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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-120 Problems

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    RogerRef


    Posts : 2
    Join date : 2021-10-24

    ST-120 Problems Empty ST-120 Problems

    Post by RogerRef Sun Oct 24, 2021 2:20 pm

    Sorry for long story...

    Built amp about a year ago.  Initial startup good.  Listened to it for few hours over several days.  With a CD playing, I heard a loud crackle through speakers and then pop and went silent.  Then the burnt electic smell.  Opened it up and the left channel switch and both resistors were smoked.  Bob asked me to check line voltage which was pretty high.  Suggested a variac and sent me replacement DPDT switch and resistors.  Replaced parts and checked everyhting else.  Found one bad solder joint on speaker terminals but that was it.  Reflowed a couple others and tried to go through startup again.  Could not adjust bias voltage which was at 0.  Original failure must have fried rectifier.  Replaced it and went though startup again.  Was fine for a minute but before I could even adjust first resistor, popped inside and blew fuse of variac.  Open it up and the green and red wires on 1 & 2 of the left channel switch again show they were overheated.  Collateral damage to the yellow wire that goes to the board.  Looks like some arching issues.  Switch was in linear mode (towards front ).  the ST120 fuse never blew.

    Bob referred it to Joe in Tucson who asked me to check pin 8 of recitfier to ground which was in 10-13 MOhm range.  

    This is where I am at.  

    I believe it was wired right as it worked for several hours.  This is my first build but was not novice to soldering.  Worked on C-130 auto pilot amps and compass systems and did fairshare of soldering then but that was a while ago.  But still understand a good solder joint.

    Am going to look at every connection for evidence of arching.  Something has to be shorted to gound somewhere.  Any common items to look at first? Quad cap?

    Roger


    Last edited by RogerRef on Sun Oct 24, 2021 2:25 pm; edited 1 time in total (Reason for editing : spell check)
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    RogerRef


    Posts : 2
    Join date : 2021-10-24

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    Post by RogerRef Sun Oct 24, 2021 9:48 pm

    i just read about yellow sheer mod and that seems like a good first step. Will try finding those parts. Small town and one electroics part store. Sure they have the diodes but worry if I need to replace ST-120 parts like quad cap.
    Bob Latino
    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    ST-120 Problems Empty Re: ST-120 Problems

    Post by Bob Latino Mon Oct 25, 2021 8:04 am

    Roger,

    Yes - If the amp worked OK for a few hours, then the amp is wired properly. That leaves tubes or a bad solder connection as the most likely culprit. Since you lost a triode/pentode ultralinear switch, you may have a bad solder connection somewhere in the amp's high voltage system. This includes the solder connections on the quad cap, SCM and ESL capacitors. Also check out pins 3 and 4 on each output tube. A bad solder connection is one that looks OK on the outside but is not making the proper contact on the inside of the connection. Bad solder connections show up later because after many heat up and cool down (expansion and then contraction) a connection can go "partial". This can happen after a few days or after months of use. Note > Contrary to what you said above, the switch facing towards the front places the amp in triode mode. A "shotgun" approach would be to resolder all high voltage connections and bring your tubes somewhere to be tested. Most guitar stores have a tube tester. They will charge you a fee. As I mentioned in an Email to you > You can always send the amp to one of our authorized VTA amp techs. They will fix the problem for a nominal fee. You will also have to pay for the return shipping costs.

    Bob

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