Teardown and Repair of an HP 8648C Synthesized Signal Generator

In this episode Shahriar attempts to repair an HP 8648C Synthesized Signal Generator from a dumpster dive. The cause of failure is investigated and traced. Alongside the repair efforts, the internal construction and components of the synthesizer are also presented. Various component teardowns are shown and described including the structure of the VCO used in the generator assembly. The functionality of the repaired unit is verified using an Agilent Power Meter and Rigol Spectrum Analyzer. Finally, the overall unit block diagram is presented and the principle of operation is described in detail.



  1. Pierre says:

    My 8648B says 616 AM modulation path failure 🙁
    Can not find A3 board schematic to check.
    Did someone has reverse engineering this side of the A3 board ?


    these blogs are very good. I have a HP 8648 that was working, until i stupidly connected an external antenna and set the rf level to zero db. to see if I could pick up the signal on my new Rigol SA.. a few mins later, heard the fan quit and the display indicating that there is a fault with the power supply.. i am hoping it is nothing more than a repairmans dream of just the fuse on the power supply.. we shall see

  3. Bob Weber says:

    I have an HP 8660C. I am not sure how to align it has anyone done this before?

  4. Marcelo says:

    Hello everybody
    I urgently need help to fix my equipment, I am getting this error on my HP 8648C

    620 COMM failure: A4
    Can anybody help me to fix it, please. as soon as possible.

  5. Mike says:

    Yuck, they went cheap sourcing those power supplies. I have an 8648B that just failed, and it’s the supply too. But in my case, it’s more sad, one side of the primary winding opened up. Not expecting much luck replacing a custom power transformer like that.

  6. ichabod mudd says:

    sell this gen, please.!
    nice vid.

  7. Ermelinda Schauer says:

    Hello! I just want to give a huge thumbs up for the good information you’ve here on this post. I can be coming back to your blog for more soon.

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  9. john senchak says:

    Excellent explanation of the block diagrams. I didn’t understand a few things, but got the overall concept of the circuitry. When the switch mode power supply was first taken out, the first thing I noticed was the ring on the top of board where the capacitor was suppose to be. My first thought was where was the that capacitor and why wasn’t it needed. When the PC board was turned over,I though it was a explosion, but then their would have been parts of the capacitor would have been all over the inside of the unit. What are you using to highlight the PDF diagrams? jls@antihotmail.com

  10. I do accept as true with all of the ideas you’ve introduced to your post. They’re really convincing and will definitely work. Nonetheless, the posts are too quick for starters. May you please lengthen them a bit from subsequent time? Thank you for the post.

  11. […] I found the following Wiha 92190 Master Technicians Bench Top Set, 50 Piece after watching a TheSignalPath video and Shahriar showed a the set he […]

    • Alexey says:

      Thanks a lot to Shahriar for showing the set and to you for the brand and “model” name. Was looking for it at ali and ebay for hours :/

  12. Great presentation; useful and educational for me in a lot of different ways! Thanks, and I’ll be looking forward to that PLL video…

  13. Brian says:

    This is a very good overview for people new to how synthesizers work. You certainly should do a video on phase noise and different kinds of frequency references/oscillators. I think you give very clear explanations but I might be biased as I am familiar with the material.

  14. […] found an HP 8648C Synthesized Signal Generator while he was ‘dumpster diving’ and set out to fix the malfunctioning lab equipment. He posted a 1-hour video on the project, which you can find embedded after the break. The actual […]

  15. Rishi says:


    I am new here.

    I do most of my browsing on the iPad, I cannot watch any of the videos. There would be a setting in YouTube enabling viewing of videos on mobile. Please enable it.

  16. Yi Yao says:

    Its not always true that the same make/model are identical in hardware. Many times, the manufacturer may go through different PCBs and BOMs as the product matures, fixing problems, jumpers, etc. None the less having a working reference is a very useful starting point.

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