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How to test a Yamaha VFET

  1. Apr 17, 2016 #1
    It looks like there is no replacement for the VFETs that were made by Yamaha or Sony. So, if a amp uses these output transistors and they have failed then the amp is not repairable.

    The amp in question is a Yamaha B-2. One side is working without issue. The other side is not working. The output transistors may be OK but because I can not replace them I want to be sure first.

    How can VFETs be tested both in circuit and out of the circuit? I can not find any data sheet for the Yamaha VFESs.


  2. jcsd
  3. Apr 17, 2016 #2
    Sounds like a straight forward case of 'compare the good-side voltages and waveforms with the bad-side ones.'

    Now to try answering your question, there are a bunch of transistor testers on the market; even some DVMs can do that. Or if you're old school use a curve tracer.
  4. Apr 18, 2016 #3
    Hi Tom,

    I guess my question is more, is there anything special about testing a VFET.


  5. Apr 19, 2016 #4
  6. Apr 19, 2016 #5
    Thanks Tom,

    I had read the link you posted. It looks like these transistors are not very cheap to make so Sony and Yamaha quit making them. I did see a data sheet for the transistors Sony made but never found one for the Yamaha transistors. I assumed that testing them would be like any other transistor but I am trying to be really sure.
    Without these transistors working the Yamaha B-2 amp is useless.


  7. Apr 19, 2016 #6
    Well... not necessarily useless but it's gonna sound different than it used to.

    I suppose you could get it working then unload it cheap with a big Black Box warning about it not being original.

    Anyhow, you still have to find out if those VFETs in the dead channel are any good.
  8. Apr 19, 2016 #7
    Agreed. The resistance measurements I made on the two sides are close. My best guess is all the VFETs are working. Those measurements were made in circuit. Problem is, I have no current way to know what any of my measurements mean, for either side.

    As I understand the design, the idea was to have very well matched pairs. They were designed to emulate a triode. The Sony 2SK82 VFET had curves that look like triodes or close at any rate. Somewhere I found a data sheet for the Sony VFET and a testing method. As of yet, I have not found anything on the Yamaha VFET.
    I have no idea if the Sony VFET and the Yamaha VFET are alike.

    upload_2016-4-19_23-34-36.png upload_2016-4-19_23-35-56.png
    The amp is constructed in such a way to make troubleshooting a bit hard to do. The PCBs are vertical and very close to other parts so I have to take each board out just to even look at it. Everything I read says it is very easy to destroy these transistors. I am proceeding carefully and in fact may send this project to someone with better skill sets than me.



    Actually selling the amp like it is may be a good idea as I can double my investment. There are a couple of other people who have half a working amp. Fixing it would bring greater rewards but comes with a good bit of risk.
    Last edited: Apr 19, 2016
  9. Apr 20, 2016 #8


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    Google "power mosfet amplifier design" and look for a design that matches your schematic. Then check what output transistors they use.
  10. Apr 20, 2016 #9

    jim hardy

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    Last edited: Apr 20, 2016
  11. Apr 20, 2016 #10
    Hi Jim,
    Funny you should mention Hexfets.
    I did some consulting work for International Rectifier back some years ago. They were having issues with their computer network which I redesigned with the help from engineers from INS. I installed routers for them in several countries in Europe and managed the telco issues. IR is a mfg of Hexfets. As for me, I know less than nothing about Hexfets.

    I have really jumped off in the deep end of the pool with the three solid state amps I am currently poking around in. The McIntosh MC 2120 is fixed and working well. I think I will be able to get to the end of the Carver...still looking for the parts to arrive. I expect to have all the parts for the Carver by next week. The Yamaha B-2 may well be more complex of an issue than I can deal with. All of this in some way is distracting me from working on the tube amps I really like to work on. The up side is being forced to learn a lot of new things about electronics in general.

    Thanks to everyone for all the help and all the education you guys are providing me with.


  12. Apr 20, 2016 #11

    jim hardy

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    I leapt straight from tubes to tightly controlled solid state with lots of negative feedback - operational if you will.

    Presumably there was a transitional period where circuit design accommodated early transistors without so much feedback ? You're running across amps from that period ?

    That's why i suggest trying out a modern device in an old amp...
    Find some Guitar-Zan audiophile writer to declare it
    and you might sell retrofit kits...

    This is a funny page.... check out their cliche generator -

    There's the payback !!!


    old jim
    Last edited: Apr 20, 2016
  13. Apr 20, 2016 #12
    LOL..it is a funny page!!

    Sort of "how to say nothing in 10,000 words or more" LOL

    As for retrofit kits...just replace all those black fiddly things with warm tubes that cast a nice light across the room late at night...

    Oh, and for those new to British English "fiddly" is defined as "complicated or detailed and awkward to do or use" LOL



    NOTE: Being from Texas I have no idea where I picked up so much British English
  14. Apr 20, 2016 #13


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    I might be wrong but I thought V and Hex FETs were essentially just an attempt to fit a bigger FET in a smaller area by making it partially three dimensional. Anyone know of other reasons/differences to standard FETs?
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