The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
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


4 posters

    Testing ST-70 Iron

    Downhome Upstate
    Downhome Upstate


    Posts : 37
    Join date : 2014-03-21
    Location : Greeley, PA

    transformer - Testing ST-70 Iron Empty Testing ST-70 Iron

    Post by Downhome Upstate Sun Mar 23, 2014 10:54 am

    Just scavenged the power and output xformers from a ST-70 carcass. I have a variac and a DMM, but no 'scope. How can I test them before I commit them to a new build?

    Thanks, folks.

    Mike
    avatar
    Guest
    Guest


    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Guest Sun Mar 23, 2014 11:35 am

    are these original built transformers? If yes, you may wish to consider using the more robust current versions, as used in the VTA ST70, but it all depends on what you're looking to get out of the ST70.
    Bob Latino
    Bob Latino
    Admin


    Posts : 3260
    Join date : 2008-11-26
    Location : Massachusetts

    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Bob Latino Sun Mar 23, 2014 11:47 am

    1. For the power transformer ... Connect the two BLACK wires to a 120 volt AC power source ..
      Measure the AC voltage across the two RED wires > 700 - 760 VAC
      Measure the AC volatge across the two GREEN wires > 6 - 7.5 volts AC
      Measure the AC voltage across the two BROWN wires > 6 - 7.5 volts AC
      Measure the AC voltage from the RED/BLACK wire to the RED/YELLOW wire > 55 to 60 volts AC
      Measure the AC voltage from the BROWN/YELLOW to the RED/YELLOW - 3 to 4 volts AC
      Measure the AC voltage from the GREEN/YELLOW to the RED/YELLOW - 3 to 4 volts AC

    2. For the OUTPUT transformer ...
       The primary side wires BLUE, BLUE/WHITE, RED, GREEN and GREEN/WHITE should have no continuity to ANY OF THE secondary wires YELLOW, ORANGE, BROWN, BLACK
       On ethe PRIMARY side wires measuring from the RED wire to each of the other 4 wires should give a resistance of 20 ohms to maybe 150 ohms
       On the SECONDARY side measuring
         From BLACK to YELLOW - maybe 1 to 2 ohms
         From BLACK to ORANGE - maybe 1/2 ohm to 1 ohm
         From BLACK to BROWM - maybe .25 ohm to .7 ohm

    Bob


    Last edited by Bob Latino on Sun Mar 23, 2014 2:51 pm; edited 1 time in total

    Sichum likes this post

    Downhome Upstate
    Downhome Upstate


    Posts : 37
    Join date : 2014-03-21
    Location : Greeley, PA

    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Downhome Upstate Sun Mar 23, 2014 12:47 pm

    Bob Latino wrote:1. For the power transformer ... Connect the two BLACK wires to a 120 volt AC power source ..
      Measure the AC voltage across the two RED wires > 700 - 760 VAC
      Measure the AC volatge across the two GREEN wires > 6 - 7.5 volts AC
      Measure the AC voltage across the two BROWN wires > 6 - 7.5 volts AC
      Measure the AC voltage from the RED/BLACK wire to the RED/YELLOW wire > 55 to 60 volts AC
      Measure the AC voltage from the BROWN/YELLOW to the RED/YELLOW - 3 to 4 volts AC
      Measure the AC voltage from the GREEN/YELLOW to the RED/YELLOW - 3 to 4 volts AC

    2. For the OUTPUT transformer ...
       The primary side wires BLUE, BLUE/WHITE, RED, GREEN and GREEN/WHITE should have no continuity to ANY OF THE secondary wires YELLOW, ORANGE, BROWN, BLACK
       On ethe PRIMARY side wires measuring from the RED wire to each of the other 4 wires should give a resistance of 20 ohms to maybe 150 ohms
       On the SECONDARY side measuring
         From BLACK to YELLOW - maybe 1 to 2 ohms
         From BLACK to ORANGE - maybe 1/2 ohm to 1 ohm
         From BLACK to BROWM - maybe .25 ohm to .7 ohm

    Bob

    Great! Thanks, Bob.

    I've got the tubes4hifi 9-pin board kit with the cap upgrade and EH 12AU7's, a Tung-Sol rectifier and a brace of Tung Sol EL-34B's waiting to go into a new, shiny Dynakitparts chassis, etc. After I put this thing together and it runs, God willin' and the crick don't rise, I've got my eye on one of your ST-120's.

    Thanks again for your help, sir.

    Best regards,

    Mike
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1840
    Join date : 2008-11-30

    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Roy Mottram Sun Mar 23, 2014 2:26 pm

    correction to Bob's voltages - if you measure from RED to RED you are going to get 700-750vac (make sure you have a DMM that can handle 1000vac !!!)
    you should have half that much from the RED/YELLOW to each RED lead (350-375vac)
    Bob Latino
    Bob Latino
    Admin


    Posts : 3260
    Join date : 2008-11-26
    Location : Massachusetts

    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Bob Latino Sun Mar 23, 2014 2:55 pm

    Roy is correct and I have changed the voltages in my post above ... The voltages I had originally posted would be correct if you measured the voltage from each red wire to chassis ground when the transformer was in an amp ..

    Bob
    Downhome Upstate
    Downhome Upstate


    Posts : 37
    Join date : 2014-03-21
    Location : Greeley, PA

    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Downhome Upstate Sun Mar 23, 2014 3:30 pm

    tubes4hifi wrote:correction to Bob's voltages - if you measure from RED to RED you are going to get 700-750vac (make sure you have a DMM that can handle 1000vac !!!)
    you should have half that much from the RED/YELLOW to each RED lead (350-375vac)

    Thanks, Roy. Darn, my meter tops out at 700 VAC. I'll make sure not to measure all the way across the secondary . . .
    Downhome Upstate
    Downhome Upstate


    Posts : 37
    Join date : 2014-03-21
    Location : Greeley, PA

    transformer - Testing ST-70 Iron Empty 14-lead PA 060 power transformer w/white stenciled lettering on end bell: "P10 62"

    Post by Downhome Upstate Mon Mar 24, 2014 4:43 pm

    Is this a 120/240 universal transformer? The lead colors seem different than the stock unit. Anybody know what 'P10 62' refers to? (Is 62 the year of manufacture?)

    Thanks,

    Mike
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1840
    Join date : 2008-11-30

    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Roy Mottram Mon Mar 24, 2014 5:07 pm

    unless you have an unusual ST70 with a 240v transformer, they are all 115-120vac input and are marked PA060
    Downhome Upstate
    Downhome Upstate


    Posts : 37
    Join date : 2014-03-21
    Location : Greeley, PA

    transformer - Testing ST-70 Iron Empty Thanks, Roy.

    Post by Downhome Upstate Mon Mar 24, 2014 9:33 pm

    I thought I counted a pair of extra leads. Wrong-o. Time to try and figure out which leads are which, given the fading & discoloration.
    avatar
    Sichum


    Posts : 5
    Join date : 2021-01-16

    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Sichum Sun Dec 18, 2022 1:58 am

    Thank you so much for this guide to test Dynaco power transformer. My Dynaco distort a little bit after a few minutes of listen. I checked the power transformer and everything were either on the spot or very close except these 2

    Measure the AC voltage from the BROWN/YELLOW to the RED/YELLOW - 3 to 4 volts AC
    Measure the AC voltage from the GREEN/YELLOW to the RED/YELLOW - 3 to 4 volts AC

    I get 0.34 volts AC on both; does that mean that wiring is shot? Can I or should I supply 3v AC from an external source? Any help/ advice is much appreciated. Thank you
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1840
    Join date : 2008-11-30

    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Roy Mottram Sun Dec 18, 2022 2:35 pm

    Sichum
    are you sure that wasn't 3.4vac instead of 0.34vac ?
    The brown yellow is the center tap of two brown wires which should read about 6.5vac between them.
    The green yellow is the center tap of two green wires which should read abot 6.5vac between them.
    The brown-yellow and green-yellow connect thru a 0.02uF cap to ground, so maybe that dual cap is bad if you get a bad reading there.

    Sichum likes this post

    avatar
    Sichum


    Posts : 5
    Join date : 2021-01-16

    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Sichum Mon Dec 19, 2022 1:21 pm

    tubes4hifi wrote:Sichum
    are you sure that wasn't 3.4vac instead of 0.34vac ?
    The brown yellow is the center tap of two brown wires which should read about 6.5vac between them.
    The green yellow is the center tap of two green wires which should read about 6.5vac between them.
    The brown-yellow and green-yellow connect thru a 0.02uF cap to ground, so maybe that dual cap is bad if you get a bad reading there.

    I'm pretty sure it was 0.34vac, I tried with 3 different volt meters. I will unsolder both of them from the 0.02uF cap and try it again when i get home. Thank you

    Here's the result of the re-read with the brown/yellow and green/yellow not connected to the 0.02uF cap
    - across the two RED wires > 752 VAC
    - across the two GREEN wires > 6.66 volts AC
    - across the two BROWN wires > 6.71 volts AC
    - from the RED/BLACK wire to the RED/YELLOW wire > 53.5 volts AC
    - from the BROWN/YELLOW to the RED/YELLOW - 0.66 volts AC
    - from the GREEN/YELLOW to the RED/YELLOW - 0.71 volts AC

    Sponsored content


    transformer - Testing ST-70 Iron Empty Re: Testing ST-70 Iron

    Post by Sponsored content


      Current date/time is Thu Mar 28, 2024 8:02 pm