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    VTA ST120 Step by Step Diagnostic Checklist?

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    tkemmett

    Posts : 14
    Join date : 2013-10-24

    VTA ST120 Step by Step Diagnostic Checklist?

    Post by tkemmett on Sun Aug 03, 2014 8:52 pm

    Howdy All,
    I have a VTA ST120 that I would like to thoroughly check over to determine if everything is ok. I have had some KT88 tubes that have "red plated" and one outright failure.
    Folks over at AK recommended that I do a detailed review of the amp before looking at the tubes as faulty.
    I am looking for a step by step checklist to measure values in each circuit. I'm pretty sure I saw checklist with target values for assessing the ST120...I just can't seem to find any sign of it. I am new to electronic diagnostic/DIY, so a list a newbie could follow would be best. I am aware of the safety issues surrounding diy but am interested in learning to take care of my system. If anyone can give me a lead or link to something like this I sure would be grateful.
    Boyotk

    Bob Latino
    Admin

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

    Re: VTA ST120 Step by Step Diagnostic Checklist?

    Post by Bob Latino on Sun Aug 03, 2014 9:18 pm

    As a check list for the VTA ST-120 > Turn the amp on with all tubes inserted and do the following voltage checks ...

    Make sure you have your meter set to AC or DC as mentioned below. Ground is any point on the chassis frame. Place the black or negative probe on the chassis and the red or positive probe on the point mentioned. All tubes should be plugged in, inputs shorted or connected to your preamp, speakers connected and no signal should be running through the amplifier. Be careful not to cross two pins with the positive probe !

    Readings very slightly above or very slightly below the range are not normally a sign of a problem. Line voltages vary slightly throughout the country. Differences in some GZ34 rectifiers can cause variations also. If a 5U4 rectifier is used in place of the GZ34 then some voltages (marked with an asterisk) will be near the low end of the range. If a solid state rectifier is used in place of the GZ34 then some voltages (marked with an asterisk) will be at the high end of the voltage range.

              GZ34, solid state rectifier or 5U4 rectifier
               Pin 4 to ground – 400 - 425 volts AC
               Pin 6 to ground – 400 - 425 volts AC
               Pin 2 to ground – 490 – 530 volts DC *
               Pin 8 to ground – 490 – 530 volts DC *

             Any 6550/KT88
                            Pin 1 to ground – approx .550 volts DC (depends on bias setting)
                            Across pins 2 and 7 – 6.0 – 6.8 volts AC
                            Pin 3 to ground – 470 - 520 volts DC *
                            Pin 4 to ground – 470 - 520 volts DC *
                            Pin 5 to ground – minus 50 to minus 62 volts DC
                            Pin 6 to ground – minus 50 to minus 62 volts DC
                            Pin 8 to ground – approx .550 volts DC (depends on bias setting)

             Quad cap – Section # 1 (SQUARE symbol) ---------- 480 – 530 volts DC *
                                 Section # 2 (HALF CIRCLE symbol) --- 470 – 525 volts DC *
                                 Section # 3 (NO or CIRCLE symbol) --- 470 – 525 volts DC *
                                 Section # 4 (TRANGLE symbol) --------- 370 – 410 volts DC *

                 Any 12AU7/12BH7/5963/6189 driver tube – across the two incoming heater wires > 6.0 - 6.8 volts AC
                 “-BIAS” on driver board > 48 – 58 volts AC
                 “B+” on driver board > 370 – 410 volts DC *

    Bob

    tkemmett

    Posts : 14
    Join date : 2013-10-24

    Re: VTA ST120 Step by Step Diagnostic Checklist?

    Post by tkemmett on Sun Aug 03, 2014 9:47 pm

    Thanks for the reply Bob,
    What are the values when using KT 120's?

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: VTA ST120 Step by Step Diagnostic Checklist?

    Post by tubes4hifi on Sun Aug 03, 2014 10:41 pm

    all voltages shown above should be the same with any output tube

    tkemmett

    Posts : 14
    Join date : 2013-10-24

    Re: VTA ST120 Step by Step Diagnostic Checklist?

    Post by tkemmett on Mon Aug 04, 2014 11:25 am

    Hello Again,
    Is the ST 70 II schematic that you have posted used for the ST 120 as well?
    How are the inputs shorted if not connecting to a preamp during this diagnostic check?
    Thanks in advance for the patient novice support.

    corndog71

    Posts : 444
    Join date : 2013-03-19
    Location : It can get windy here

    Re: VTA ST120 Step by Step Diagnostic Checklist?

    Post by corndog71 on Mon Aug 04, 2014 1:36 pm

    tkemmett wrote:Hello Again,
    Is the ST 70 II schematic that you have posted used for the ST 120 as well?
    How are the inputs shorted if not connecting to a preamp during this diagnostic check?
    Thanks in advance for the patient novice support.

    In order to short the inputs you simply need to get a pair of RCA plugs and on each connect a wire from the pin to the shield. Then plug them into the inputs.


    sKiZo

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    Join date : 2013-04-01
    Location : Michigan USA

    Re: VTA ST120 Step by Step Diagnostic Checklist?

    Post by sKiZo on Mon Aug 04, 2014 5:39 pm

    I think what he's looking for is the step by step assembly instructions shipped with the kit. Not sure if there's a PDF available for that.

    There's a red plating issue with the amp, and I suggested going through those one by one and checking things off as you would with a virgin build. Easier to target a potential problem that way then just randomly poking around. Run the list, check each connection for long tails, missing or cold solder, etc. , check it off, and move on. Once that's done, THEN do the voltage checks.

    And yes, the voltages supplied apply to all tube types. Only difference on the KT120s is, first, your amp needs to be able to drive them properly (no problem with the ST120, but a stock Dynaco doesn't have the iron for them), and I find they sound fuller with the bias bumped up to 60mV. Bob tested his up to 75mV with no issues, but also no noticeable improvement at that level.

    tkemmett

    Posts : 14
    Join date : 2013-10-24

    Re: VTA ST120 Step by Step Diagnostic Checklist?

    Post by tkemmett on Mon Aug 04, 2014 11:14 pm

    Using the voltage chart posted, I found all measurements are within range except for the B+ reading of 375vdc (400-440 is normal) and section #4 of the quad cap reading 373vdc (normal is 400-440). For those in the know, how are these reading interpreted when troubleshooting?
    Secondly, how are the driver tube pins numbered... clockwise/counter clockwise? I'm not sure of the driver tube voltage readings as I did not see any numerical labeling on/near the sockets.
    Again, thanks for your replies!

    tkemmett

    Posts : 14
    Join date : 2013-10-24

    Re: VTA ST120 Step by Step Diagnostic Checklist?

    Post by tkemmett on Tue Aug 05, 2014 10:42 am

    I failed to include readings of 6.9vac across pins 2 and 7 in all 4 power tubes (normal 6.0-6.5vac)...
    Thanks for your help.

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: VTA ST120 Step by Step Diagnostic Checklist?

    Post by GP49 on Tue Aug 05, 2014 10:44 am

    Tube pin convention is clockwise as viewed from the bottom, Pin 1 counting up, starting with the first pin clockwise from the gap in tne pins, or from the keyway slot.

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