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Thread: Power transformer low output

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    Power transformer low output

    Hi all, Iím working on an old Teisco 72R amp. It was having issues, namely the reverb and tremolo not working. Voltages looked suspect, and I realized that the PT is putting out a lower voltage in the secondary than itís getting in the primary.

    Here is a pic. With the transformer completely disconnected from the amp, wall voltage (120v) connected to the 117v and 0v primary connections gets me ~106v across the 250v secondaries. The heater secondaries read correct.

    Is this just one way a transformer can go bad? Iíve seen one burnt up that put out no voltage, but never seen this.

    Thanks,
    Matt

    Click image for larger version. 

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    Lifetime Member Enzo's Avatar
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    Measure resistance of that secondary, is it real high? An open winding can still induce some voltage to "fool" your meter.

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    Primary measures 7.3 ohms, secondary measures 16.1 ohms.

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    Lifetime Member Enzo's Avatar
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    well, doesn't sound open. You could have a shorted turn or two. I usually expect that to affect the other winding, but...

    You are powering this up unloaded and watching voltages. OK so far, but watch the mains current draw. Is it aanything above zero? An unloaded transformer ought not draw but the tiniest current. A shorted turn or two might raise teh mains draw.

    Also go over to RG Keen's geofex web site and look up his handy and simple transformer tester.

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    Supporting Member The Dude's Avatar
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    I think the transformer is mislabeled (not defective) since there doesn't seem to be a short or open. The transformer itself is made by Teisco and maybe they labeled it for the resulting supply? It's a Japanese made amp, so a labeling error wouldn't surprise me. I couldn't find a schematic for the 72R, but I found one for a 73R and they use a 120V transformer and a voltage doubler. I wonder if that's what's going on here. Do you have a schematic for the 72R? I couldn't find one, but here's the 73R.

    http://members.iinet.net.au/~mij_60s...Schedmatic.pdf

    Edit: If your supply looks like the one in the 73R, and your supply voltages are low, I'd look at those 2 electrolytics right off the rectifier.

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    Last edited by The Dude; 05-14-2020 at 05:31 AM.
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    Thanks Dude, I think you nailed it. Don’t have a schem for the 72R but the power section is similar, and the voltages look similar with it re-installed. Could’ve sworn something looked lower when I first went down this rabbit hole, but I’m thinking the PT is fine, just mislabeled. Thanks! Now to figure out why it’s not ‘verbing or tremoloing.

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    Is it anything above zero? An unloaded transformer ought not draw but the tiniest current.
    There will always be the primary magnetizing current (aka exciting current) of 100mA to 200mA independent of load.
    Hammond sometimes specifies it.
    E.g.:
    https://www.hammfg.com/files/parts/pdf/290DHX.pdf
    https://www.hammfg.com/files/parts/pdf/269AX.pdf

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    Last edited by Helmholtz; 05-14-2020 at 02:57 PM.
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    Lifetime Member Enzo's Avatar
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    Yep, that's a pretty tiny current.

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    Quote Originally Posted by Enzo View Post
    Yep, that's a pretty tiny current.
    Well it can be almost 50% of full power current in a small PT.

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    This one was drawing about 70mA unloaded.

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    Quote Originally Posted by Drummer4gc View Post
    This one was drawing about 70mA unloaded.
    Fine, no winding short.

    But why did you expect to see a 250V secondary? The schematic provided by the Dude shows a 120V secondary and a voltage doubler circuit.
    A drawing of your rectifier circuitry would be helpful.

    Edit: Just saw that it says 230V or 250V on the PT's label.

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    Last edited by Helmholtz; 05-14-2020 at 08:10 PM.
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