIII: Q3772 and/or EP701 on Main Unit

Here are a couple of tests to determine if the CI-V port has indeed failed:

Measure for continuity between the ground side of the CI-V connector and the chassis. It should be close to, if not, zero-ohms. Otherwise, the RF-bead/choke (or jumper) may be open. Although, another possibility is the ground pin of the CI-V jack has a cracked solder-joint, due to mechanical stress on the connector.

You should measure about 4.5VDC across the CI-V port, with no interface connected. If the voltage is much lower (assuming the RF-bead/choke is OK), then the transistor may be at fault.

Here are the components in the IC-756PROIII:

MMZ1608Y102B SMD Ferrite Bead (EP701, under Main Unit)
2SC4081T106R SMD Transistor (Q3772, topside of Main Unit)

Both of these components were available from Digi-Key and Mouser at the time I posted this.

Here are component locations for the IC-756PROIII:

CI-V_Q3772

CI-V_EP701

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
www.n1eq.com

April 28, 2014

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

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):

ctrl_a

IC-756PRO CTRL Unit (bottom view):

ctrl_b

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:
756pro1

IC-756PRO CTRL Unit with modification:
756pro2

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
www.n1eq.com

Posted December 27, 2008

Updated May 15, 2019

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.

ic151_a

ic151_b

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:

ic151_c

ic151_d

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:

drv1

drv2

drv3

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:

746p_sa

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:

 hrx

 

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
www.n1eq.com

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

ICOM IC-756PRO Receiver Noise

If your IC-756PRO’s receiver has lots of noise, with crackles and sweeping heterodynes, especially noticeable on AM, it may be due to a faulty electrolytic capacitor in the noise-blanker circuit (C287 on Main Unit). I’ve seen this happen to several 756PRO’s, and replacing the capacitor solved the problem.

Note: Icom used a few different values for this cap. In the original PRO, C287 is usually 22uF (although the service manual says it is 100uF). The PROII service manual says it’s 47uF. So I’ve been using 47uF as a replacement.

756p_nb

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
www.n1eq.com

March 18, 2009