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

    Not quite sure, but I would like to test the final drivers in circuit, both to know how and to make sure they are OK?


    Hmm. The service manual says 1.5 amps per. My reading indicates that this is the idle current that was blowing finals and the rumor is that Yaesu now sets them to 1.25. I set mine to 1 amp even and have seen mentions of folks using that. Is that enough of a difference to affect linearity? And would the biggest affect of non-linearity be splatter?
     
  2. KA5IPF

    KA5IPF Ham Member QRZ Page

    What are the finals?
     
  3. KK6RPX

    KK6RPX Ham Member QRZ Page

    The finals are what amplify your signal right before it heads to the antenna. In this case it is a pair of MOSFETS, RD100HHF1.
     
  4. KA5IPF

    KA5IPF Ham Member QRZ Page

    Those are spec'ed for 1a so you should be good.
     
  5. KK6RPX

    KK6RPX Ham Member QRZ Page

    Thanks! And sorry about the misunderstanding, you clearly know what finals are. :)
     
  6. G4COE

    G4COE Ham Member QRZ Page

    Don't know if it's me... with these mod-con FET stages, I reckon they pick a number, double it and use that, always seem to be set on the high side. Could be to cover the spread of FET characteristics. Was never like this with audio power amplifiers in my servicing day's it was always down to the last mA, mind you these are a different kettle of fish.

    Dave
     
  7. KN4TS

    KN4TS Ham Member QRZ Page

    My PA's need replacing as well. When replacing the finals, the tech manual says to disconnect J4001, but I'm not sure how it comes off (straight pull, twist?). Did you do that step, and if so, do you remember how to disconnect the coax at J4001?

    Many thanks,
    Bruce
     
  8. KK6RPX

    KK6RPX Ham Member QRZ Page

    I didn't have the tech manual. I didn't disconnect the coax or jumpers. I just un-soldered and replaced. The only jumpers that I did remove was during setting the idle current for each side.
     
  9. KN4TS

    KN4TS Ham Member QRZ Page

    Okay and thanks for the quick response. I'm guessing they have that step just for extra caution. Looking at the schematic, the coax is the signal input to the HF PA board. With the radio turned off and ample time to discharge, probably not a big factor.
     
  10. KK6RPX

    KK6RPX Ham Member QRZ Page

    Good luck and let us know how it turns out. I found that setting the driver current evenly took a little doing, the pots are single turn and they are quite fussy. I imagine that was part of the problem with the early run of these rigs.

    I have quite a few hours on mine since I replaced my finals and it is still going strong.
     

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