Icom 756 Pro III - tuner question

Discussion in 'General Technical Questions and Answers' started by K2QI, Nov 14, 2010.

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  1. K2QI

    K2QI Ham Member QRZ Page

    I'm playing around with a loaned 756 Pro III, and have a question regarding its tuner. I have no experience with Icom rigs, so please forgive me if this comes across sounding stupid.

    Here's what happens - my antenna system gives me a good match with a fairly low SWR; probably in the 1.2 - 1.5:1 range. There are some losses in the transmission line due to a poor ground system. I'm using a 1:1 balun at the antenna feedpoint to compensate and to also decouple the feedline.

    When I turn the Icom's tuner on, power output is reduced from 100W to about 85W as verified on a calibrated forward power meter. I test by sending an unmodulated FSK carrier.

    When I turn the Icom's tuner back off, I get full power out.

    Now I've experienced output loss from built-in tuners before, but not this much. I have my K3 setting next to the Icom, and when its tuner is inline, I might lose 1 to 2 watts tops depending on band. The Icom on the other hand reduces drive by 15%.

    Any thoughts?
  2. K9XR

    K9XR Ham Member QRZ Page

    Ater You QSY try holding the TUNER button down for about 2 seconds and see if that makes any difference. It sounds like the tuner "is out of tune" and causing the finals to fold back a little. I believe there are several modes on the menu for the tuner. I think at least one tunes with just the receiver, one tunes when you hold the mic button down for 2 seconds and I believe one requires yiou to manually push the button.
  3. K2QI

    K2QI Ham Member QRZ Page

    I've done that; hold the button for 2 seconds thereby initiating a manual tune. Results are the same. I'm tending to agree with you - something seems to be out of whack with the tuner.

    I'm still digging through the menus and haven't found anything about adjusting the tuner. Any suggestions?
  4. K2QI

    K2QI Ham Member QRZ Page

    OK, here's the even stranger bit.

    If I purposely detune the antenna, SWR obviously increases but so does output power when the tuner is inline. Retune the antenna so that SWR is flat, and output power drops unless I turn the tuner off.

    I don't get it. *scratches head*

    P.S. It does the same thing into a dummy load.
    Last edited: Nov 14, 2010
  5. W5DXP

    W5DXP Ham Member QRZ Page

    The forward power (that you are measuring) certainly increases as SWR increases. That can happen without increasing the generated power. Assuming a lossless system:

    Generated power = load power = (forward power - reflected power)

    Assume generated power = load power = 100 watts and constant

    If the SWR = 5.83:1, the forward power will be 200 watts and reflected power will be 100 watts. You will measure 200 watts on your forward power meter.

    If the SWR = 9.9:1, the forward power will be 300 watts and the reflected power will be 200 watts. You will measure 300 watts on your forward power meter.

    When you understand that forward power is not the same thing as "output power", everything will become clear.
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    What Cecil wrote is true: When you measure "power" into a mismatched load, you're actually measuring the sum of forward + reflected power (pretty much "always," with any kind of power meter), and the actual power is the Pf measured minus the Pr measured.

    That, assuming your transmitter can deliver 100W into a mismatch, should always sum to 100W.

    Also, if the power "drops" from 100W to 85W and that is a real power decrease, it's only 0.7 dB, which is likely within spec for the tuner (most are rated ~ 1 dB within normal tuning range). I wouldn't sweat this at all.
  7. K2QI

    K2QI Ham Member QRZ Page

    Well, I guess what I'm trying to say is that a 15% insertion loss when the tuner is active seems way too high. It doesn't matter whether the rig sees a nominal 50 ohm load into a dummy load or resonant antenna. Other IC756Pro3 owners have said that there insertion loss usually averages 2%.
  8. W5DXP

    W5DXP Ham Member QRZ Page

    Are you sure that it is "insertion loss" rather than the transmitter reducing its output power because of a mismatch that the tuner cannot handle? My IC-756PRO manual says the autotuner will only handle impedances in the range of 16.7-150 ohms. Are you sure that the impedance you are trying to match is within that range?
  9. K2QI

    K2QI Ham Member QRZ Page

    Yes. Dummy load is 50 ohms, and resonant antenna is similar. I confirmed both with an MFJ-259.

    The K3 sitting next to the Icom has no problem or odd behavior when its tuner is active and connected to the same.
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Maybe a failed component in the tuner?

    However 0.7 dB insertion loss in a 50 Ohm system still doesn't seem terrible; although if you run a full duty-cycle mode like RTTY and stuff in the tuner is dissipating 15 Watts, that might eventually damage a component. On SSB or CW, probably not.
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