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    Problem with SP-8 blowing fuse

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    Mike16W

    Posts : 39
    Join date : 2009-11-03

    Problem with SP-8 blowing fuse

    Post by Mike16W on Fri Feb 25, 2011 6:53 pm

    I am building the Sp-8 preamp and the fuse keeps blowing once power is applied. I have removed every line, switch, and wired the AC directly to the board. Even with the secondaries of the transformer removed, it still blows the fuse. I am using the large Torrid power transformer and the primaries blacks are tied together and soldered to the board and the same with the Red wires. I am only getting about 3.5 ohms across the primary. Is there some memo I missed about wiring the primary different or ? Any ideas would be appreciated.
    Thanks
    Mike

    Luddite

    Posts : 235
    Join date : 2009-02-04
    Age : 66
    Location : Texas

    Re: Problem with SP-8 blowing fuse

    Post by Luddite on Fri Feb 25, 2011 10:52 pm

    Mike16W wrote: I am building the Sp-8 preamp and the fuse keeps blowing once power is applied. I have removed every line, switch, and wired the AC directly to the board. Even with the secondaries of the transformer removed, it still blows the fuse. I am using the large Torrid power transformer and the primaries blacks are tied together and soldered to the board and the same with the Red wires. I am only getting about 3.5 ohms across the primary. Is there some memo I missed about wiring the primary different or ? Any ideas would be appreciated.
    Thanks
    Mike

    Are you sure the primaries are supposed to be soldered to the board? Ordinarily, the primary windings are connected directly to the incoming AC line, while the various secondary windings are connected to the (power supply) board. Not having a schematic nor a transformer wiring diagram to refer to, I can only make two suggestions. First, carefully check the schematic to be absolutely certain all primaries and secondaries are connected properly. Secondly, if the problem still occurs, then it's time to consult Roy Mottram, the designer!

    Best Regards,
    Charlie

    Mike16W

    Posts : 39
    Join date : 2009-11-03

    Re: Problem with SP-8 blowing fuse

    Post by Mike16W on Sat Feb 26, 2011 8:17 am

    Charlie,
    Thanks for the reply. I am going to rewire the primary side exactly like what it says in the instructions and see what happens. There maybe something there I don't see, even though it's a pretty simple circuit. I will let you know what happens.

    Thanks Again,
    Mike

    crxman

    Posts : 4
    Join date : 2011-01-04

    Re: Problem with SP-8 blowing fuse

    Post by crxman on Sun Feb 27, 2011 6:51 pm

    Hi,

    My preamp did exactly what you describe. I had taken the secondaries from the large torroid and accidently hooked the yellow and white from the same coil to the CT (centertap), and hooked one of each of the other yellow / white to the other locations on the PS board. When I powered it up with the variac, it blew the fuse before you could even see power in the secondary side. I tracked it back and fixed it and walla... a perfectly operating pre... I even had Roy stumped. It isn't sometning that would even be considered but I was working with the help of a friend and got careless...

    Arnie from Philly....


    Last edited by crxman on Mon Feb 28, 2011 4:47 pm; edited 1 time in total

    Mike16W

    Posts : 39
    Join date : 2009-11-03

    Re: Problem with SP-8 blowing fuse

    Post by Mike16W on Sun Feb 27, 2011 9:48 pm

    Arnie,
    Thanks for your suggestion. Mine actually blows the fuse with the secondaries removed. I have pulled the primary wires off and checked to make sure they are not shorted and everything seems fine. I can't see a solder bridge but will continue checking.

    Mike

    Mike16W

    Posts : 39
    Join date : 2009-11-03

    Re: Problem with SP-8 blowing fuse

    Post by Mike16W on Mon Feb 28, 2011 12:37 am

    Well, I rewired the power supply section and used a 2 amp fuse and everything seems OK. Now to find the tubes I bought last year for this preamp. Thanks everyone for the help.

    Mike

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Problem with SP-8 blowing fuse

    Post by tubes4hifi on Mon Feb 28, 2011 3:10 pm

    hopefully you are using a slow-blo fuse, if you were using a 1 amp fast blow fuse you'd go thru several dozen!!

    Mike16W

    Posts : 39
    Join date : 2009-11-03

    Re: Problem with SP-8 blowing fuse

    Post by Mike16W on Mon Feb 28, 2011 3:15 pm

    Yes, slow blow. It does seem to be working now. I ohmed out the primary and secondary leads from the transformer and re-soldered them. I'm not going to think it much beyond that since I have other soldering to do. I want to try the phono section on the SP-8 to determine if it's worth while purchasing a stand alone PH-11 or PH-12. Thanks for the help.

    Mike

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