Teardown and Repair of an Agilent E4421A Synthesized Signal Generator

In this episode Shahriar repairs an Agilent E4421A synthesized signal generator purchased from eBay. The unit displays the error messages “UNLOCK” and “UNLEVEL”. The cause of failure is investigated and traced. Alongside the repair efforts, the internal construction and components of the synthesizer are also presented. A short tutorial on the advantages and disadvantages of using a Darlington transistor is also presented as well as the schematic of a simple Darlington based voltage regulator. The documents for this tutorial can be found here.



  1. Walter Robert Hopf says:

    Hello my friend Shahriar,

    the contribution to the topic was very interesting. I think that one can come to this kind of high-quality measuring instruments. I like Ebay actually not so much. But your example clearly shows again that one hardly comes around as a small investor to Ebay.
    It was another post with good Sytematik and good entertainment! I hope that you stand by a long time and such posts on the internet ask. You can not possibly imagine quite how much enthusiasm you bring your friends and fans so.
    I hope to interpret the Google translator is really in a position my feelings (?)!

    Thank you for that!
    Friendly greetings from Germany
    Walter Hopf

    P. S. Maybe you can occasionally contribute to Infiniivison MSO3000X T series of Keysight do, but only if you are convinced …. It is perhaps the last oscilloscope I want to buy in my life.

  2. Scott says:

    Nice job on the videos. Very interesting.

  3. Paul Hamilton says:

    That was a superb video. Thank you for your efforts and selflessness in sharing.



  4. douglas lee says:

    I would be very interested to know what you would estimate the value of the generator being now that you’ve fixed it. Since you paid about $500, how many fold do you think that “relatively” short repair job will be worth, if looked at in terms of “return per time unit”?



  5. paul says:

    I just wanted to leave a very warm thank you here for the great work and all your spare time you put into sharing your knowledge. very good job =)

  6. Kelvin Tan says:

    Have been waiting for your video, and I am glad to see them again. Lots of educational values. Keep them coming.

  7. Sebastian says:

    Ahhh…I’m Addicted to this Feeling, when it Finally works. 🙂

    I Repaired a R&S SM300 Signal Generator myself. Bought it Broken for ~600EUR. N-Connector was broken, and Output Stage Transistor was Blown. I think It’s worth 3000 now.

    • John says:


      I have a problem with my R&S SM300. Do you have a schematic diagram of output stage?

      When the Pout =27dBm signal is disappeared, when I increasing the output power. I think that output stage was broken, I would like to repair it. Could you give me some information which devices I would need, or with type of its did you used to repair yours generator?

      Thank you so much in advance for you r help.

      Best regards,

      • Mike says:

        My SM300 had thesame problem.
        I replaced the gain block AGB3303 by an BGA6589 (also a SOT89).
        The MMIC is located at 16cm from the N-connector.
        Best regards,

      • Mike says:

        Forgot to mention;
        I also decreased the current by removing the 27 ohm resistor in the current source, located 1cm above the amp block.

  8. John Wallace says:

    Congratulations on another excellent video – keep it up, your work is much appreciated.

    I’ve just purchased one of these units myself – it was also “bounced” in transit (to Australia) – resulting in one of the back feet being bent and the rack mount flanges on the front being ripped off!! Fortunately it still works – which is testimony to just how rugged these units are!! Easily fixed with just another bit of boring metal work!!

  9. tw says:

    It’s great to see another video! I’ve been looking forward to another one for several months and was thrilled to see a new one. The quality and content were top noch as usual. Glad to hear that you’ll be making more videos.

  10. Rupert says:

    Did you know that your page has a “No Fax Payday Loan” div at the top (immediately visible if javascript is turned off)?

  11. bbutler says:

    Ditto on Anne’s kudos.
    I learn more in your videos than nearly all others combined.

    Beck’s, Red Wine and lots of Knowledge for consumption. Can it get any better than this?

    Your style, presentation and ability to impart understanding truly makes a difference to this old community member.

    Thank you. Its great to ‘see’ you back.

    • Rich Stoer says:

      I agree completely with this post. Very informative videos which I learn a lot from.
      Add my thanks to the others.

  12. Anne says:

    Dear Shahriar,

    I recently discovered your Youtube channel and I’m amazed by your video’s,
    they truely stand out from others (Dave, Mike, etc.) on educational level.
    Instead of quickly showing “what” and rushing through “why”, you explain all
    reasoning behind in great detail, and that is by far the most important to me.
    I love how your take me along with your thought and explain things that’re
    not common-knowledge to hobbyists like me!

    For example your DAC video gave me hours of pleasure building my own DAC
    and going through the same steps you did.

    I also love your video style: Showing the stuff that matters in detail and skipping
    the obvious things (unscrewing the case, soldering simple components, etc.)
    Learning about the darlington transistor is far more important to me than
    watching howto unscrew the case, which is generaly quite obvious.

    Please keep up the great work and don’t apologise for posting less frequently!
    I rather see one of these video’s then tens of others!

    Best wishes from the Netherlands!

    • smling says:

      Mike’s video is not bad. A style of his own, and humor is done in a good taste.
      Dave’s is a mixed bag, but drifting more and more into life-blog away from eevblog. But I just run it in the background. For Sha and Mike, I gave them full attention and safekeep a copy in my library, and many times, watch them more than once.

  13. dk says:

    great to have you back, spearheading the electronic repair thriller genre, thanks for charing the knowledge!!

  14. solipso says:

    Respawned! HOORAY!
    Great video, btw.

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