The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Help! I shorted my ST-35, one channel dead

    Share

    Raul
    Guest

    Help! I shorted my ST-35, one channel dead

    Post by Raul on Mon Apr 04, 2011 4:23 am

    Yesterday I took off from the shelf the nice Dynaco ST-35 that I bought several months ago from ebay to give it a run. It was restored from the seller that replaced the two printed circuit boards and installed also an Audio Sound Lab cap board to replace the old can cap. The amp played very good, even though he had put Tesla tubes and 715P (not 716P) caps on the tube boards. Then I decided to replace at least the 6BQ5's with some better quality tubes that I had at hand. It sounded well, better than before, so I decided to let the new tubes in. And now I did my dumb action, I tried to put again the cage on the amp without turning it off. To make a long story short, I don't know where the cage touched when I tried to put it into place (probably something on a board) but I saw a spark, heard a pop and a channel died. One speaker continued to play music while the other didn't. All the tubes still lighted. The fuse didn't blow. Then I checked the voltages on the 4 EL84 and they were good. I wasn't able to read the voltages on the ECC832 by checking the numbered ponts under the printed circuit board (I don't know if I did this in the right way).
    The first thing I feared was to have blown OPT? So I checked the impedance and they were still good (less than 200 ohm on the primary side and less than 2 ohm on the secondaries) and were about the same on both OPTs so these seems save.
    What other should I check to try to understand what happened? I am a newbie when it comes to electronics troubleshooting so I would need a step-by-step guidance.
    Any help would be very appreciated.
    Thank you very very much.
    P.S.Please don't insult me for my stupid action, I already did it myself for acting like a moron

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Help! I shorted my ST-35, one channel dead

    Post by Bob Latino on Mon Apr 04, 2011 8:11 am

    1. First try swapping tubes from one channel to the other to see if the problem moves to the other channel. If it does, you just blew a tube. Just because you measured the proper voltages on the EL84's doesn't mean the tube is still good. Try swapping the 12DW7 driver tubes first. Next try swapping the two EL84 output tubes from one channel to the other.

    2. The problem is not on the main power supply because both the power transformer and the quad cap are common to both channels.

    3. The output transformer on that channel is more than likely OK if you have continuity on both the primary and secondary sides.

    4. With the amp OFF > Check the resistances across each resistor on one board and then check THE SAME resistor on the other board. If you come across a major difference, you may have found your problem.

    5. With the amp OFF > If you have a capacitor meter check each capacitor on one board and compare it to the same capacitor on the other board. If you come across a major difference, you may have found your problem.

    9 pin socket pin outs as viewed from the bottom of the driver board are below.



    Bob

    Raul
    Guest

    Re: Help! I shorted my ST-35, one channel dead

    Post by Raul on Mon Apr 04, 2011 12:39 pm

    Thanks Bob,
    just swapped the tubes and by swapping the ECC832 the problem moves to the other side, so it's probably that tube that's gone (while the EL84s don't change anything). What's odd to me (but maybe it isn't) is that with my emission tube tester both ECC832 still seems equal and good, with no shorts and about the same values.
    I already checked the resistors and the values are the same on both boards so it's probably only the tube that is gone bad with the short (maybe the cage touched one of the upper points of the printed circuit that comes from one of the tube's pins?)

    Sponsored content

    Re: Help! I shorted my ST-35, one channel dead

    Post by Sponsored content Today at 6:20 pm


      Current date/time is Sun Dec 04, 2016 6:20 pm