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    ST 70 Sproing

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    Bugs

    Posts : 91
    Join date : 2009-04-17

    ST 70 Sproing

    Post by Bugs on Thu Oct 15, 2009 9:22 pm

    What does it mean when you turn on your amp and you hear the electrical version of 'SPROING' (imagine the sproing sound with a little more 60 cycle hum than a mechanical spring sproing)?

    The fuse is good and changing the rectifier made no difference.

    Started doing this yesturday and when I get home tonight a light sproing and then no power, at least the tubes don't light up. I haven't taken a close look at it yet, but thought see if it had any definate meaning to anyone?

    I am getting to know the inside of an ST 70 pretty well, but after my last transformer transplant when what I needed was a new quad cap, I figured I ought to ask first.

    Thanks,
    Doug

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: ST 70 Sproing

    Post by GP49 on Thu Oct 15, 2009 10:58 pm

    Usually that's caused by laminations in the transformer vibrating in sympathy with the surge in current through the transformer as you turn on the amplifier. Normally it is not a real concern unless that sort of thing bothers you. I have a Mark II that has been doing that since the mid-1980s when I first rescued it from a junkpile and modified/upgraded it. That amp is still in my main system today and is playing music right now.

    However your amp ISN'T working any more, so further investigation is warranted. Check the fuse first. I forget if the ST-70 calls for a slow-blow type; if it does, and you put in a standard-blow type, it may have blown on that initial current surge. If a new fuse doesn't restore the amp to life: with the amp unplugged, check the primary and secondary windings of the transformer for continuity. You may find that one of them has opened up, which would be bad news. If you feel up to it, unplug the rectifier tube, plug in the amp to AC, and switch it on; CAREFULLY (high voltage on the B+ winding!!!) check the secondaries for AC voltage. Let us know what you find.

    I'm a bit concerned that your amplifier did NOT make the SPROING noise previously.

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: ST 70 Sproing

    Post by Bugs on Fri Oct 16, 2009 12:24 am

    GP49 wrote:I'm a bit concerned that your amplifier did NOT make the SPROING noise previously.

    It has been making the sound lightly for awhile. The amp has been a real good performer since I rebuilt it, however it is still using the original power transformer. Yesterday and maybe the day before I remember the sound being louder. When I came home from lunch it was very noticable and then when I returned from work tonight it was quieter, but then it didn't power up.

    I did check the fuse and swapped out the rectifier which didn't change anything. I'll be going out of town this weekend so I'll give it a better going over when I return.

    Thanks,
    Doug

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    sprong noise

    Post by tubes4hifi on Fri Oct 16, 2009 2:13 am

    another possibility is the quad cap is getting weak, especially likely if you notice it is warm when the amp is running, once those old caps get past 20 years or so they start leaking due to higher current flowing thru lowered internal resistance. The extra current is contributing to the mechanical noise of the transformer laminations.
    The ST70 uses a 3 amp SLO-BLOW fuse.

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: ST 70 Sproing

    Post by Bugs on Sat Oct 17, 2009 12:20 am

    I must be getting old or senile or both. Rolling Eyes

    I turned the receiver over and checked voltages and everything looked good. I took a peak at the tubes and all were on. I hooked it up to the preamp and low and behold it was back to its wonderful self.

    It could have been a cold solder joint. If it happens again I'll give it a good going over.

    Thanks to those that replied, sorry for the false alarm.

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