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Parts replacements.

Discussion in 'Electronic Repair' started by [email protected], Apr 12, 2007.

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    Maybe I am wrong here, but when we're talking three and four terminal
    devices, I don't see the problems some face. I have tried diligently
    to get my co-workers to get the hell out of the ECG book and compare
    things for themselves.

    The appilacation should give you the needed specs. If this is an RF
    power transistor, which it was in the last post I read, you can go on
    a datasheet search and find what you need usually.

    One of the things people in the field need to realize though, is the
    basic difference between a switching transistor vs. one that is meant
    to operate in it's linear range.

    If we are taling an RF transistor here, they almost always operate in
    class C, although this is not true of small signal RF transistors.

    As people ask for mundane things, like outputs for an audio amp and
    stuff like that, that is all easy. Personally the first thing I do is
    to find out if I can get the originals. THEN instead of looking in an
    ECG book, I look at the circuit.

    For example, I got this Mac amplifier, I know it uses darlingtons, not
    from them telling me, just from the cicruit that drives it. It looked
    wimpy, and the thing had a decent damping factor, s those are
    darlingtons. Sansui used them as well, their SJ series of transistors
    were darlington.

    But we weren't talking about that.

    Let me roll out a newer experience. Reloze that I work for an ISC and
    info is sometimezs lacking.

    A newer Sony came in, I looked around, had never seen this model
    before. Had a shorted HVOT.

    Now I am figuring the 2SC5150 I was going to put in it would run long
    enough for me to cop a waveform. And if it didn't stay bad I would
    reconnect the flyback output.

    Well, when I saw the zener diode across the source and gate I knew. I
    never thought MOSFETS would get up to the voltages required for HOTs
    and HVOTs. but the proof is in the lack of a driver transformer, but
    that is not conclusive. What is conclusive is the totem pole SMD
    transiastors that run the gate drive, so I knew I could not throw my
    favorite 2SC5150 in there, and we had nothing like an FET that would
    handle it.

    We found out subsequently that the FET cost as much as the flyback,
    and this was the HVOT, not the HOT.

    One acceptable way to go. Get two FETs and one flyback, if the FET
    deos not blow with the new fly, it was the fly. If the new FET blows
    again disconnect everything from the fly except the feedback loop. If
    the waveform stays bad, disconnect the feedback loop and watch for the
    good wavweform without the feedback (whatever it is) disconnected.

    So, back to the OP, you have to know the frequency range, in what band
    does this operate ? Oops, that is another thead.

    You need to know a few things, what voltage is it fed and meant to
    control. how much current and at what frequency ? Also you must
    consider the drive circuit of course.

    When I have all that info, it is not hard to select a replacement.

    Now when you get to obsolete types that need to replced with modern
    equivalents, you will most likely have to deal with lower
    intetrelectrode capacitance and higher speed devices. If tolerances
    were built into the circuit design you might be off scot free.

    All you have to do then is choose the best device with the same case
    style and of course the type.

    In the specsheets you will find the description of your basic bipolar,
    if so, when it says the original is like "meta diffused" you should
    use a replacement that is "meta diffused". Some processes are designed
    to make sweitching transistors and some are used to make linear
    transistors.

    Same with MOSFETs, GEMFETS, all of it. It needs the base or gate drive
    that it needs, it is specialised.

    Talking ICs ? yes, you need a datasheet, but three terminal device I
    really don't have a problem.

    My latest problem is finding those 1200-1500 volt HOTs for direct
    views in the TO220 package. I want to know what the best one is, and
    buy a bunch of them. I'll just put up a sign "here they are".

    I don't do alot of direct view sets anymore, but the company has my
    100+% when I am there. I am used when I can get like 10 parts that can
    fix alot of sets.

    There is one thing I haven't been able to figure out yet, what is so
    special about the old Toshibe outputs, the 2SD2553 ? I have never
    found an adequate sub for them, even for temporary testing
    procedures.

    And another thing, watch those lot munbers. I have replaced 2SC5148s
    with 5150s, and had them fail in warranty.The 5150 is the same family
    of transitors as the 5148, and has the exact same specs except for
    more current. Cost us alot of money, and you couldn't even trust the
    ones from B&D. Changed again they last, so that is proven.

    I actually put 2SD1887s in PTK195s and they have not failed. They run
    even cooler as well.

    So what I am saying in the end here, you do whatever you need to do.

    Post the usage of this device you wish to replace, is it a final out
    on an amp of any kind ? Well what are the main filters rated at ? If
    RF. what frequency are we talking here ? Megs, Gigs what ?

    You zero in on these things, and who knows where you start, but you
    need to zero in on the right solution. That is why some of us get the
    big bucks.

    JURB
     
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