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    ST70 pop on turn off

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    sailor

    Posts : 269
    Join date : 2011-04-04

    ST70 pop on turn off

    Post by sailor on Fri Mar 23, 2012 5:18 pm

    I have a ST70 that works just fine but it pops on turn off. Is this normal? Is the switch going bad and needs to be replaced or should it be shunted with say a .01/400volt cap?

    Bob Latino
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    Re: ST70 pop on turn off

    Post by Bob Latino on Fri Mar 23, 2012 6:23 pm

    Hi Sailor,

    Both of your solutions will probably work. Either replace the switch OR run about a .02 uF cap @ 200 volts or higher across the two terminals. Personally - I would replace the switch. Radio Shack has a suitable replacement SPST slide switch that will fit right in there. The only problem with the Radio Shack switch is that it is not threaded like the original switch. An easy solution is to just use a 4-40 nut over the end of the two screws that hold the switch on.

    Bob

    sailor

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    Join date : 2011-04-04

    Re: ST70 pop on turn off

    Post by sailor on Fri Mar 23, 2012 8:58 pm

    Thanks Bob, you make this the best audio board on the net.

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: ST70 pop on turn off

    Post by sailor on Fri Mar 30, 2012 3:59 pm

    I replaced the the switch and added a shunt cap for good measure the pop is now completely gone.
    While I was soldering on the bottom I decided it was time to replace the original bios caps.
    When I turned the amp. on I was greeted with far more detailed sound and it seemed to have better Bass.
    The problem is once you change 2 things out you don't know which made the difference.
    I would assume that either the switch was so bad that it was limiting the amount of power to the transformer or more likely, the 2 caps were very noisy and changing them out cleaned up the sound. I had no idea that the bios caps could make that much of a difference?

    Tube Nube

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    Age : 53
    Location : Calgary, AB

    Re: ST70 pop on turn off

    Post by Tube Nube on Fri Mar 30, 2012 4:05 pm

    Can you tell us a bit more about what you replace, and what you put in as a substitute?

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: ST70 pop on turn off

    Post by sailor on Fri Mar 30, 2012 5:01 pm

    Hi Tube Nube, A shunt cap is a low value cap around .01 to .02mfd 400volts or higher, across a switch to help stop a pop when you turn electronics on and off. Most amps and preamps already have this cap. The ST70 does not. I may not have needed it with the new switch replacement but I was already soldering so I put it in.
    As far as the 2 electrolytic caps in the bios supply, The original ones are either green or gold and I think 50mfd/75 volts. I changed them out with new 100mfd/100volt. I was holding bios just fine before they were changed out. But once removed there poor condition became apparent.
    I think all would agree: If you get an old ST70 at the very least you should replace the quad cap and the 2 bios caps as they are very likely to fail and do damage to your tubes and choke. I did not change the bios caps because I thought they were in better shape than the were. I won't make that mistake again.

    Tube Nube

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    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: ST70 pop on turn off

    Post by Tube Nube on Fri Mar 30, 2012 6:04 pm

    "Hey there Sailor", (Couldn't resist saying that, and having done so, it feels a little too "Mae West")

    Seriously, thanks for that explanation. I wondered of I should be considering replacing caps like you did, but I'd guess not since my ST70 is one of Bob's, and is only a couple of years old.

    As for the shunt cap . . . that might be a good idea!


    tubes4hifi
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    Re: ST70 pop on turn off

    Post by tubes4hifi on Fri Mar 30, 2012 11:49 pm

    TubeNube,
    it's just like the cap on your preamp power switch, I use 0.01uF 1000v rated capacitors

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: ST70 pop on turn off

    Post by sailor on Sat Mar 31, 2012 12:15 pm

    A picture is worth a thousand words.
    Nube your caps are just fine. Electrolytic caps have a shelf life of about 20 to 25 years. The caps in the old Dynaco equipment are closer to 50 years which makes them a ticking time bomb. They are going to fail just a matter of when.

    fedoragent

    Posts : 35
    Join date : 2013-09-14

    Re: ST70 pop on turn off

    Post by fedoragent on Fri Dec 20, 2013 5:42 pm

    tubes4hifi wrote:TubeNube,
    it's just like the cap on your preamp power switch, I use 0.01uF 1000v rated capacitors

    This photo is a little bit out of focus.  Are there certain pins that this cap needs to be on?  I'm going to use a .01 Orange Drop that is rated for 630V.  This should do the job I hope.

    tubes4hifi
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    Re: ST70 pop on turn off

    Post by tubes4hifi on Fri Dec 20, 2013 10:52 pm

    there are two wires to the power switch, one lead of the cap goes to one each of the wires

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