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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


    Dynakit Stereo 70 Stopped Audio Out

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    tndave123

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    Join date : 2020-09-22

    Dynakit Stereo 70 Stopped Audio Out Empty Dynakit Stereo 70 Stopped Audio Out

    Post by tndave123 on Tue Sep 22, 2020 2:18 pm

    Hey Forum! Hope you're all well!

    I have a Dynakit Stereo 70 Amp and Pre. I was listening to a record and the sound stopped. I was out of the room when it happened so not didn't see anything or hear anything funny, just came in and the record was going and there was no sound. Power is working on both. Tubes appear to be working on both. I tried an alternate (line-in) input and that's not working either.

    I've gotten ok at tube replacement and bias work and i'm pretty good as a troubleshooter in general but my experience with analogue audio repair is pretty much non-existant in terms of replacing other parts.

    Do you have any suggestions in terms of how to narrow down the problem further or steps I may have overlooked?

    Thanks so much for you help. This system is crucial to my life! Smile

    David
    peterh
    peterh

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    Location : gothenburg, sweden

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    Post by peterh on Tue Sep 22, 2020 3:20 pm

    tndave123 wrote:Hey Forum! Hope you're all well!

    I have a Dynakit Stereo 70 Amp and Pre. I was listening to a record and the sound stopped. I was out of the room when it happened so not didn't see anything or hear anything funny, just came in and the record was going and there was no sound. Power is working on both. Tubes appear to be working on both. I tried an alternate (line-in) input and that's not working either.

    I've gotten ok at tube replacement and bias work and i'm pretty good as a troubleshooter in general but my experience with analogue audio repair is pretty much non-existant in terms of replacing other parts.

    Do you have any suggestions in terms of how to narrow down the problem further or steps I may have overlooked?

    Thanks so much for you help. This system is crucial to my life! Smile

    David

    You mention "AMP and PRE".
    You could pinpoint if the fault is at the pre ( what brand is it ?) or the ST70 by
    loosing the input cables to the st70. A large hum will sound if the ST70 is ok . If nothing
    is heard then the st70 is at fault. Try the mains fuse first !
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    tndave123

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    Post by tndave123 on Tue Sep 22, 2020 5:33 pm

    Hi Peter.

    Thanks so much for the help.

    The preamp / receiver are both Dynakit. My father put it all together back in 62. I tried loosening the inputs on the amp and the pre and didn't get a hum.

    Dynakit Stereo 70 Stopped Audio Out Img_4710
    Dynakit Stereo 70 Stopped Audio Out Img_4711
    Dynakit Stereo 70 Stopped Audio Out Img_4712

    I removed the amp fuse and it looks good.

    David
    Bob Latino
    Bob Latino
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    Post by Bob Latino on Tue Sep 22, 2020 7:30 pm

    Check the bias on your Dynakit ST-70. If you get no bias voltage on both channels, then probably the rectifier tube is bad. Note - Just because a vacuum tube "lights up" is no indication that the tube is actually "good".

    Bob
    peterh
    peterh

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    Post by peterh on Wed Sep 23, 2020 2:05 am

    By loosening inputs i mean to slightly pull the front located inputs on the st70, just so much that the
    circumference ground connection is lost.

    If this wont cause large hum then the ST70 is dead.
    Get the dynaco manual ( or you already have it ) it has a fault finding section that will be of value.
    A good guess is the rectifier tube, but still a guess.
    You will need a decent voltmeter and VERY CAREFUL fingers when measuring the voltages in the amp.
    You should not be alone doing this as voltages are lethal inside. If in doubt do take it to
    a repair shop.
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    tndave123

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    Post by tndave123 on Wed Sep 23, 2020 8:35 am

    Thank you so much everyone! I do have a person here in Brooklyn who does some basic repair. I'll try and check the rectifier tube / bias as you mentioned Bob. Probably will take it to someone if it gets beyond that.
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    tndave123

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    Post by tndave123 on Wed Sep 23, 2020 11:44 pm

    Hi Bob.

    Thanks for your help. I only get bias voltage from one channel. when i switch the small tubes by the bias checking area, the voltage moves with the tube. Do you think that might mean it's one of the small tubes (I think the 7199s?). I did replace the rectifier tube once a year or two ago and i did have to move the amp in the last few month (up a 4 story walkup which was no picnic).

    Also, others have mentioned that my amp looks bad and I'm willing to have it professionally refurbed, just not sure where to take it. I'm in Brooklyn.

    Much appreciated.
    David
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    tndave123

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    Post by tndave123 on Wed Sep 23, 2020 11:45 pm

    Also, one of the 7199 flashes whenever i turn on the stereo and I thought it was weird, but it always worked and there was no hum, so I thought maybe it was ok.
    PeterCapo
    PeterCapo

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    Post by PeterCapo on Thu Sep 24, 2020 10:52 am

    Here's some reading material you might consider studying...

    Original manual https://www.dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf

    Recent lab study with recommendations http://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf

    Preeminent source for parts https://www.dynakitparts.com/

    Service facility https://www.dynakitparts.com/links/

    If you were to thoroughly study the written materials at the first two links, you could potentially repair or restore it yourself.  Depends how comfortable you are with the idea, I suppose.
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    tndave123

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    Post by tndave123 on Thu Sep 24, 2020 12:51 pm

    Thank you Peter. I'll take a look. I'm not terribly bad with putting things together with instructions and things like that, however I have developed a tendency to let professionals do work when i really want something done right the first time. In this case, because I rely on listening to records so much, I'd probably prefer to have someone do the work. With something like this I'll almost always make a mistake and I don't really plan on becoming an audio repair person, as neat as it seems. I'm afraid i'll shock myself to death actually. Smile So, tubes, bias, fuse I feel cool with. Soldering, caps, things like that feel dangerous.
    PeterCapo
    PeterCapo

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    Post by PeterCapo on Thu Sep 24, 2020 1:02 pm

    There are different ideas floating around about how to deal with an amplifier like yours.  If/when you send it to someone, my suggestion would be to make no assumptions about exactly what will be done to it.  I'd have a detailed, proactive discussion with whomever is going to do the work and agree on what he will do with it.  I'd also print out and pass along to him the two documents linked in Post n°9, as they are excellent references...

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