Teardown, Repair & Analysis of a Rohde & Schwarz FSH3 3.0GHz Portable Spectrum Analyzer

In this episode Shahriar examines a faulty R&S FSH3 100kHz – 3.0GHz Portable Spectrum Analyzer. This exceptionally dirty unit does not power on and is missing a power supply and battery. After verifying the correct operation of the battery charger circuit and keypad the problem is traces to a damaged SOT-23-6 MOSFET device which is part of a fly-back DC-DC converter. A replacement MOSFET with a lower current rating proves to be a failure as the device fails on startup. A higher rated MOSFET resolves the problem and device begins to operate correctly.

The block diagram of the system as well as a detailed block diagram of the RF section is presented and correlated with the teardown of the RF section. The super-heterodyne down-conversion circuits and components are examined.

6 comments

  1. Josep says:

    I have an FSH6 with this same problem.
    On my equipment the fault occurred at the moment of changing the old battery pack with a new one.
    Just at the moment of connecting the new pack, smoke has immediately emerged from the R79 resistor.

    Searching the internet I found the service circular #10640 that comments this problem and how to solve it:

    http://www.signalhound.eu/download/Rohde_Schwarz_FSH_Mod_FET.pdf

    Thanks Shahriar for yours videos.
    Josep Carles

  2. samir habbaki says:

    dear sir good morning
    i have 2 pcs fsh3 and 6
    the fsh6 analizer portable do not turn on and now i have problem in mother board i open it i found the 651f dual transistor switching supply are shorted circuit so pls advice if you have parts or an refurbuished module
    and also i need several used refurbuished measurement in future
    pls advice

  3. Justin says:

    As always, great video.

    Wondering what caused the original mosfet to fail. Perhaps just a faulty part. And curious if the new replacement is running hot.

    The overlay message during the RF section walk thru could have been taken down sooner as it detracted a little from the walk thru.

    Really enjoyed the approach with the resistor and the bigger transistor to gently power up the device.

    Thanks Shahriar.

    Justin

  4. Vasily Ivanenko says:

    Great example of problem solving. Thanks

  5. Steve C says:

    Hi,

    Sorry if asked before, but what camera do you use to make these wonderful videos? I’m impressed by the ability to autofocus when you move to close-ups. Thanks.

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