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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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DavidR
ImFritz
corndog71
Bob Latino
peterh
cci1492
Mr C
11 posters

    ST-120 no left channel - fixed !

    Mr C
    Mr C


    Posts : 99
    Join date : 2019-07-11
    Age : 69
    Location : Visalia CA.

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by Mr C 17th December 2019, 12:26 pm

    Bob Latino wrote:My question to Mr. C is > "Did you at any time ever use any driver tube OTHER THAN a 12AU7/12BH7/5963/5814/6189 on this board ?" If you did, it is possible that you have taken out the LM334 IC current regulator for that channel. This will cause a dead channel. You can't use 12AT7, 12AX7, 12AV7, 12AY7, 5751 tubes on the VTA driver board. Although these tubes are plug compatible with a 12AU7 type tube, they have other characteristics that are incompatible with the board. I have had a couple of customers plug the wrong driver tube into the board and as a consequence found themselves with a dead channel. Both times we found that the LM334 IC for than channel was now bad. I can't say for sure that this is your problem but it could be ?

    Bob

    Hi Bob. The only tubes I have are the RCA 5963/12AU7s supplied with the kit. Other tubes I've had in the amp are KT-120s, A Weber WS1-T and a Mullard GZ37/CV378.
    Mr C
    Mr C


    Posts : 99
    Join date : 2019-07-11
    Age : 69
    Location : Visalia CA.

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by Mr C 17th December 2019, 12:29 pm

    anbitet66 wrote:Mr. C,

    A quick way to provide the (possibly) missing bias voltage to your 12AU7 tubes would be to use a 2.2k to 2.7k resistor and bridge from the V+ terminal to chassis ground (the V- terminal) across the LM334 IC to provide an approximation of the bias current.  This is just a quick test to see if sound is restored.  If you can try this, you'll either have your answer, or need to look elsewhere.  If Bob is right, this would be the quick way to prove it.

    Tony

    If I do have one it'll be 1/4 watt. Will That do?
    Bob Latino
    Bob Latino
    Admin


    Posts : 3280
    Join date : 2008-11-26
    Location : Massachusetts

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by Bob Latino 17th December 2019, 1:22 pm

    Mr C wrote:
    Hi Bob.  The only tubes I have are the RCA 5963/12AU7s supplied with the kit.  Other tubes I've had in the amp are KT-120s, A Weber WS1-T and a Mullard GZ37/CV378.  

    Try this with an INSULATED plastic pen > Play the amp in the upside down position on a doubled up towel on a table at a lower volume. Tap on every connection on the left (dead) channel with your insulated pen. This also includes all the left channel driver board connections. If you hear a "crackle" noise and/or the left channel comes back on even for a split second, you may have found a bad solder connection. Shut the amp off and resolder that connection. Try the amp again after the connection has been resoldered.

    Bob
    Mr C
    Mr C


    Posts : 99
    Join date : 2019-07-11
    Age : 69
    Location : Visalia CA.

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by Mr C 17th December 2019, 2:20 pm

    Bob Latino wrote:
    Mr C wrote:
    Hi Bob.  The only tubes I have are the RCA 5963/12AU7s supplied with the kit.  Other tubes I've had in the amp are KT-120s, A Weber WS1-T and a Mullard GZ37/CV378.  

    Try this with an INSULATED plastic pen > Play the amp in the upside down position on a doubled up towel on a table at a lower volume. Tap on every connection on the left (dead) channel with your insulated pen. This also includes all the left channel driver board connections. If you hear a "crackle" noise and/or the left channel comes back on even for a split second, you may have found a bad solder connection. Shut the amp off and resolder that connection. Try the amp again after the connection has been resoldered.

    Bob

    I'll go over it this afternoon after I run some errands. I did some of that early on with a chop stick but didn't find anything.
    Mr C
    Mr C


    Posts : 99
    Join date : 2019-07-11
    Age : 69
    Location : Visalia CA.

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by Mr C 17th December 2019, 3:22 pm

    I found a 2.2k 1/4 watt resistor. Red, red, red? Right? I haven't tried tacking it in yet. I want to verify if that's okay to do. It seems like its pointing at the LM334 as the offending component. Mouser has TI LM334Z/NOPD TO92-3 in stock. Is this the correct part?
    anbitet66
    anbitet66


    Posts : 143
    Join date : 2009-12-23
    Location : Valley Stream, NY

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by anbitet66 17th December 2019, 5:07 pm

    Mr. C,

    The quarter watt resistor will work.  Just try it for a couple of seconds.  Attach it first, then power up the amp as normal and see if you hear sound from both channels.  The part from mouser sounds correct but I'll let Bob chime in on that.

    Tony
    cci1492
    cci1492


    Posts : 331
    Join date : 2016-05-09
    Age : 64
    Location : NJ

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by cci1492 17th December 2019, 5:10 pm

    Mr C wrote:I found a 2.2k 1/4 watt resistor.  Red, red, red?  Right?  I haven't tried tacking it in yet.  I want to verify if that's okay to do.  It seems like its pointing at the LM334 as the offending component.  Mouser has TI LM334Z/NOPD  TO92-3 in stock.  Is this the correct part?  

    http://resistor.cherryjourney.pt/
    anbitet66
    anbitet66


    Posts : 143
    Join date : 2009-12-23
    Location : Valley Stream, NY

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by anbitet66 17th December 2019, 5:18 pm

    Cool app.  Red red red is 2.2k or 2200 ohms.  Give it a try, the power dissipated is much lower that the resistor's rating unless there is a short in the tube. Then it becomes a fuse.
    Bob Latino
    Bob Latino
    Admin


    Posts : 3280
    Join date : 2008-11-26
    Location : Massachusetts

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by Bob Latino 17th December 2019, 5:20 pm

    Below is the Mouser Electronica link of the Texas Instruments LM334Z IC as used in all the VTA amp driver circuits.

    https://www.mouser.com/ProductDetail/Texas-Instruments/LM334Z-NOPB?qs=%2Fha2pyFadujjVi5WsIAqfoCVcpUDzIHkk04mZSbxMnyNwbNAPRkosQ%3D%3D

    Bob
    Mr C
    Mr C


    Posts : 99
    Join date : 2019-07-11
    Age : 69
    Location : Visalia CA.

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by Mr C 17th December 2019, 7:00 pm

    I tacked in the 2.2k resistor. The left channel came alive! I placed an order for the LM334, I ordered several actually. You guys are awesome and a big thanks to the collective knowledge of this forum. A special thanks to MechEngVic, rjpjnk, anbitet66 and Bob. And thanks for your patients with me. I hope I'm not premature with this but it looks like it's been diagnosed and the fix is on its way. We may never know why the LM334 failed.
    anbitet66
    anbitet66


    Posts : 143
    Join date : 2009-12-23
    Location : Valley Stream, NY

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by anbitet66 17th December 2019, 11:01 pm

    That is great news!  Happy to help.  Now I have to explain to the wife why the goofy smile and that my head is swollen again... Cool
    MechEngVic
    MechEngVic


    Posts : 107
    Join date : 2019-01-16

    ST-120 no left channel - fixed ! - Page 2 Empty Re: ST-120 no left channel - fixed !

    Post by MechEngVic 18th December 2019, 12:28 pm

    I just completed work on my amp, upgrading several resistors, it is such a relief when it fires up and all is well, so I know how you're feeling right now having conquered your amp. If the bias circuit on the ST120 is wired anything like my series ii, then your previous issue with the bias circuit on the other channel may have damaged that voltage regulator. In order to help deter future damage to the bias circuit, tubes, coupling caps, and especially that bias resistor, you might look into screen stability resistors. http://tronola.com/html/maximize_tube_life.html
    Mr C
    Mr C


    Posts : 99
    Join date : 2019-07-11
    Age : 69
    Location : Visalia CA.

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    Post by Mr C 19th December 2019, 5:06 pm

    The LM334 arrived today.  Took just a minute to solder it in place.  And...  OMG!!!!  The ST-120 sounds beautiful once again!  What a lovely amp.  I agree, it was such a relief when it fired up.  So much better then that ugly sickly feeling I got when the channel failed.  One thing, I rather enjoyed chasing down the problem and fixing it with your assistance.  I learned a little bit.  It was fun.  Thank you once again!

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