preamp component smoked - RF Concepts 2-417

Discussion in 'Amateur Radio Amplifiers' started by KD2QED, Nov 17, 2019.

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  1. KD2QED

    KD2QED XML Subscriber QRZ Page

    Hi,

    I purchased a RF Concepts 2-417 and learned RF sensing is suboptimal for keying detection. This resulted in the preamp no longer functioning.
    I opened it up and have identified one of the components in here that looks burnt up, but I can't tell what it is. I've attached a picture of it.
    I have the schematic, but I'm having some trouble understanding it.

    Manual - http://www.radiomanual.info/schemi/ACC_PA/RF_Concepts_RFC-2-315_RFC-2-417_RFC-3-312_user.pdf
    Schematic - http://www.radiomanual.info/schemi/ACC_PA/RF_Concepts_RFC-2-315_RFC-2-417_sch.pdf

    I measured it at 3 ohms and the diode test returned 0.003 V.

    Can someone give me some advise on what this is and where it is in the schematic?
    I'm hoping it's simply a matter of replacing it to get it back running.

    Thanks,
    Jason
    KD2QED

    20191117_163809.jpg

    Full album: https://photos.app.goo.gl/Sn2LdLrhTTYsKqed6
     
  2. AA7QQ

    AA7QQ Ham Member QRZ Page

    A more vertical pic would be better. It might show where the traces go. Also, an underside pic with the lands identified would help.

    Ed
     
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

  4. WR2E

    WR2E Ham Member QRZ Page

    Looks like what's left of a metal film resistor.
     
  5. KD2QED

    KD2QED XML Subscriber QRZ Page

    Hi Ed,

    Thanks for the quick reply. Let me know if these are what you're looking for. I added some more to the album too.

    It's a challenging area to photograph. I will see what it takes to get this board off of the chassis to post the underside.

    20191117_170605.jpg 20191117_170756.jpg
     
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd guess that's either a small form factor molded inductor or one of the preamp protection diodes D13, D14.(it looks more like a small inductor to me, but the protection diodes might be more likely to actually blow). Really hard to say based on what the manual and schematic shows. I'd use careful visual inspection and an ohm meter to follow the traces on either side of that burnt component to see where they go. That should help you pin down where it is in the schematic.
     
    Last edited: Nov 17, 2019
  7. WR2E

    WR2E Ham Member QRZ Page

    The metal end caps are typical of the construction of metal film resistors. I've seen them burnt up and that's what they look like. I can't say I've seen diodes or molded inductors that are burnt to a crisp like that.

    On the other hand, if it was a resistor, it would likely be OPEN and not read 3 ohms.

    So yeah, a diode might make more sense, they tend to fail shorted.

    But an inductor IS a DC short.
     
    Last edited: Nov 17, 2019
  8. WR2E

    WR2E Ham Member QRZ Page

  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I was thinking molded inductors like these that have a similar construction. But yes, a resistor probably fits what we can see better though I'd expect most resistors to burn up in open circuit mode and not show 3 ohms as the OP posted (though I guess most inductors would burn up in open circuit mode as well :) )
    [​IMG]

    Either way unless someone else has one of these and can verify what that part is, the best bet is to trace the circuit to figure out where the mystery device connects.
     
    Last edited: Nov 17, 2019
    WR2E likes this.
  10. WR2E

    WR2E Ham Member QRZ Page

    I think you're right...
     

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