FT-991, blown finals or drivers?

Discussion in 'Radio Circuits, Repair & Performance' started by KK6RPX, Nov 12, 2018.

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  1. KK6RPX

    KK6RPX Ham Member QRZ Page

    Hi All,

    Well, looks like I blew the finals in my FT-991. I pulled the cover and did the idle current check. One side is 400mA and the other is 1800mA. I tried to adjust the side with 1800mA down and it would only go down to 1500mA. So my questions is do these readings indicate bad finals or bad final drivers? And if it is the finals, would you replace both?

    Thanks!

    KK6RPX
     
  2. K1LKP

    K1LKP Premium Subscriber QRZ Page

    HI KK6RPX,

    I RECOMMEND YOU GO ON LINE AND
    FIND A FT 991 USERS GROUP.
    THEY SHOULD BE ABLE TO HELP
    YOU RESOLVE THIS ISSUE.
    kangaroo2[1].gif
    thats all folks.jpg
    73 - K1LKP
    CARMEN
     
  3. K1LKP

    K1LKP Premium Subscriber QRZ Page

    JONATHAN
    TRY THIS LOCATON

    http://foxtango.org/foxtango001.htm

     
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    What did you do that makes you think you blew the finals ?

    Disconnecting the B+ from them should clue you to where the current is going.

    A finger feal may tell you what is getting warmer than normal.

    Good Luck.
     
    Last edited: Nov 13, 2018
  5. KK6RPX

    KK6RPX Ham Member QRZ Page

    Carmen, I did join the FT-991 group and posted there. I joined Fox Tango too but haven't been accepted yet. Thanks for the suggestions.

    KA9JLM, I did some poking around on the net and the symptoms fit. A little power on the higher bands like 20, less to almost no power on the lower bands. The idle current certainly isn't right. But I am by no means certain it is the finals and so I am asking here for help!
     
  6. N8VIL

    N8VIL Premium Subscriber QRZ Page

    I would check to see what the bias voltage going to each of the final transistors is. If one is higher and cannot be adjusted down, then you may have a bias control issue. If not, one of the finals may have failed. If one of the finals are defective, I usually replace in pairs.
     
    Last edited: Nov 13, 2018
    US7IGN likes this.
  7. KK6RPX

    KK6RPX Ham Member QRZ Page

    I am at work for another 24 hours but as soon as I get home I will check.

    Thanks!
     
  8. KK6RPX

    KK6RPX Ham Member QRZ Page

    Got the new finals and put them in and the radio works! I set the idle current to 1 amp, lower than Yaesu recommends and am getting 90 watts out. Perfect!

    Still would like to know how to check the final drivers with a 'scope but as long as it works!
     
    N8VIL likes this.
  9. VE1IC

    VE1IC Ham Member QRZ Page

    as a octopus circuit / component tester / curve tracer?
     
  10. KA5IPF

    KA5IPF Ham Member QRZ Page

    I would suggest you set the idling current to spec. That determines the linearity.
     

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