FTDX-1200 TX red light flashing

Discussion in 'Radio Circuits, Repair & Performance' started by YC1UTC, Apr 14, 2019.

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  1. WD8BWW

    WD8BWW XML Subscriber QRZ Page

    Pretty much any of the modern radios will do this if there is an external PTT condition on turn on. Such as Mic hold down switch is in TX position. CW key paddle is engaged or key plug is backed out a bit. KR4J you did the right thing unplugging the key.
  2. YC1UTC

    YC1UTC Ham Member QRZ Page


    Finally, I solved my problem but I should go to the service manual thoroughly since all the advice way stated on the internet nothing hep.

    First of all I should check the PTT pin in the microphone connector, I got the voltage PTT closed is 0V and PTT open is 1.4 Volt. So I decided to verify this condition with FTDX 1200 Service Manual which we can find from the net.

    In every modern radio, there is a processor which always in standby mode when power supply plugged in. This processor usually has responsible to control the PTT, from the Block and Wiring Diagram in Service manual, I found that is the Q3031 in the Control Unit.

    Routed wiring from the PTT pin till to the pin #83 of Q3031 (a pin which have function as PTT key in processor) and checked what is wrong with logic voltage. This step need some electronic knowledge and skill to perform and off course need a careful wiring check and be patient. I will not explain detailly how to do that, it is long step and need a tricky way, but my conclusion is the PTT function of the processor pin #83 is no problem.

    But in short story, this pin #83 in existing situation is not reacted with PTT state condition (PTT close or open). If we analyze the wiring diagram, voltage at the PTT in mic connector in open condition should have at least 3 Volt and 0 Volt for close condition, so there is something wrong with wiring connection or electronic part related with this pin.

    FTDX 1200 have another plug as EXT PTT in the back of the radio, this pin has related connection even though not directly with PTT in mic connector. So I checked the state condition for this pin too, 1.6 volt for open and 0 V for close.

    After thoroughly check, I found there is a blocking diode related with PTT Mic pin which not function properly, there is leakage current internally. This diode should maintain the bias voltage at PTT mic pin almost to 5 volt. This defective diode is D8001 in the SW-A-Unit wiring diagram. The actual module of the SW-A-Unit Board is inside of the Radio Front Panel. To replace that, we should open radio front panel. So after replace the diode, state condition back to normal again, to 3 Volt.

    The problem is not finish yet, since state condition at the PTT pin in EXT PTT connector at back of the radio still low (1.4 V). Again after thoroughly checked, there is diode as blocking voltage and a capacitor to be defective due to leakage. This diode is D1076 and C1712, both components located on the Main Unit wiring diagram, and actually in the Main Unit Board close to the EXT PTT connector. So after replaced both part, the open voltage on this PTT pin is 2.5 Volt.

    I used available diode component 1N4148 and ceramic capacitor as replacement, off course need a little arrangement to put such components on SMD mounting (LOL).

    Finally, the radio back to normal again till now without any issue, only with cents of dollar.

    Sorry for simple wording, hope easy to understood and to be helpful. Good luck

    Nuril - YC1UTC

    Attached Files:

    K5ABB and EA4BPN like this.
  3. WR2E

    WR2E Ham Member QRZ Page

    Quite understandable!

    I wonder if you have been able to determine the root cause of the failure of these components? It seems very unusual to me that these components would simply all fail at the same time.

    Did anything happen previous to the failure that you might be able to assign a root cause to the failure? For example, possible ESD issues?
  4. KQ0J

    KQ0J XML Subscriber QRZ Page

    Good job!
  5. G3YRO

    G3YRO Ham Member QRZ Page

    Yes I'm thinking that somebody probably accidentally plugged some high voltage into the rear PTT socket . . . maybe the TX switching line from a valve amp like an SB200 or FL2100 - that would certainly blow those components !

    Anyway . . . well done for tracking it down !

    Roger G3YRO
  6. YC1UTC

    YC1UTC Ham Member QRZ Page

    I have same curiosity why this happened. Nothing specially to what I did previously except just an normal use just change the microphone from standard hand held mic with desk mic MD200 and vice versa. Failure not happened just after replace the mic but a view days after when I turned on radio again. When I tried to find out the answer of this problem, I saw many same issue happened on FTDX 1200 and 3000 and some on 5000.

    Failure caused by ESD is possible, but the faulty part are seemed not sensitive component.
    WR2E likes this.
  7. KK9W

    KK9W Ham Member QRZ Page

    I just encountered the same issue of the red flashing light on my Yaesu. The cause was a 3.5mm stereo plug accessory cable for the tuner was not fully inserted.

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