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    Assistance with ST-120 problem?

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    skriefal

    Posts : 118
    Join date : 2011-09-20
    Location : Utah, USA

    Re: Assistance with ST-120 problem?

    Post by skriefal on Thu May 09, 2013 1:22 am

    Sal recommended some additional tests which I completed earlier. I found that the amp is blowing fuses on each power up whenever any rectifier is installed, regardless of which power tubes are (or are not) installed. Also tried an alternate set of 12AU7s with the same result (blown fuse). The two(?) times that the amp powered up without a blown fuse seem to have been flukes. So I can't re-test the voltage on pin 8. There's something else going on. Total guess on my part, but... maybe a bad tube socket?

    nmchiefsfan

    Posts : 33
    Join date : 2012-03-21

    Re: Assistance with ST-120 problem?

    Post by nmchiefsfan on Thu May 09, 2013 5:50 am

    Quad Cap, perhaps? A section could have gone bad. Have you measured the cap or possibly the chokes?

    skriefal

    Posts : 118
    Join date : 2011-09-20
    Location : Utah, USA

    Re: Assistance with ST-120 problem?

    Post by skriefal on Thu May 09, 2013 9:59 am

    I don't know how to measure either of those.

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: Assistance with ST-120 problem?

    Post by Sal on Fri May 10, 2013 7:13 pm

    Scott, I sent you a PM, give me a call tonight before 11pm eastern time.

    Regards,
    Sal

    skriefal

    Posts : 118
    Join date : 2011-09-20
    Location : Utah, USA

    Re: Assistance with ST-120 problem?

    Post by skriefal on Fri May 10, 2013 8:45 pm

    nmchiefsfan wrote:Quad Cap, perhaps? A section could have gone bad. Have you measured the cap or possibly the chokes?

    Your guess is probably correct. Looks like a probable bad/failed section in the quad cap. Replacing the quad cap is beyond my current skills level, so I'll be sending in the amp for repair.

    I'm curious to know whether the failed quad cap is a symptom (Weber rectifier failed and took out the quad cap), or whether the failed quad cap is the source of the problem. Or if it's even possible to determine this?

    The amp will be singing again soon, so there's something to look forward to!

    Thanks Bob and Sal!

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