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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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Bob Latino
buschfsu
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    is my power xformer dead (ST-70)

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    • Post n°1

    is my power xformer dead (ST-70) Empty is my power xformer dead (ST-70)

    Post by buschfsu Thu Aug 23, 2018 11:37 pm

    picked up a inexpensiveish ST-70 (no tubes).  output iron tests good.  power iron, well...

    so my current limiter 250w light bulb in series with the hot glows immediately

    there is 1.9ohms acorss the primary

    am i about to be 150 dollars poorer due to needed new power iron?
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    Bob Latino
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    • Post n°2

    is my power xformer dead (ST-70) Empty Re: is my power xformer dead (ST-70)

    Post by Bob Latino Fri Aug 24, 2018 7:27 am

    buschfsu wrote:picked up a inexpensiveish ST-70 (no tubes).  output iron tests good.  power iron, well...

    so my current limiter 250w light bulb in series with the hot glows immediately

    there is 1.9ohms acorss the primary

    am i about to be 150 dollars poorer due to needed new power iron?

    Turn the amp ON with no tubes in there and see if you can measure the following ..

    1. Across the two RED wired > About 720 VAC
    2. Across the two WHITE wires > About 5 VAC
    3. Across the two GREEN wires > About 6.3 VAC
    4. Across the two BROWN wires > About 6.3 VAC
    5. RED/BLACK wire to chassis ground > About 55 VAC
    6. GREEN/YELLOW wire to chassis ground > About 3.15 VAC
    7. BROWN/YELLOW wire to chassis ground > About 3.15 VAC

    If you DO get (approximately) these voltages, then most likely the power transformer is OK. If you don't get these AC voltages, post up a photo of the amp's inside wiring on the forum here so we can take a look at the wiring.

    Bob
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    Peter W.
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    • Post n°3

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    Post by Peter W. Fri Aug 24, 2018 7:38 am

    In supplementing Bob's suggestions:

    a) 1.9 ohms across the primary is not 0 ohms. Generally, I look for "not Zero" on the primary side and move on from there.
    b) At full load, these beasts draw up to 190 watts. Without the secondary loads, I would guess that it might draw as much as 60-100 watts. That is more than enough to 'glow' a 250 watt lamp.

    Do the tests Bob suggests, and please let us know.
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    j beede
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    • Post n°4

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    Post by j beede Fri Aug 24, 2018 10:30 am

    A 250W "dim bulb"... that's large. I usually use a 25W or 40W incandescent with a Stereo 70. I didn't get a sense of how bright the 250W lamp lights up from your description, but if "glows immediately" means the lamp is lit as if it were being supplied by the mains, then you have significant current flowing. I re-read your posting and it is not clear to me whether you powered on with or without tubes in place. If your amplifier will light a 250W dim bulb with no tubes in place my guess is that--before long--you would be able tell where the current is going, either visually or via olfaction.
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    Peter W.
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    Post by Peter W. Fri Aug 24, 2018 11:13 am

    j beede wrote:A 250W "dim bulb"... that's large. I usually use a 25W or 40W incandescent with a Stereo 70. I didn't get a sense of how bright the 250W lamp lights up from your description, but if "glows immediately" means the lamp is lit as if it were being supplied by the mains, then you have significant current flowing. I re-read your posting and it is not clear to me whether you powered on with or without tubes in place. If your amplifier will light a 250W dim bulb with no tubes in place my guess is that--before long--you would be able tell where the current is going, either visually or via olfaction.

    250 watts is massive. And pretty much anything more than a glow (as described) would be a warning to me.
    A 25-watt lamp would light fully, even at no-load, however.

    I use a metered iso-variac that gives me ample warning of overload/over draw - and a tool that should be on every bench. There are many variations from many manufacturers, but the one I use is:

    http://www.byan-roper.org/steve/steve-at-play/antique-electronics-and-2/heathkit-ip-5220-variable.html

    With thanks to Bryan Roper.
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    WLT
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    Post by WLT Fri Aug 24, 2018 11:37 am

    I assume you tested the 117 winding and called that 1.9 ohms reading as the primary. Not the red/red leads as primary

    One test that should be done prior to turning it on. Then follow testing per Bob's recommendation. The most likely winding to fail is the secondary high voltage winding. Without plugging it in measure the resistance from red lead to ground and compare that with the other red lead to ground. Both should be about the same value (+/- 4 ohms) and be a significant number like maybe 60 or 80 ohms. If they are significantly different from each other you will probably have a problem with that transformer. Even if it tests well it still could be a problem but I have had good luck with that test for over 50 years. I have a collection of dead power transformers and none will pass this simple test.
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    Post by buschfsu Fri Aug 24, 2018 5:34 pm

    Ton of great info thanks!

    Primary im testing is 117v yes. 1.9R
    I got rid of the current limiter bulb and the 3A fuses pops after a second. I will look for shorts to ground and test B+ Secondary ohms as recommended

    None of my tests have been w any tubes

    Few more readings and ill also post a wiring pic

    Thanks!!
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    Post by buschfsu Fri Aug 24, 2018 5:35 pm

    Current bulb idea came from this fellow

    https://youtu.be/wRFRwOnLsZI
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    Post by buschfsu Fri Aug 24, 2018 6:01 pm

    Ok red to gnd =24r for both (attached to rectifier socket)

    Red to red = 6.4r

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    Post by buschfsu Fri Aug 24, 2018 6:09 pm

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    WLT
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    Post by WLT Fri Aug 24, 2018 9:17 pm

    Stop with further energized testing. I pulled the covers off my ST 70 hand have the following info. Red to ground 28 ohms, other red to ground 30.8 ohms. Red to red 58.8 ohms. It works fine

    yours - With red to ground of 24 ohms and the other red to ground of 24 ohm the red to red resistance should be double or 48 ohms. With only 6 ohms red to red you have a shorted winding. Not good!

    I feel you have found the problem and a replacement transformer is in order. Please re-measure a few times to be sure but I think you found the problem.

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    Post by buschfsu Fri Aug 24, 2018 9:30 pm

    Man thanks for checking values of your unit. Thats above and beyond! Very Happy lI have repeated. Same values. Thanks all. Anyone selling used working iron?
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    Peter W.
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    • Post n°13

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    Post by Peter W. Mon Aug 27, 2018 7:53 am

    Ummm........

    There is enough carbon and blackness on both the PT and the choke that I would be seriously concerned with both. It is clear from your tests that the PT is toast - but I think the choke needs attention as well.

    A source for the PT is here: http://triodeelectronics.com/mq060.html or here: https://www.dynakitparts.com/shop/pa060-power-xfmr/

    Choke: https://www.dynakitparts.com/shop/c-354-choke/
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    anbitet66
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    • Post n°14

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    Post by anbitet66 Mon Aug 27, 2018 10:18 am

    Busch,

    My recommendation would  be to use the transformer from dynakit parts.  I have the transformer from triode electronics and unless you reuse the old bottom cover (at least you have one to use) the transformer is supplied without a cover.  I bought mine many years ago and had to also purchase the bottom cover separately.  The one they sell has the holes for the wires in the wrong place.  Yes, annoying since that is not "original" and not seen, but still annoying.  Note, the picture on their website doesn't show the bottom of the transformer.

    The one from dynakit parts comes complete and is less expensive (had to stop myself  from saying cheaper...) to boot.

    Otherwise, either would be a good choice.
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    • Post n°15

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    Post by buschfsu Sun Sep 02, 2018 10:45 pm

    guys

    thanks for all the help.. i did go with dynakit parts and got my power xformer and choke along with the multicap.  i also replaced the rectifier and bias and went with orange drops.  i am really impressed with how it came out.  amp is dead quiet but i have 60cycle coming from the recevier.  cant change plug orientations because turntable and preamp and receiver all use 3 prongs and my new 2 prong in the st70 has fat blade to fuse.  at a loss but happy that the sound is amazing!
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