Icom IC 746 non pro internal antenna tuner not tuning

Discussion in 'Radio Circuits, Repair & Performance' started by AC7AV, Apr 18, 2017.

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  1. AC7AV

    AC7AV Ham Member QRZ Page

    I have an Icom IC746 that the internal antenna tuner is not working correctly. I purchased the radio used and I noticed it was not working well when I received it. I don't normally use the tuner, but I wanted to make sure everything is in good working condition. I have read a lot of information about the tuner issues on this model, mainly the stepper motors. I did have one that I had to repair thinking this was the only problem. Well after reassembling the radio for testing after the repair of the motor, the tuner will still not tune. Both motors are fully turning the capacitors with no noise now, so the repaired motor is working. The SWR almost never gets to a match under 2:1 on the radio and the radio will eventually give up and turn off the tuner. This was tested into a dummy load and to a known antenna. The radio will not tune on any band, but it seems to be going thew all the motions. Is this tuner prone to having relay problems? It seems like this may be the area that may be having a problem but I don't know, Anyone with some insight that has experienced this problem? By the way, the radio is transmitting full power, so its not a transmitting power output problem.
  2. G3YRO

    G3YRO Ham Member QRZ Page

    I'm not familiar with the circuit of this rig's tuner . . . but if it's anything like the one in my TS930SAT it's pretty complicated !

    I've often thought it would be a nightmare to suss out the fault if it ever goes wrong. There are so many things that could cause a problem . . .

    I guess you have to test everything, and eliminate them one by one. If the actual SWR sensing circuit is OK, I would certainly start with the relays.

    Does the Service Manual not give you some suggested testing procedure?

    Roger G3YRO
  3. KA1MDA

    KA1MDA Ham Member QRZ Page

    Have you tried the tuner on 6 meters as well as HF? IIRC, there is a relay on the tuner board which adds inductance to the tuner circuit for HF, and one that bypass inductance on 6 Meters. If you've been testing the tuner on HF, try testing it on 6 meters and see if it works.

  4. AC7AV

    AC7AV Ham Member QRZ Page

    Tom, thanks for the information on the 6 meter section. I did try it on 6 meters as well and it will not tune even into the dummy load.

    Just so its clear, the SWR meter does work correctly when the tuner is off. The dummy load shows a 1:1 match on all bands 160-6 meters.

    There is a circuit thew the tuner when its turned on, because I don't loose the receive when on an antenna, and I see power output on an external meter. Although not full power when in tune mode, and you can see the power level going up and down when the radio is trying to tune. Receive sensitivity is also reduced when the tuner is engaged, most likely because the tuner is not tuned correctly.

    I don't have a service manual for this radio. If someone has a PDF of one or knows where one could be downloaded, that would be great.
  5. KA1MDA

    KA1MDA Ham Member QRZ Page

    I sent you a private message here you may want to check out. The fact that it won't tune on any band is interesting. Any chance the tuner-in and tuner out cables got swapped? Also, is there any way you can watch the stepper motors or tuner caps to make sure they are traveling through their entire range of motion? Wondering if there is some kind of alignment process that needs to be done when replacing stepper motors to make sure the position feedback (if any) matches the position of the capacitor plates?

    KB3WG likes this.
  6. AC7AV

    AC7AV Ham Member QRZ Page

    Thank you, I got the message and attachment.
    To answer your question on the tuner, I don't think you can hook it in a way to swap the input and output but Ill look closely at it. I took hi quality pictures as I was going threw it, so there wouldn't be much of a question on where the wires went. I think It only had 3 RF cables to remove and two ribbon cables the speaker plug and one other 2 conductor cable that had to be unplugged from the bottom of the main board. It wasn't much. Oh, well I had to un-solder and re-solder the coax connections on the back of the radio in the process.

    Yes I can view the stepper motors and see that they are both turning, and the couplers are connected and turning the tuning capacitors. Depending on the band they turn more or less, but Ive seen them both move the full range from full open to full closed. Each motor shaft has a key on it that goes form full closed stop to full open stop, so each motor only has a movement of 180 degrees. The radio runes the motors to full closed stop when the tuner is off, so the radio must know the position of the motor at startup. These motors do not have anything in them like a sensor, so it has no way of telling the computer the exact position of the tuning capacitor, its probably all done but time measurement.

    Ill shoot a video of it in action later today, and look threw the attachment.
  7. W4KVW

    W4KVW Ham Member QRZ Page

    I also have an ICOM 746 (Original) & the built in tuner does not work.It acts like it is trying & then it turns itself off.It does the same thing on all bands including 6 meters.The radio works great other than the tuner not functioning at all.I have Full Output on all bands into the dummy load.


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