Teardown and Repair of an Agilent E3634A Power Supply


In this episode Shahriar investigates the cause of failure of an E3634A DC Power Supply. The video features a step by step troubleshooting and teardown of the damaged power supply. Various schematics and block diagram of the power supply are presented. Will he be able to fix it?



  1. adri says:

    hi Shahriar and all,
    a few weeks ago I bought a non working E3634A and found the problem is very similar to the one shown in Shahriar’s video, i.e. a failing 80C51BH, (also part nr is the same: 34401-88804)
    I discovered it as the 5V regulator was so heavily loaded that could not supply the -12,4V rail so after replacing one by one all the component using that rail , I found the problem was (the one I hoped was not): the mask ROM 80C51
    Having read the same part nr is also used on the 34401A multimeter, and hoping to stumble into the right one I have already bought, to no avail, a couple of damaged 34401A display panels, (in one case it was completely different NEC processor, in the other case, the processor was right but the Agilent part nr was only “close”)
    I soldered it in place anyway and I could see for the first time, the power supply came alive, giving me the feeling the power supply, with the right processor would work
    Question: does anyone have the right part nr (34401-88804) for sale ? Or does anyone knows what 34401A display panel version does use that part nr ? Thanks

  2. Alessandro says:

    Hi Shahriar,

    I’m desperate, I bought an E3631A with the display turned off, but I repaired it,
    but those who have tried before me, broke the encoder and the solution
    of -16,4V -11,4V as the microcontroller is powered, the two lines towards the microcontroller
    went to the ground, which has two ports blown so I’m looking the parts, but I don’t find
    P / N 34401-88804, ( it seems a part of the multimeter, but it is the power supply )
    or all I need the firmware, because the microcontroller is a N87C51 to 12 MHz.
    What advice can you give me? I contacted the keysight Italy, without success.
    Thanks for any reply

  3. Hi Shahriar,

    I hope you don’t mind me asking, I have recently bought a Agilent E3634A which I need to repair but I can not seem to find a schematic diagram. Is this something you would be willing to share? You will see that I also do an EE blog of sorts, I will probably would like to do a teardown and repair too. Any help much appreciated and great blog by the way


  4. Congratulations to your success. I am sitting here with the same unit but a different problem…

    Where did you find the schematics? I only seem to find service manuals for the E3634A with the schematics removed…


    • Never mind, I finally found a service manual with the schematics included. The damage in my unit was more severe, however. Both power MOSFETs are burnt (short between all three legs!) and the 50mohm current shunt has let out the gray smoke, which drives all electronics.

      These are only the visible and easily accessible damages – probably more parts in the floating part of the power supply will not have survived the major catastrophy which did this damage!


      • Shahriar says:

        That is terrible. Will you attempt a repair?

        Can you please send me the service manual with the schematics? I would appreciate it.

        • I will try it – it seems to be a nice piece of equipment. I already ordered the power MOSFETs, but I don’t think it will be easy to remove them from the board, because they are mounted on this gigantic heat sink. By the way, I don’t think they were first soldered and then screwed onto the heat sink behind the capacitors. Rather I believe that the complete heat sink assembly with the transistors alread attached was soldered into the board. Yes, the heat sink itself is soldered…

          I haven’t found an accessible source for the Kelvin-connected 50mohm 0.5% shunt yet. For my own purposes, however, I suppose a 50mohm resistor with an external Kelvin connection to the lead wires will suffice.


  5. vic says:

    Thanks for a nice video.

    Another nice thing on the way to hot air rework station are those gas driven soldering pens you find in hobby stores. Usually the tip can be removed exposing the flame. A bit of a blunt tool but together with some aluminum foil screening it can be used to remove larger components.

  6. n2o says:

    More of these awesome repair videos! And please do post the failures also so we mortals won’t feel that bad 😀

  7. Gerd says:

    The moment I saw your pliers I thought “be careful with the pads”…

    Here is how I desolder SOT23 (and small 2 pinnners like Rs and Cs): I use a well tip (sometimes called miniwave) completely filled with solder, place it from above onto the component and wipe it away. The molten tin in the tip touches all pins at once and melts the solder there.

    I have a JBC soldering station. The C245-931 tip works for 0603 and 0805, the C245-938 for 1206, 1210 and SOT23.

  8. cJones says:

    Very interesting. You didn’t mention anything about discharging the big caps. Were you just carful around them or did you short them during disassembly?

  9. jer says:

    Hi Shahriar,

    Great video! Unfortunately, my problem is usually just getting hold of this sort of stuff. Unfortunately, most of this equipment just gets thrown in the bin when it isn’t working (or at least as far as I can tell). Any tips for the younger players still building up their lab on where to find this stuff ?

    • Shahriar says:

      For broken stuff, start with eBay. Unfortunately, there will be an investment to start. If you save up about $2000 you can buy a LOT of stuff, new! You can’t build a whole lab overnight. It will take some time. But if you are serious and passionate about it, it will be worth it! :)

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