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Extending the life of a Tek 2465?

Discussion in 'Electronic Repair' started by DaveC, Nov 21, 2003.

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  1. DaveC

    DaveC Guest

    The U400 hybrid IC in the Tek 24xx 'scope models is infamous for its short
    life.

    Has anyone attempted heat-sinking it (if it is not already) and blowing it
    with a small fan (a la pentium and PowerPC)?

    Just crossed my mind in between random other thoughts...
     
  2. Jim Yanik

    Jim Yanik Guest


    I haven't heard of anyone doing it,but it would seem to lend itself to
    being heatsinked,the metal substrate is exposed on the topside of the
    IC,and there's threaded studs to secure a HS. I don't think there's enough
    clearance for a fan(cabinet-HS clearance)

    It certainly couldn't hurt.
    Although,I suspect it's some problem in the die construction itself.
     
  3. DaveC

    DaveC Guest

    Is U400 accessible w/o removing any PCB's? It sounds like it's on the outward
    side of the main board, just beneath the case skin. Is this true?

    Thanks,
     
  4. Jim Yanik

    Jim Yanik Guest

    Yes,the metal substrate is on the TOP of the IC,exposed,completely
    accessable. (but soldered in,40 pin DIP)
    All one has to do is coat with HS compound,and bolt the HS to the
    studs,IIRC,they're 4-40 thread.I don't think the HS could be more than
    perhaps .375" high,though.(SWAG)
    You don't want it to contact the cabinet,as I'm not sure if the substrate
    is electrically neutral or not.
     
  5. DaveC

    DaveC Guest

    -=-=-=-

    Hey, what gives? I wasn't expecting this:

    http://home.covad.net/~peninsula/U400.jpg

    Isn't this already heat-sink'd? I wasn't expecting all the metal. I don't see
    how a heat sink could help this situation much. It seems pretty cool, as is.

    This is on a 2465DMS, s/n B051xxx. (And yes, it works. FIne.)

    Comments?

    Thanks,
     
  6. mike

    mike Guest

    Well, it's not a 40-pin dip.
    The board sez channel2 preamp.
    Methinks you have the wrong U400.

    But when you find the right u400, please post that pix.
    I don't really want to take mine apart.
    mike

    --
    Bunch of stuff For Sale and Wanted at the link below.
    laptops and parts Test Equipment
    Honda CB-125S
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  7. DaveC

    DaveC Guest

    I have the Tek svc manual. It says there is only one U400. Maybe it's a
    40-pin under that massive sink?

    I think that "channel 2 preamp" is the description for U200. Look in the
    lower left corner of the picture: next to "U400" is the description
    (partially hidden by the cable) "channel switch" which is what U400's
    function is.

    Jim Y. described its location accurately, as did the manual's component
    location page, and that's where I found it. Just not the size/form factor I
    was expecting...
     
  8. Jim Yanik

    Jim Yanik Guest


    That's not the horizontal output IC being discussed,that's the CH2 preamp
    hybrid IC.

    Odd,but most of the hybrids with this type of HS don't even need a HS,they
    don't get warm at all,but the H-output IC doesn't have a HS,yet does get
    warm. I guess it's just a fancy package to clamp the hybrid substrate to
    the PCB socket.
     
  9. Jim Yanik

    Jim Yanik Guest

    IIRC,it was U800 that is the 40 pin DIP horiz output IC.
    (and not a hybrid)
     
  10. John Miles

    John Miles Guest

    My understanding is that the troublesome 246x part is U800, the
    horizontal driver. This is a (28-pin?) DIP with an integrated heat
    spreader on the bottom that's bolted to the board.

    Does U400 cause a lot of problems, too?

    -- jm
     
  11. DaveC

    DaveC Guest

    My mistake :-(

    I'll look at U800, then.

    Thanks,
     
  12. DaveC

    DaveC Guest

    OK, I've done it. I added a heat sink to U800 in my 2465DVS (w/DMM option):

    http://home.covad.net/~peninsula/Before.jpg

    http://home.covad.net/~peninsula/After.jpg

    The IC is on the main board which is located on the under side of the scope,
    easily accessible once the case is removed (by removing 6 screws on the rear
    of the scope).

    This heat sink fit just perfectly. It is a stepped design, probably for a
    TO-220. This allows it to raise up above the neighboring components without
    interference.

    The IC is already bolted to the PCB with nuts on two #4-40 studs. I had to
    notch the ends of the sink to sit under the nuts, but that was trivial. It
    was important to keep below the height of neighboring real estate so the heat
    sink won't contact the case when reinstalled. I used a *light* coat of white
    heat sink compound under the sink.

    Who knows if this will extend the life of this fragile IC, but I consider all
    semiconductor failures heat-related. So to my mind, this will help in some
    incalculable way.

    Enjoy,
     
  13. mike

    mike Guest

    A couple of thoughts.

    What's on the backside of the IC? Do the studs connect to some metal
    structure that can act as a heat sink?

    What was the temperature of the tab before and after the modification?
    You can also stick a probe thru the hole and measure the top of the IC
    before and after just for grins.

    I don't know anything about this particular IC.
    BUT
    In general, when there's a tab, virtually ALL the heat flows out the tab.
    Heat sinking the plastic probably won't do any harm and may do some good
    IFF you don't hinder the heat flow out the tab.

    Can't tell from the picture...did you leave the nut on the tab and add
    another for the heat sink? If you just used one nut, you probably
    REDUCED the heat flow out the tab into whatever is on the backside
    by a LARGE amount.

    A couple of temperature measurements should tell the story. I wouldn't
    run the scope until you measure that tab temp.

    More mechanical details please.
    mike
    --
    Bunch of stuff For Sale and Wanted at the link below.
    laptops and parts Test Equipment
    Honda CB-125S
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  14. DaveC

    DaveC Guest

    The tab seems to run through the middle of the IC:

    http://home.covad.net/~peninsula/Before.jpg

    http://home.covad.net/~peninsula/After.jpg

    On the right end (as viewed in the photo), the tab runs out like a TO-220 and
    is held to the PCB with a stud and nut. I checked the level of the top of
    this nut and found that it was above the level of the plastic IC case, which
    would have not allowed a flush-fitting heat sink.

    On the left end, the plastic is notched (you can see a bit of the notch just
    to the right of the left nut) which exposes a bit of the integral tab (Jim Y.
    calles this the "metal substrate" of the IC). This nut is not in contact with
    the metal, but I presume this nut's purpose is to hold the metal in contact
    with whatever heat sink is below it the chip.

    I replaced the right nut with one that was a bit shallower in hight, so it
    could continue to hold the tab down, but also so the heat sink would clear
    the top of the nut. The original nut incorporated a star lock washer, and the
    replacement didn't (it would have been too tall). I used a *very small*
    amount of Loctite thread locker (blue) on the top of the plain nut (where it
    meets the stud) after installing it, to keep it from loosening.

    I notched the ends of the heat sink where they intersected the #4-40 studs.

    I put a *very small* amount of white heat sink compound on the top of the IC
    and placed the heat sink on top of it, moving it around a bit to get good
    distribution.

    Then added the original nuts and tightened carefully.

    Enjoy,
     
  15. Jim Yanik

    Jim Yanik Guest

    On this particular IC,the metal substrate is on the TOP side of the IC.Just
    right for adding a heat sink! The PCB does not dissipate any heat from the
    IC except for what comes thru the leads.I suspect the studs were intended
    for a HS that someone else thought unnecessary,or an extra expense.

    I would not overtighten those nuts,it might cause IC failure
    itself;mechanical strain.
     
  16. mike

    mike Guest

    I thought I understood until I read this. According to the pictures and
    the previous description, the tab looks like it sits on the board.
    There's room to add a nut who's top sits at the level of the IC top???

    If there's any room between the tab and the board, the mounting stresses
    on the package would be enormous. This assumes the studs go thru the
    board. Just occurred to me that the studs could be pressed into the tab
    and not go thru the board at all???

    If the chip sits on the tab, the optimum setup might be to put a
    threaded spacer on the left end that fits inside the notch, without
    binding the plastic, to get a solid conductive path to the heatsink
    on that end. Contact with the top of the chip should be irrelevant
    if all the heat is coming out the tab. Also might want to grind the
    anodize off the heat sink (keeping it FLAT) for better thermal contact
    with the stud material.

    I'd still like to see the temperature numbers. The question would be,
    "What mode should the system be in for max heat?"
    High sweep rate would put the most stress during sweep, but holdoff
    would reduce the duty factor. So, tests at different sweep speeds
    while activelly triggered might be in order. Just letting it free-run
    at high sweep won't maximize the duty factor. Which brings up another
    issue.

    Back when I was designing pulse generators, I read an article that
    suggested that the primary failure mode of output transistors was
    thermal cycling. Given the thermal time constants in the package,
    the worst case for thermal cycling was in the 10-100Hz. rep rate range.

    So, if you leave the scope sitting untriggered in auto-trigger mode,
    you're running the sweep circuits near maximum thermal cycling stress.
    From now on, I'm gonna turn off auto-triggering on my scopes when I'm
    not using 'em.

    So, DaveC, can you give us some temperature readings in various modes?

    Thanks, mike

    --
    Bunch of stuff For Sale and Wanted at the link below.
    laptops and parts Test Equipment
    Honda CB-125S
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  17. DaveC

    DaveC Guest

    No. I'm not opening it up any more. It was risky to open it to do the
    heatsink, and I'm not going to risk ESD or any other tomfoolery I might do
    accidentally to this important piece of equipment in my lab. I just wanted to
    do *something* to settle in my mind that I've *possibly* extended the life of
    this component. I've done what I can do. Maybe not in your mind, but I'm
    satisfied. I'll let y'all know if anything comes of it.
     
  18. Jim Yanik

    Jim Yanik Guest

    the studs are mounted to the PCB.I don't even know why the engineers felt
    they needed them.You could leave the nuts off and it would make no
    difference.The bottom of the IC plastic package sits right on the PCB,the
    substrate IIRC,doesn't hang past the ends of the IC,there's notches in the
    ends of the IC for the studs. If I remember right.
     
  19. DaveC

    DaveC Guest

    One end of the substrate hangs out like a TO-220 tab. On the other end, it
    does not; instead the plastic is notched for the stud.

    See the photo here:

    http://home.covad.net/~peninsula/Before.jpg
     
  20. Vince

    Vince Guest

    Dave,

    Where did you get this heatsink? I'd like to get one to add to my U800.

    I spoke with a Tek engineer who told me that excessive heat was the
    failure mode for U800 and that heatsinking it is a good idea, so I think
    you're on the right track.

    I know someone else who heatsinked his U800 and hasn't had any problems
    so far. The only caveat he said is the studs are at -5V so one has to be
    sure that the heatsink doesn't touch the case...

    -V-
     
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