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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


    Sudden Intense Buzzing in St 70 (transformer?)

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    Skantily

    Posts : 6
    Join date : 2020-12-19

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    Post by Skantily on Mon Dec 21, 2020 4:20 pm

    Hi All
    I am a new hifi listener - so please forgive my lack of expertise.. I put together a system with the help of a friend. I’ve been listening to the Dynaco ST70 amp (purchased rebuilt in great condition on eBay with new genalex tubes) with Klipsch heresy speakers and an NAD preamp. All has been great for a month or so And then the Actual amp suddenly started intensely buzzing and rupturing the sound.) The sound sounds like it’s coming out of the transformers, and is more intense when I slightly lift/touch the unit. There is a possibility that I accidentally shorted it since I realized that a speaker filament attached to the C connection might have touched the vacant 4OHM connection next to it. Any ideas of how to remedy this? I have been trying to call/write to places that repair these amps to no avail. I can try to get some help possibly replacing the transformer if it needs replacement. Please help!

    Thanks

    Sanya
    peterh
    peterh

    Posts : 1479
    Join date : 2012-12-25
    Location : gothenburg, sweden

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    Post by peterh on Mon Dec 21, 2020 5:06 pm

    Skantily wrote:Hi All
    I am a new hifi listener - so please forgive my lack of expertise.. I put together a system with the help of a friend. I’ve been listening to the Dynaco ST70 amp (purchased rebuilt in great condition on eBay with new genalex tubes) with Klipsch heresy speakers and an NAD preamp. All has been great for a month or so And then the Actual amp suddenly started intensely buzzing and rupturing the sound.)  The sound sounds like it’s coming out of the transformers, and is more intense when I slightly lift/touch the unit. There is a possibility that I accidentally shorted it since I realized that a speaker filament attached to the C connection might have touched the vacant 4OHM connection next to it. Any ideas of how to remedy this? I have been trying to call/write to places that repair these amps to no avail. I can try to get some help possibly replacing the transformer if it needs replacement. Please help!

    Thanks

    Sanya
    If you say where you live maybe someone can help.

    Can you describe the sound better ? Is it a steady humming independent of music ?
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    Skantily

    Posts : 6
    Join date : 2020-12-19

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    Post by Skantily on Mon Dec 21, 2020 5:09 pm

    I’m currently in upstate New York by Hudson/great barrington mass. The transformer buzzes intensely while connected and disconnected from speakers and preamp. When the buzzing got really bad the sound coming from the speakers came to a kind of distorted abrupt break, at which point I turned it off.
    peterh
    peterh

    Posts : 1479
    Join date : 2012-12-25
    Location : gothenburg, sweden

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    Post by peterh on Mon Dec 21, 2020 7:05 pm

    Skantily wrote:I’m currently in upstate New York by Hudson/great barrington mass. The transformer buzzes intensely while connected and disconnected from speakers and preamp. When the buzzing got really bad the sound coming from the speakers came to a kind of distorted abrupt break, at which point I turned it off.
    You should not disconnect speaker cables, a tube amp without load might be damaged.

    Arn't bob somewhere there ? ( i'm unfamiliar with us locations ) If so you might drop it
    off with him.

    WLT
    WLT

    Posts : 145
    Join date : 2013-07-13
    Location : Rochester NY

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    Post by WLT on Mon Dec 21, 2020 8:27 pm

    It does not sound very good at all. One quick test. Do not turn on the preamp but keep the interconnect cables connected. Connect the amp to the speakers. Turn on the amp just long enough to determine if the noise is still there. Turn off if it is. If it is you need to get this amp to someone for further help. If no noise let us know.

    There used to be a few hifi shops in the Albany area that had techs that could work on your amp. I do not know of any now. I am in Rochester NY but that is a good 4 ½ -5 hour drive from where you are. I would help you but the trip seems unreasonable if you can get help closer to where you are.
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    Skantily

    Posts : 6
    Join date : 2020-12-19

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    Post by Skantily on Mon Dec 21, 2020 8:54 pm

    Thank you! Will try that... do you guys have any idea of what might be wrong?
    WLT
    WLT

    Posts : 145
    Join date : 2013-07-13
    Location : Rochester NY

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    Post by WLT on Tue Dec 22, 2020 11:53 am

    Unknown but it does not sound good. If the there is a major short it should blow the fuse. You do have the correct size fuse in there - please check. If you have a replacement fuse that is very large it may not blow on a dead short.

    Assuming the fuse is correct and not blown. It could be a transformer problem that is partially shorted but not enough to blow the fuse. Remote but maybe. Or something else that may sound like the transformer but is not.

    Do you have a DMM and know how to use it inside one of these amps? We could walk you thru some simple tests on the transformer but you have to know the amp wiring some. No testing when it is energized with high voltage but you do have to know how to read the schematic and find the corresponding parts inside. Let us know.
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    Skantily

    Posts : 6
    Join date : 2020-12-19

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    Post by Skantily on Tue Dec 22, 2020 4:23 pm

    Hi guys thanks so much for your help. I found a Dynaco person nearby with good feedback on eBay: Rudy129.

    Im just scared to mess it up even further and want to get in into the right hands.
    S

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    solderblob
    solderblob

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    Age : 75
    Location : Placerville, California

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    Post by solderblob on Wed Dec 23, 2020 12:56 pm

    Great that you found somebody.  Before you or he go tearing into things, try cleaning the tube sockets with Caig Deoxit.  And make sure the pin contacts are nice and tight.  I had a similar incident with my Mk IIIs and that fixed the problem.

    dave

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