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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 products

    Noisy ST-70 transformer


    Noisy ST-70 transformer Empty Noisy ST-70 transformer

    Post by BaxterT on Tue Dec 09, 2008 6:07 pm

    My old Dynaco ST-70 has developed a mechanical buzz, vibration, hum ? that comes from the power middle (power?) transformer of the three black transformers. Now there is a little hum coming out of the speakers but this is coming from the middle transformer. When you put your ear within like two feet of the amp you can hear it. Does this mean that this power transformer is bad? Or is it going to die soon ? If so, what do you recommend I should do to remedy this situation ? BaxterT
    Bob Latino
    Bob Latino

    Posts : 2976
    Join date : 2008-11-26
    Location : Massachusetts

    Noisy ST-70 transformer Empty RE: Noisy ST-70 transformer

    Post by Bob Latino on Tue Dec 09, 2008 7:40 pm

    Many of the old ST-70's are approaching 50 years old. The power transformer was one of the weak areas of this amp. It would barely put out enough current to run the amp and after 40+ years many of them simply run out of gas and need to be replaced.

    Try these two things first. Take off the bottom cover and retighten the four 8X32 nuts that hold the transformer to the chassis. If you are lucky you will find them not so tight and maybe just snugging them up a little can solve the issue. The transformer *could* be a little loose on the chassis. Next try replacing the GZ34 rectifier tube. Sometimes a bad or weak rectifier which is not supplying the proper B+ voltage can cause an improper "draw" on the power transformer and the power transformer will actually vibrate in protest. I had an old original ST-70 years ago that this DID happen to. I replaced the rectifier and the vibration went away.

    If neither of there two things work you should replace the power transformer. It's not that hard a job. Just unsolder all the power transformer wires making note of the wire's color (or colors) and where the wire's terminal point is. Go to - They sell an excellent replacement Dynaco ST-70 transformer. It has a higher stack lamination and will put out much more current than the original PA-060 power transformer. The wire colors are exactly the same as on the original transformer. Download the pictorial that I have listed at the link below. The wires are colored in for you so you can't really make a mistake.

    Let us know how you made out ..


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