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Yaesu FT-736R : calibration audio level

Discussion in '"Boat Anchor" & Classic Equipment' started by KC3MBK, Oct 15, 2021.

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  1. KC3MBK

    KC3MBK Ham Member QRZ Page

    I believe I have confirmed that the FTS-8 daughter board that Yaesu called an "option" does get involved in transmitting the PLTONE. And it also handles the receive Tone Squelch.

    There are a couple of web sites that have information on replacement tone encoder boards. PIEXX Computers sells a replacement that does tone transmit and receive, it's their model FTS-8px.

    There is also a guy that built his own for transmit only, it's on the pic microcontroller web site and the project is "FTS-8 subtone encoder". The output does have a "level" control for setting the deviation of the tone.

    Now if I could only find the schematic for the Yaesu FTS-8 board...
  2. KC3MBK

    KC3MBK Ham Member QRZ Page

    OK - I did find the schematic, and it also shows a VR1 after a Q3. I'll try to include it here for others.

    And I think I found the calibration of VR1. With just the tone being transmitted the deviation should be 250 - 300 Hz. However, VR1 on the TX Unit should be set first (I think).
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I specifically stated FTS-8 and it's VR1.

    That is on the FTS-8 board and adjusts only the CTCSS audio level (which becomes deviation in the FT-736R).

    Schematic below. The instructions are here:
  4. KC3MBK

    KC3MBK Ham Member QRZ Page

    Yep - see my message above.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Sorry, I posted my last one in reply to your earlier post and didn't see what you had posted just after that!

    Timing was close.:p
  6. KC3MBK

    KC3MBK Ham Member QRZ Page

    What did Kenny Rogers say.. "You gotta know when to fold 'em"

    I think I'm at the point with this rig where I really don't know where the distorted audio is coming from. Convinced myself today that it has nothing to do with the FTS-8 module. I went back and retested 2 meters, with no PLTONE. Laptop and SDR receive the carrier just fine from the dummy load. 736 set to about 3 or 4 watts out. If I record the audio with just a simple 1800 KHz tone on the MIC jack it sounds OK but audio seems low. But when I connect a Yaesu MIC and record speech it sounds pretty bad. Tracing the fixed tone audio through the TX Unit, the sine looks clean all the way to Vr1 on the TX unit (deviation adjust).

    The one thing I have noticed, but could be normal, is how soon the Green ALC indicator comes on bright (with no PLTONE). With the MIC level at the 9:00 position it's indicating max modulation with a an input of 15 mv. And with PLTONE enabled, the Green ALC indicator comes on instantly in transmit (MIC at minimum). This does not seem correct to me. And I don't have the equipment to verify the PLTONE deviation.

    The 127.3Hz PLTONE will open the repeater if enabled. So I don't think frequency response is an issue.

    If I record JUST the PLTONE with no other MIC input I can hear the recorded tone but it sounds a bit raspy (but who knows, could be my ears).

    So "time to fold 'em" ? I went into this repair with a positive attitude but now I'm convinced it's beyond my ability and the equipment I own.

    I'll be putting the covers back on and setting it aside for awhile.

    Steven, Thanks for your assistance.


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