Teardown, Repair & Experiments with an Agilent N4901B 13.5Gb/s Bit Error Rate Tester BERT

In this episode Shahriar repairs a non-functional N4901B 13.5Gb/s BERT mainframe. This instrument is equipped with both the generator and analyzer modules which makes it fairly valuable. The instrument does not power on and does not react to the soft power button.

Teardown of the unit reveals a TD-Lambda Vega power supplies series which is a fully configurable switching power supply design. Even though the power supply in the instrument is completely custom, it is made from generic sub-blocks readily available on eBay. A replacement front-end is purchased to repair the power supply after detecting the problem.

The instrument also shows a minor problem with falsely detecting an overload condition on the delay control port which is by passed by installing an overwrite switch. The instrument performance is verified by generating and measuring various eye diagrams through a back-plane PCB board.


  1. Christian Servais says:

    In your video, you proved that you could defeat the invalid control voltage detection system and swapped the modules to get a functional input at the front panel. As you said, most probably, the message box comes from an interrupt generated from the module with the sick input, but now unused. Each of the two modules being able at anytime to generate this interrupt.

    So, why don’t you replicate internally inside the instrument on the now unused but still sick input the trick you made with your external supply? It should be easy and have no effect on the instrument function except getting rid of that nasty dialog box.

  2. Jan says:

    Hello Shahriar,

    I love all your videos and they have made me finally dare to move from digital into the analog and RF electronics world, which is awesome!

    The depressing part is that most of the instruments you show are pretty much out of reach financially for me (and probably many others), even used/broken.

    I was recently looking for an affordable vector network analyzer, and found that quite a few ones have appeared in the <$500 range, for example:

    XaVNA: https://xaxaxa-dev.com/xavna.cppsp
    PocketVNA: https://pocketvna.com/
    N2101SA (also in aliexpress): https://www.banggood.com/-p-1090983.html
    miniVNA tiny: http://miniradiosolutions.com/54-2/

    I think comparing a few of these against your top-of-the-line equipment would make a great video and be very useful for many people. Maybe even tear down one or two and explain what is different compared to high-quality instruments?

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