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    ST-70 quit working

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    daveshel

    Posts : 148
    Join date : 2011-11-06
    Location : Tucson AZ USA

    ST-70 quit working

    Post by daveshel on Sun Nov 06, 2011 8:50 pm

    My (rebuilt) ST-70 is not working. The initial symptom was that it would start out OK then stop making sound after about 10 minutes. Now it has no output. All the tubes light up except the 5AR4/GZ34 rectifier tube. I looked to see if any caps or other components looked burnt or damaged and everything looks good. Seems to me the likelihood is that the rectifier tube has failed and simply needs to be replaced. Is this a typical point of failure or do I need to look further?

    Thanks,
    Dave, new guy on the forum.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 quit working

    Post by Bob Latino on Sun Nov 06, 2011 9:24 pm

    Hi Dave,

    Yes - your GZ34 rectifier has probably gone bad. I should also mention that a GZ34 rectifier can also be bad even if it lights up. The thing to do is to check the bias voltage. If you get NO BIAS reading and no sound then usually the rectifier is bad.

    If you put in a new rectifier and that one blows pretty quickly, then probably there is an issue with the high voltage system .. Usually a partially shorted quad cap .. In that case - replace the quad cap ..

    Bob

    daveshel

    Posts : 148
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: ST-70 quit working

    Post by daveshel on Mon Nov 07, 2011 1:07 am

    Thanks, Bob.

    So if I was to measure the BIAS voltage I might avoid wasting another rectifier tube? My quad cap could be 40 years old.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 quit working

    Post by Bob Latino on Mon Nov 07, 2011 5:27 am

    daveshel wrote:Thanks, Bob.

    So if I was to measure the BIAS voltage I might avoid wasting another rectifier tube? My quad cap could be 40 years old.

    Dave,

    If your quad cap is original it could be as old as 52 years, since the ST-70 came out in 1959. I would replace it. Kevin at Dynakitparts sells replacement quad caps for the ST-70 ..

    Bob

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: ST-70 quit working

    Post by tubes4hifi on Mon Nov 07, 2011 4:57 pm

    I'd replace any quad cap over 10 years old, and I'd do the rectifier tube diode mod which I've now posted as a "sticky"

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