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    Dynaco ST-70 060Power Transformer question

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    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
    Location : Upstate New York

    Dynaco ST-70 060Power Transformer question

    Post by cc43fan on Wed Mar 14, 2012 8:25 am

    I'm using an orginal power transformer, and I'm having a hard time matching the leads correctly. It seems age has discolored the white, red, etc. Only the green leads seem the most obvious. Can I connect the 117VAC black leads to a variac, bring the power up slowly and measure the red, white, gr, br, etc voltages? Will doing this harm the transformer in any way?
    Is there a better method to verify the leads before I begin wiring up?

    Thanks,
    Chuck

    anbitet66

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

    Re: Dynaco ST-70 060Power Transformer question

    Post by anbitet66 on Wed Mar 14, 2012 10:11 am

    That would be your best bet. I'd first use an ohmeter to find each winding (filaments, plate, bias, and their respective center taps, etc...) then a voltmeter to confirm each winding for what it is. No harm will result as long as you don't use your fingers to hold the wires to your meter probes affraid , or cause any shorts while checking. A power transformer doesn't need a load to operate safely.

    Tony

    Bob Latino
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    Re: Dynaco ST-70 060Power Transformer question

    Post by Bob Latino on Wed Mar 14, 2012 10:13 am

    Hi Chuck,

    Yes - You can do it that way as you suggest and it won't harm the transformer in any way .. Just be very careful that none of the leads are touching any other leads. Turn the transformer upside down on a table and apply your 117 - 120 VAC to the two BLACK leads.

    Two GREEN wires - 6 to 6.8 VAC as measured across the two leads
    Two BROWN wires - 6 to 6.8 VAC as measured across the two leads
    Two WHITE wires - 4.8 to 5.5 VAC as measured across the two leads
    Two RED wires - 700 to 750 VAC as measured across the two leads
    GREEN/YELLOW to RED/YELLOW 3 to 4 VAC
    BROWN/YELLOW to RED/YELLOW 3 to 4 VAC
    RED/BLACK to RED/YELLOW 50 to 60 VAC

    Depending on your incoming line voltage and the fact that there would be no load on the power transformer, some of the voltages could be slightly out of range. If one reading is WAY OFF then that could indicate a problem with the PT.

    Bob

    tubes4hifi
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    Re: Dynaco ST-70 060Power Transformer question

    Post by tubes4hifi on Wed Mar 14, 2012 11:00 am

    I'm a pretty cautious guy, and I would do that but with only 50% or about 60vac on the input.
    Besides, not many meters will measure more than 600vac.
    Also, somewhere on this forum Bob had posted info on the approximate meter readings for the various taps of a transformer, although sometimes it's difficult to measure less than 3-4 ohms on a winding.

    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
    Location : Upstate New York

    Re: Dynaco ST-70 060Power Transformer question

    Post by cc43fan on Wed Mar 14, 2012 6:02 pm

    Bob Latino wrote:Hi Chuck,

    Yes - You can do it that way as you suggest and it won't harm the transformer in any way .. Just be very careful that none of the leads are touching any other leads. Turn the transformer upside down on a table and apply your 117 - 120 VAC to the two BLACK leads.

    Two GREEN wires - 6 to 6.8 VAC as measured across the two leads
    Two BROWN wires - 6 to 6.8 VAC as measured across the two leads
    Two WHITE wires - 4.8 to 5.5 VAC as measured across the two leads
    Two RED wires - 700 to 750 VAC as measured across the two leads
    GREEN/YELLOW to RED/YELLOW 3 to 4 VAC
    BROWN/YELLOW to RED/YELLOW 3 to 4 VAC
    RED/BLACK to RED/YELLOW 50 to 60 VAC

    Depending on your incoming line voltage and the fact that there would be no load on the power transformer, some of the voltages could be slightly out of range. If one reading is WAY OFF then that could indicate a problem with the PT.

    Bob

    Hi bob,
    Thanks for the info, it worked like a charm. I also went with "tubesforhifi's" suggestion and only went to 60VAC and then doubled my DVM's readings.

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