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.

10 comments

  1. Luther Beasley says:

    I have a FSH6 was working fine – today the keyboard buttons stop working, the wheel still works. I checked the PCB connectors seems good. the selector is on frequency the wheel changes the center frequency. The power button works.

    • Gingerpale says:

      I have the same problem! The row with keys “-“, “0”, “.” ,”Hz” and the “preset” are not working. I have checked the keyboard itself and this is OK. Signal of these keys enter the main PCB. I’ve noticed that the tracks and via’s for the keyboard are very thin on the main PCB. So my suspicion is that due to the extensive heat of the power supply components the tracks are cracked somewhere. But I can’t find where yet. I want to try to follow the keyboard tracks. According to the block diagram I could find, the keyboard should be connected to the “risc controller”. Which part is that?
      Did you find by any chance the solution yet? Any help is highly appreciated…

  2. omar says:

    Buenas tardes, tengo una pegunta, y espero que puedan apoyarme, e perdido la antena del equipo FSH3, con que antena puedo sustituir la????

  3. Junior TSHONGO says:

    I like this video TSP #58
    I’m having the same issue, please can you advise me the transistor that can be used on place of the 651B. Cause my FSH3 cannot ON and I can see after opening the divice the same scenario with what I’ve seen in the video TSP#58.

  4. 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

  5. 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

  6. 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

  7. Vasily Ivanenko says:

    Great example of problem solving. Thanks

  8. 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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