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    Testing ST-70 Iron

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    Downhome Upstate

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

    Testing ST-70 Iron

    Post by Downhome Upstate on 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

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: Testing ST-70 Iron

    Post by MontanaWay on 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
    Admin

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

    Re: Testing ST-70 Iron

    Post by Bob Latino on 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

    Downhome Upstate

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

    Re: Testing ST-70 Iron

    Post by Downhome Upstate on 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

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Testing ST-70 Iron

    Post by tubes4hifi on 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
    Admin

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

    Re: Testing ST-70 Iron

    Post by Bob Latino on 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

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

    Re: Testing ST-70 Iron

    Post by Downhome Upstate on 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

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

    14-lead PA 060 power transformer w/white stenciled lettering on end bell: "P10 62"

    Post by Downhome Upstate on 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

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Testing ST-70 Iron

    Post by tubes4hifi on 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

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

    Thanks, Roy.

    Post by Downhome Upstate on 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.

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