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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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    VTA ST120 - Lost right channel, help needed

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    willythree


    Posts : 4
    Join date : 2020-01-14

    VTA ST120 - Lost right channel, help needed Empty VTA ST120 - Lost right channel, help needed

    Post by willythree Fri Sep 23, 2022 11:41 pm

    I recently attempted to change the power tubes in my 120, with apparently at least one bad 6550 on the front right, which redplated. After shutting down and reinstalling four known good KT120s (and then swapping those for four new 6550s from a reputable supplier), there is no sound from the right channel. Left channel plays fine. Switching the positions of the three driver tubes or swapping them out with three others has no effect. I have the yellow sheet mod with IN4007 diodes installed, but can't seem to get a consistent ohm reading on them.

    I made the following measurements following Bob's diagnostic checklist:

    All four 10 ohm resistors at the bias points measure 10 ohms.

    B+ on driver board: 412 volts DC (slightly high)

    GZ33/5AR4s (three different rectifiers measure similarly)
    Pin 4: 218 volts AC (very low)
    Pin 6: 220 volts AC (very low)
    Pin 2: 517 volts DC
    Pin 8: 518 volts DC

    Quad cap
    #1 (square):     515 volts DC
    #2 (half circle): 510 volts DC
    #3 (circle):       510 volts DC
    #4 (triangle):    428 volts DC (a bit high)

    Sovtek 6550s (all four measure similarly)
    Pin 1: .500 volts DC
    Pin 3: 509 volts DC
    Pin 4: 508 volts DC
    Pin 5: -62 volts DC
    Pin 6: -62 volts DC
    Pin 8: .500 volts DC
    Across pins 2 and 7: 6.9 volts AC (slightly high)

    Any advice would be much appreciated!
    Bob Latino
    Bob Latino
    Admin


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    Location : Massachusetts

    VTA ST120 - Lost right channel, help needed Empty Re: VTA ST120 - Lost right channel, help needed

    Post by Bob Latino Sat Sep 24, 2022 8:16 am

    The first step (if all your amp voltages are pretty much normal) in fixing your music system is to eliminate everything else in your music system. Check your source components, preamp, interconnects, speakers and speaker wires as a cause of the problem FIRST.

    1. source components > CD player, streaming service, turntable etc
    2. Interconnects
    3. speaker wire
    4. speakers

    The easiest way to do this is to swap the interconnects or speaker wire from one channel to the other. If you see the problem change to the other channel, you most likely have found the problem component.

    You can also try tapping with an insulated object (plastic pen ?) on all the connections between the components. If you hear a crackle sound or if the sound comes back for even a split second, you may have found the problem ?

    Bob
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    willythree


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    Join date : 2020-01-14

    VTA ST120 - Lost right channel, help needed Empty Re: VTA ST120 - Lost right channel, help needed

    Post by willythree Sat Sep 24, 2022 11:03 am

    Thank you for your quick reply, Bob.

    I tried switching the interconnects from channel to channel. Then I swapped everything out completely with different RCAs, speaker wire, preamp, source component and speakers, and the problem remains the same: no signal output from the 120's right channel. The issue started after I lost control of the right front power tube.

    Bill
    Bob Latino
    Bob Latino
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    VTA ST120 - Lost right channel, help needed Empty Re: VTA ST120 - Lost right channel, help needed

    Post by Bob Latino Sat Sep 24, 2022 12:28 pm

    willythree wrote:Thank you for your quick reply, Bob.

    I tried switching the interconnects from channel to channel. Then I swapped everything out completely with different RCAs, speaker wire, preamp, source component and speakers, and the problem remains the same: no signal output from the 120's right channel. The issue started after I lost control of the right front power tube.

    Bill

    Hi Bill,

    OK - Take the bottom cover off and turn the amp upside down on two layers of a towel. Have everything connected and play some music through the amp. Get the plastic pen and tap on every connection on the right channel including the right channel area on the driver board. If you get a crackling sound or the music comes on your right channel even for a split second, you may have found a bad or cold solder connection.

    Bob
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    rebellovw


    Posts : 73
    Join date : 2022-01-23

    VTA ST120 - Lost right channel, help needed Empty Re: VTA ST120 - Lost right channel, help needed

    Post by rebellovw Sun Sep 25, 2022 12:43 pm

    Perhaps when you removed the tube from the socket something came loose. Clean the socket and make sure the socket pins are fine. Also make sure the tube is going in the slot and plugging in correctly - the tube sockets can allow the tubes to go in wrong.
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    willythree


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    Join date : 2020-01-14

    VTA ST120 - Lost right channel, help needed Empty Re: VTA ST120 - Lost right channel, help needed

    Post by willythree Sun Sep 25, 2022 2:58 pm

    Thank you Bob and rebellovw for your sugestions. I found a loose connection at pin 3 of V7. After resoldering it I got faint sound coming from the right channel for a short time, then back to silent. I haven't been able to find any other loose connections that create static or sound in the right channel.

    Forum reading suggests that replacing the LM334Z IC on the right channel would be the next step?
    Bob Latino
    Bob Latino
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    Posts : 3172
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    VTA ST120 - Lost right channel, help needed Empty Re: VTA ST120 - Lost right channel, help needed

    Post by Bob Latino Sun Sep 25, 2022 3:15 pm

    willythree wrote:Thank you Bob and rebellovw for your sugestions. I found a loose connection at pin 3 of V7. After resoldering it I got faint sound coming from the right channel for a short time, then back to silent. I haven't been able to find any other loose connections that create static or sound in the right channel.

    Forum reading suggests that replacing the LM334Z IC on the right channel would be the next step?

    Yes - If the LM334 IC goes bad then you will get very low volume on that channel. Sometimes a bad driver tube OR the use of an improper driver tube can take out the LM334 on one channel. Only use 12AU7, 12BH7 or the military industrial versions of the 12AU7 (5963, 5814 or 6189) in your amp ! Do NOT use 12AT7 or 12AX7 driver tubes in place of a 12AU7 in a VTA amp. The use of improper driver tubes can take out the LM334. LM334's are inexpensive. You can get three pieces for very little money at the link below.

    LM334 IC's on Ebay

    Side note > Early VTA amps up to the end of 2011 used a different driver circuit that did use 12AT7 driver tubes.

    Bob
    Seamus
    Seamus


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    Join date : 2020-03-17

    VTA ST120 - Lost right channel, help needed Empty Re: VTA ST120 - Lost right channel, help needed

    Post by Seamus Wed Sep 28, 2022 11:44 am

    You can buy genuine TI LM334 @ Digikey for $1.34

    There is so much fake garbage out there it is not worth saving a few cents. Manufacturers like TI & AD will not accept failed parts unless you can provide provenance.

    Pin 3 or 8 on v1 & v3 should both have about 10.5 volts according to the schematics. If they do, the LM334 is OK.

    Check the speaker terminal connections are well soldered.

    Unplug the amp and disconnect the speakers. Remove the power tubes. Ensure there is no voltage across the SCM. Now measure the resistance from B+ to pins 3 & 4 on each power tube and note the values. They should be the very close on each channel. If not, OPT maybe toast.

    Next measure resistance on the output side from GND to 4Ω, 8Ω and NFB. Values should be very close. If not, the OPT is toast.

    If you have a voltmeter and a computer you can generate a steady state signal follow the A/C signal through the amp.
    Online Tone Generator


    USE CLIP LEADS AND KEEP ONE HAND IN YOUR POCKET
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    willythree


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    Join date : 2020-01-14

    VTA ST120 - Lost right channel, help needed Empty Re: VTA ST120 - Lost right channel, help needed

    Post by willythree Wed Sep 28, 2022 1:27 pm

    Thank you Seamus for the link. After checking Mouser and the TI site and finding that both currently have the LM334 on backorder for many months, I bought some on Amazon (which may indeed be fake). But I installed one an hour ago and my 120 is back in business. Thanks again to everyone who chimed in, especially Bob.

    New2Tubez, kFish and rebellovw like this post

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    rebellovw


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    Join date : 2022-01-23

    VTA ST120 - Lost right channel, help needed Empty Re: VTA ST120 - Lost right channel, help needed

    Post by rebellovw Wed Sep 28, 2022 11:47 pm

    That is great news!

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