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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    ST-120 LM334 burnout

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    grosen


    Posts : 20
    Join date : 2020-05-10

    ST-120 LM334 burnout Empty ST-120 LM334 burnout

    Post by grosen Wed Dec 15, 2021 7:38 am

    Hi all,

    I noticed yesterday that I've got low output on one channel of my ST-120.  This has happened twice before — not always in the same channel — and both times replacing the LM334 regulator did the trick.  I've ordered a new part, and I'm sure that's the problem this time.  It's an easy enough fix, so I don't mind the occasional replacement.  But I do wonder if I should be looking for the underlying cause.  The unit does got hot, and it may be that the problem is triggered by long sessions where the amp is on all day and gets hotter than usual.  But it sits on a top shelf in free air, so maybe that's not the issue.  Any ideas?  

    — Gideon
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    DavidT1


    Posts : 3
    Join date : 2016-02-17

    ST-120 LM334 burnout Empty Re: ST-120 LM334 burnout

    Post by DavidT1 Wed Dec 15, 2021 10:46 am

    Hello Gideon,

    I relate to your problem, and admire your dedication to maintenance - well worth it! Here are some thoughts, from easy to hard.


    1) Hot air rises, since it is less dense than cooler air. As proof, some people use ceiling fans to distribute hot air near the ceiling.

    You can locate a small quiet fan near the amp to help reduce this heating effect, assuming you cannot move the amp.

    2) Install the LM334 under the PCB to shield the device. Use taller feet to raise the chassis and thus subtlety improve air flow.

    3) Install a small TO-92 heat-sink on the LM334 using thermal grease. (Both available at Mouser and/or Digikey). This will help the LM334 dissipate more heat.

    HTH, David

    grosen likes this post

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    grosen


    Posts : 20
    Join date : 2020-05-10

    ST-120 LM334 burnout Empty Re: ST-120 LM334 burnout

    Post by grosen Thu Dec 16, 2021 7:42 pm

    Thanks, David. Excellent ideas. Will try them all.
    Gideon
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    DavidT1


    Posts : 3
    Join date : 2016-02-17

    ST-120 LM334 burnout Empty Re: ST-120 LM334 burnout

    Post by DavidT1 Thu Dec 16, 2021 8:11 pm

    Gideon, you are welcome.

    David

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