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    2x original Dynaco ST-70's with VTA board with continued issues after service

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    Sigjunk

    Posts : 2
    Join date : 2020-03-17

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    Post by Sigjunk on Fri Mar 20, 2020 7:12 pm

    Hi All,

    I have 2 original Dynaco ST-70's.

    1. I bought as a good working item along with a full Dynaco setup as my first real tube stereo.  It already had the VTA kit on it, so is really the only Dynaco I know.

    2. I bought a 2nd-hand stock ST-70 and I challenged myself to do the VTA mod on it.  I can follow directions and solder, but am truly a novice when it comes to circuitry.  Bob helped me out with a few outstanding questions and was super supportive--as it seems he always is.

    After the upgrade on #2, I hooked these up to my system using them to power 2 sets of speakers--which I swapped out a bunch of different times until I found a system I liked.  Over time (a matter of months), i noticed that #2 was starting to exhibit a loud 60 cycle hum.  It wasn't consistent at first, but after a while made it unusable.  At the exact same time, #1 started to have sporadic white / pink noise out of the left channel and a little quieter noise out of the right.  Upon closer listen, I could also hear a 60-cycle hum in addition to that.  The noise had gotten very loud and also made it unusable.  

    At this point, I needed help with troubleshooting.  I have normally brought my gear to Deltronics locally (Suburbs location).  They have always been very nice, but I find that I bring my gear back to them multiple times under warranty as the initial fix rarely was 100%.  Vintage gear is finicky, so I get it.  I realize that I want an expert to review these.  I had noticed that tube4hifi offered repair service if I covered the shipping at a very reasonable bench rate.  Although I knew that after shipping two of these items both ways + repair would likely mean that these items are no longer worth what I have into them, but I want to keep them and I want them to be perfect--because when they are functioning, I love the sound.

    I talked with Roy and sent them over to him to troubleshoot > fix.  He was super communicative and helpful.  He noticed some obvious issues:
    Most tubes need to be replaced (not surprising as I hadn't done this yet on either), some cold solder points and I had messed up part of the wiring on #2.  He fixed them up in a week and shipped them back.  

    While I was waiting, I decided to bite the bullet and buy 2 new sets of matching tubes just to have everything pristine when they got back. (Mullard EL34, Tung-Sol GZ34 / 5AR4).  I installed them, biased and connected to my rig.  

    Turned them on and instantly noticed that the hum and the noise was there on both items.  Literally, nothing had changed!  I sat on it as I was about to explode.  After all the effort and $700 in repairs and tubes, I was listening to the same damaged amps.  I replaced the speaker wire, removed the preamp - no change.  I swapped out the power conditioner, brought them to a different room and a different set of speakers, no change.  No input is connected at all, so it definitely is an issue with the amps or power.  I am thinking less and less that the house's power or other environmental things are leading to the issue as they both have the same, yet different issues.  And, I don't notice anything like this on my other gear.

    I contacted Roy and he was also surprised--he actually recommended that I come here to look for help.  The noise is loud and instantly obvious and he said that they were both 100% silent in his shop before they returned them to me.  I trust that he is the expert and that all was tested before the return.  I just have no idea what to do now.

    Pic of Oscilloscope (iPhone) and vid of white noise below

    Any ideas for obvious troubleshooting? What can you recommend?

    I appreciate any expertise you can provide.  (like I said, I am a novice w/ rudimentary skills + testing gear)



    2x original Dynaco ST-70's with VTA board with continued issues after service St-70_10
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    Hops

    Posts : 23
    Join date : 2019-09-21
    Location : South Detroit, Michigan

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    Post by Hops on Fri Mar 20, 2020 8:10 pm

    I bought some overpriced shorting RCA plugs for the INPUTS of my VTA-120. Generally useless, but nice for when I want to ensure that there is NO noise on the inputs of my VTA-120. If I am curious about a noise, I put on the shorting plugs and crank the amp up. Usually I am greeted with silence.
    Bob Latino
    Bob Latino
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    Post by Bob Latino on Sat Mar 21, 2020 9:13 am

    What Hops said <> When you test for noise in a Dynaco type amp, you have to make sure that the input jacks are not OPEN. If they are open, they act like an antenna and can pick up noise from the power transformer. Disconnect the interconnects that go from the preamp to the amp at the preamp.. Use the flat blade of a screwdriver to short between the center pin and the outer shield on both interconnects that were just removed from your preamp. What you now hear from the speakers is the residual noise level of the amp. The amp should be almost silent like this. As Hops said > You can pick up "shorting plugs" on Ebay relatively cheap to do the same thing. Another thing you can do is use the shorting plugs on the UNUSED RCA inputs on the back of your preamp. Doing this can sometimes reduce slightly any noise pick up of your preamp. Check the link below ..

    RCA shorting plugs on Ebay

    Bob
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    Sigjunk

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    Join date : 2020-03-17

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    Post by Sigjunk on Sat Mar 21, 2020 12:34 pm

    Thanks for the ideas. Had some shorting plugs around, but unfortunately (or fortunately) no change in the signal. 2x original Dynaco ST-70's with VTA board with continued issues after service 6377dd10
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    Dale Stevens

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    Post by Dale Stevens on Sat Mar 21, 2020 6:24 pm

    What are "shorting plugs on unused RCA inputs on the pre" ?? Dale
    Bob Latino
    Bob Latino
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    Post by Bob Latino on Sat Mar 21, 2020 6:28 pm

    Dale Stevens wrote:What are "shorting plugs on unused RCA inputs on the pre" ??   Dale

    You can use shorting plugs on unused inputs on your preamp. Sometimes this can lower the noise level of the preamp especially if it is an older vintage preamp.

    Bob
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    audiobill

    Posts : 420
    Join date : 2014-03-13
    Location : Albany, NY

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    Post by audiobill on Sun Mar 22, 2020 6:18 am

    Sounds like it may be the input tubes - try switching them around. The center tube, as a voltage amplifier, is most suspect....

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