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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-70 Strange Behavior

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    moodyj2000


    Posts : 18
    Join date : 2023-01-18

    ST-70 Strange Behavior Empty ST-70 Strange Behavior

    Post by moodyj2000 Wed Apr 24, 2024 1:04 pm

    I had a nice untouched 70 show up on my bench. It has been stored a long time so needed work. I replaced the PCB board, can capacitor, bias circuit, and purchased all new tubes. Left channel is dead silent but right channel has a hum. Put a scope on it at the outputs and measured about 120 Hz so I went to checking grounds and solder joints.

    I resoldered most of the joints and put a 1000 Hz sine wave on it from a signal generator. Outputs connected to speakers (with me and ear plugs). I have dummy loads but just wanted to hear if anything crackled. I started clamping on signal points inside the cabinet with my scope and noticed when I touched the case itself with my hand the hum dimmed. It didn't go away completely but definitely reduced in level. I clamped the scope onto the casing itself and I had 1.2V at fluctuating frequency but mostly around 120 Hz.

    I feel like this is a grounding issue but everything is brought back to a single point on the chasis. Any thoughts?
    Mr Ridesglide
    Mr Ridesglide


    Posts : 13
    Join date : 2024-04-17

    ST-70 Strange Behavior Empty Re: ST-70 Strange Behavior

    Post by Mr Ridesglide Thu Apr 25, 2024 8:22 pm

    well it doesn't sound like a tube issue per se, but you could roll some tubes through it.
    I had this once with a guitar amp and it was indeed a loose pot, thus a ground issue. Your amp doesn't have any pots, but it does have jacks. I'd start on the right side jacks and go inwards perhaps. I'm not expert, but I have de-bugged a few things!

      Current date/time is Wed Dec 04, 2024 9:49 pm