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    VTA ST-120 Problems - solved !

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    stewdan

    Posts : 160
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    VTA ST-120 Problems - solved !

    Post by stewdan on Sat Jul 09, 2016 10:03 pm

    Hi -- I recently acquired an older (2008) High Gain Version of the ST-120 that was built as a kit and had/has problems. The quad cap connections were originally "mangled" and the unit was non functional. The Underside was a bit of a rats nest of wiring.

    So, based upon the documentation I put the quad cap stuff back together.  Put all the tubes in (except the Rectifier) and powered it back up with a variac at low voltage and nothing blew up.   At 50% to 60% voltage, the tubes were warm and the filaments lit up. After an hour or so, I powered it down and put the GZ34 in.  I attached an IPOD for music and powered everything up (Variac Voltage was at 80%).  

    Sounded nice for 10-15 minutes, then the left channel dropped in volume and it had a hum.  Powered down and moved the left side tubes to the right side and vice versa.  Powered back up and had nice music again, then after 10-15 minutes had the reduced volume on the left channel and a hum.  I took a pencil and tapped all over the chassis and the tubes and all the mechanical noise of the tapping came thru both speakers.

    With music playing, tapping the left side of the chassis results in a scatchy (not clear) sounding output from the Left channel. Powered down, let the unit cool off and opened up the bottom cover.  I spent the next 2 hours looking everything over and resoldering connections on the VTA Board and tube socket connections, also went over all the grounding connections with my soldering iron.  Powered back up at 80% voltage and the music sounded better then before, but after 10+ minutes the left channel had reduced volume and a hum. Tapping stuff as before resulted in the mechanical  noise coming thru both speakers.

    Powered off, checked all solder points and made sure nothing was touching something it should not be touching and that no solder runs flowed to where they should not be. Did not find anything.

    HELP!!!  Any ideas or suggestions would be greatly appreciated.

    Thanks in advance.

    arledgsc

    Posts : 330
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: VTA ST-120 Problems - solved !

    Post by arledgsc on Mon Jul 11, 2016 10:16 am

    It is difficult to troubleshoot a through a bunch of bad wiring.  Best of luck.  But here are some things to check.  

    Did you swap the small signal tubes as well??  If so you may have a leaky coupling cap in the left channel.  Check all resistors for char, bulging, or cracks - including the 10 ohm cathode resistors on the power tubes and plate resistors as well.  Is bias consistent when the issue is happening and when not?

    To help isolate where the issue is happen pull the center tube.  If no noise look around the center tube circuit area.  If noise exists pull the phase inverter tubes.  If noise still present look at all connections in the power amp area.  Check the tube sockets and re-tension the pins.  Check bias on each tube pull to be on the safe side.

    It is a process of elimination.  Find what works and what does not to zero in on the problem.

    stewdan

    Posts : 160
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    VTA ST-120 NO Problems NOW

    Post by stewdan on Mon Jul 11, 2016 11:51 am

    Hi - Thank you for the response.  

    The other day, when I moved the left side tubes to the right side and vice versa, I also flipped the L/R 12AT7's but still had the HUM and the Reduced Volume on the Left Channel and the mechanical noise from tapping tubes and chassis.

    To me, it sounded like some sort of grounding or non-grounded issue. So, for the last few days, I have been checking and rechecking connections and re-flowing solder all over the place.  All to no avail.  The hum and the reduced volume and the mechanical noise of tapping persisted??

    This morning, after a re-soldering session, when I reconnected the RCA cables on my preamp and the ST-120 to try it again, I noticed that the RCA plugs were rotating on the jacks. So, I took a pair of long nose pliers and squeezed the outer part of the RCA plug that makes contact with the outer part of the RCA jack (and the grounding tab of the jack).  

    When all the plugs no longer rotated around the jacks, I powered up and played music ----- NO HUM and NO REDUCED VOLUME and NO MECHANICAL NOISE FROM TAPPING THINGS!!!

    So,  success!!!!

    Thanks again!!

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA ST-120 Problems - solved !

    Post by Bob Latino on Mon Jul 11, 2016 12:40 pm

    Hi Stew,

    Glad to hear you found the issue with that older VTA ST-120 that you purchased ..

    I realize that you did not build this amp but > The instructions mention to TIGHTEN the nut securely on each input jack and to make sure that the outer shield area of the input jack does not touch the chassis and that the ground tab does not touch the chassis. The ground tab should be grounded ONLY to the ground eyelet on the driver board. If the outer shield of the input jack or the ground tab touches the chassis, then you WILL have a ground loop set up and you will get hum from the amp.

    It is possible that over the years (the last 8 years from 2008 to 2016) that inserting and removing interconnects multiple times loosened the input jack or the ground tab from the backing board setting up a partial contact issue that caused the hum. I have a feeling that the nuts on the back of each input jack were probably not secured tightly when the amp was built ..

    Bob

    Gregg R.

    Posts : 80
    Join date : 2014-02-20
    Age : 71
    Location : Land of 11,842 Lakes

    Re: VTA ST-120 Problems - solved !

    Post by Gregg R. on Tue Jul 12, 2016 2:17 am

    When I first built my VTA ST-70, I had a similar problem. I usually insert the RCA cables and turn them 1/8 turn to the right. This caused the grounding lug to touch the chassis; rotating the plugs to the left solved the issue. I later tightened up the nut and added a small drop of crazy glue to keep the nuts in place.

    Sometimes the simplest things cause the most problems!!

    Dale Stevens

    Posts : 77
    Join date : 2014-07-06
    Age : 67
    Location : Loris, SC

    Re: VTA ST-120 Problems - solved !

    Post by Dale Stevens on Wed Jul 13, 2016 2:59 pm

    Stewdan, Good Tip. I found one channel a tad lose. Should be part of our regular tuneup. Dale

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