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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'm slowing bringing a "new to me" 736 back to life. Based on a report of bad audio I have finally gotten the Technical Supplement. I'm a bit confused on the transmitter alignment (TX Unit). Specifically on Page 61 - Step E it calls for setting the audio level on the MIC jack to 15 mv and 1 kHz.

    Do you think they are specifying 15 mv rms? Or would this be peak-to-peak?

    The very next step is to adjust the deviation, both Wide (normal) and Narrow.

    Clearly I'm new at this.

    Thanks for any replies.
     
  2. K7JEM

    K7JEM Ham Member QRZ Page

    Should be rms, unless specified as peak to peak.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Before doing any transmitter alignment I'd want to be certain the "bad audio" wasn't caused by something else.

    My FT-736R is from 1987 and I purchased it new so I'm the only owner. It's 34 years old and I've never done any kind of alignment to it. I've changed the memory battery twice.:)

    The 736R is known for having a bad ground connection between the front panel connector threads and the circuit board ground behind it; if that ground comes loose, "bad audio" can definitely occur. The fix has nothing to do with alignment.
     
  4. KC3MBK

    KC3MBK Ham Member QRZ Page

    I don't know the history on this rig and up to this point I have not changed any alignment. I have injected an audio signal into the mic jack and followed it through the circuits, in both FM mode and SSB mode. I have not seen any issue with the audio getting all the way to VR01 after Q03 quad op amp (FM mode). The sine wave looks clean.

    I have seen one other issue, which I have ignored, and that is when the Transmit Tone is enabled for repeater use, the green busy indicator comes on instantly with transmit (with my audio generator turned off). So right now I have that feature turned OFF.

    Thanks for the reply. I will take a look at the ground. And I did a battery replacement on this unit when I got it.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The green LED varies in brightness with modulation, including on FM.

    If the CTCSS tone is turned up too high, it will turn green but probably "dimly" green, compared with your voice modulation when you're speaking into the microphone.

    Is that what happens?
     
  6. KC3MBK

    KC3MBK Ham Member QRZ Page

    Without any additional modulation from the microphone, if [ENC] is turned ON the green busy indicator turns ON with transmit and it is at almost full brightness. So that if you add microphone modulation there is almost no additional brightness with mic gain.

    It's my understanding that the FTS-8 option unit does NOT generate the transmit PLTONE, is this correct? So it could be removed if you jumper pins 4 & 5 of J20 and still have transmit tone?

    In addition, I have no information on the FTS-8 unit, no schematic, and nothing mentioned in the Technical Supplement, even though there seems to be an adjustment for something on the FTS-8.

    Lastly, You mention "If the CTCSS tone is turned up too high", where is this adjustment? Or are you referring to tone squelch?

    I didn't get much done on the 736 yesterday. I plan to spend some time on it today.

    Forgot to mention, the grounds behind the MIC jack look OK. Looks like previous owner may have re-soldered all these connections and there is a wire from the small board that runs under the mounting nut.

    Thanks for your comments.
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I thought the FTS-8 was factory stock in the units shipped to the U.S. Maybe not, but my old FT-736R, purchased new in 1987 by me, came with it installed and I didn't order it separately. Possibly they changed something after that.

    If you turn OFF the T-ENC does this problem disappear?

    I've never adjusted the encoder tone modulation in mine, but there must be a way. When I have a chance, I'll see if I can find a Service Manual and look for that adjustment. I "think" there is a service manual, I just don't have it. Likely on line someplace.
     
  8. KC3MBK

    KC3MBK Ham Member QRZ Page

    I thought I was buying a "Service Manual" for mine. But instead I got something called "Technical Supplement". It has board layouts, schematics, calibration procedure, and an upfront warning that I'm not qualified to remove the covers. It also has signal tracing flow diagrams.

    Here is a pic of the carrier w/o modulation
    carrier (Small).JPG

    Looks a little strange. And here is a pic with modulation at 1KHz, mic level at 12:00.

    Modulation_1K (Small).JPG

    All of these are w/o PLTONE. And here is the first carrier NULL, which happened at 1.998KHZ, 15MV sine input to MIC jack. Results are pretty close to the required 1.871KHz according to calibration procedure.

    FirstCarrierNull (Small).JPG

    Later today I'll grab some pics of what happens with PLTONE enabled.
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I found the FTS-8 instructions, and there is a deviation level adjustment for the CTCSS tone output: It's VR1, which adjusts the output of Q3 over a huge range from literally "nothing" to "full."

    If someone played with that or the pot failed or got bumped somehow, it's possible the tone deviation is set way too high.
     
  10. KC3MBK

    KC3MBK Ham Member QRZ Page

    Are you referring to VR1 on the TX Unit? That's on the output from Q3 the quad op amp. The calibration procedure for FM Deviation on page 61 of the Supplement has Vr1 adjusted for a total deviation of 4.5 KHz. So I believe this is for total audio not just the PLTONE.

    However, there is an adjustment on the FTS-8 daughter card plugged into the CTRL Unit. Do you have documentation on this?
     

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