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    Bad Transformer?

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    j4570

    Posts : 141
    Join date : 2010-08-30

    Bad Transformer?

    Post by j4570 on Sat Dec 07, 2013 11:27 am

    When I rebuilt my ST-70, I replaced the power transformer.  I found the old one recently cleaning up the garage, and tested it.  I used a variac set at 60 VAC, so I doubled the readings (the readings below have been doubled already).  All the voltage readings are in spec per Bob's list EXCEPT:

    Green/Yellow to Red/Yellow read between 6.3-6.4 VAC (instead of 3 to 4)
    Brown/Yellow to Red/Yellow read between 8.8-9.2 VAC (instead of 3 to 4)

    So it looks like the transformer is bad to me.  Only the white wires had naked wire inside the bell, so I heat shrunk them prior to testing.

    I have no problem tossing it if it's really bad, I'll save the two bells and bolts/nuts if someone wants them for a restoration.

    You guys concur this thing is toast?  I'm thinking so.

    Jason


    Last edited by j4570 on Sat Dec 07, 2013 11:28 am; edited 1 time in total (Reason for editing : Clarification)

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bad Transformer?

    Post by Bob Latino on Sat Dec 07, 2013 11:35 am

    Hi Jason,

    The RED/YELLOW wire is the power transformer's connection to chassis ground.

    The GREEN/YELLOW (left channel) and the BROWN/YELLOW (right channel) are the center taps of the two 6.3 volt AC lines and should measure about 1/2 the 6.3 volts AC or about 3.15 volts AC

    Try the GREEN/YELLOW and the BROWN/YELLOW directly to chassis ground. You may get the same readings as before ? If you do - something seems wrong there ? A bad winding ?

    Bob

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: Bad Transformer?

    Post by j4570 on Sun Dec 08, 2013 10:03 am

    Bob,

    The tranny is out of a chassis, it's the old one, I got a new one from you when I rebuilt the amp ground up. When I measured across the Green/Yellow and Brown/Yellow, it gave about 14-15 volts, which indicates to me the windings are indeed bad. I really didn't chase it down on the schematic to figure out it was center tapped, but since it is, that tells me there is something wrong inside. Oddly on both legs of this tap. So I'm going to trash the tranny. Anybody need the bells?

    This would help explain why the amp was not working when I got it (and I knew that) and what caused failure. One channel all the tubes were shot. He does have a sheet in there where he added up what it would cost to fix, and one note was a new transformer to permit replacement of the 5AR4 (why I don't know, but he had parts lists from Panor).

    I have the paperwork for the amp (original) including notes on it being built in 1964 by the original owner, he made very good notes in the book. He notes it took him 10 3/4 hours to assemble and finish the amp. I need to scan it, as it's really neat to look at all the notes, and the checks he made. What's really cool is he used this amp for about 30+ years before replacing it.

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bad Transformer?

    Post by Bob Latino on Sun Dec 08, 2013 1:33 pm

    Don't measure across the GREEN/YELLOW and BROWN/YELLOW wires.  If you do that, you are measuring into two separate windings. Measure from GREEN/YELLOW to chassis ground and then from BROWN/YELLOW to chassis ground. Two measurements ..  If the power transformer is out of the chassis measure from each of the two wires to the RED/YELLOW wire. Let me know what you get ?

    Bob

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: Bad Transformer?

    Post by j4570 on Thu Dec 12, 2013 9:35 pm

    I tested the transformer about 3 times and got the same readings. Upon closer inspection, I could see where the epoxy (or whatever) coating had melted on some of the windings and ran to the bottom of the frame pooling up. So I cut the cardboard off where the taps were, and it was a mess. The Transformer went in the trash. I just don't believe it was safe to use. I don't want someone else to end up with it and have an issue either. I looked at it this way, I have a new transformer in my amp that should give many years of service, and it's not worth the risk over a few dollars.

    I did keep the bells if someone needs them.




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