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    Time for a new board? ST70

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    wildiowa

    Posts : 126
    Join date : 2012-03-19

    Time for a new board? ST70

    Post by wildiowa on Fri May 04, 2012 10:45 am

    Have re-done almost everything on the ST70, finally received the 10k and 15k 1w resistors and put them across terminals 1-2 and 3-4 as bias would not come into range, was way too high. Fired it up and bias still high have turned bias pots totally counterclockwise, voltage about 2-2.2 volts. Lower than it was before adding resistors but can't get it into range.

    I also ordered a couple 27k 1w resistors as I read that value might be added between terminal 3 and 4 rather than the 15k but not sure if even that will bring it into range. Only thing I have not done on this amp is trannys and a new PC board. From previous photos I have been advised to get rid of that board it is pretty funky. Perhaps that will solve issues?

    Amp seems stable and quiet and sounds pretty good, but bias won't correct. With time and $$ invested would like to get it right. Thx. IA

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Time for a new board? ST70

    Post by GP49 on Fri May 04, 2012 11:46 am

    Is the "voltage about 2-2.2 volts" being measured across the stock common cathode resistor value of 15.6Ω?

    If so, check for around NEGATIVE 32 volts at Pin 5 of all four output tubes, all should be approximately the same. If that's incorrect, check the voltage at various points along the bias circuit. You are looking for something around NEGATIVE 30-40 volts where it goes to the circuit board (eyelets 6 and 21), variable with the setting of the bias pots.

    If what you get is NOT NEGATIVE ENOUGH, make sure you put the 10KΩ resistor between lugs 3 and 4 on the terminal strip, and the 15KΩ between lugs 1 and 2 (assuming your wiring is identical to the stock Dynaco shown on the Pictorials available on this site). The other way round on a stock Dynaco Stereo 70 will not provide sufficient bias voltage and won't bring the bias current measurement down low enough.

    You DID replace the selenium rectifier with a modern silicon rectifier, didn't you?

    If all that is not right, nothing you do elsewhere will fix the problem you report.

    wildiowa

    Posts : 126
    Join date : 2012-03-19

    Re: Time for a new board? ST70

    Post by wildiowa on Fri May 04, 2012 3:09 pm

    SUCCESS! Upon a suggestion here from GP49 to see if the 10k and 15k resistors were on correct terminals, discovered I had the 10k between 1 and 2 and the 15k between 3 and 4. Wrong! Switched them and things balance.

    One issue left- although it's within range now the left bias pot has to be turned completely counterclockwise but seems to be stable at 1.5v -- I can not get it any lower. The right side has more latitude, and you can rotate that pot up and down to get 1.5v or so somewhere in the middle. I have a 27k resistor....would using that in place of the 10k or 15k give me the latitude I need on the left side? Actually things are finally OK so I hesitate to mess with it as all seems to be alright at this point. And again, thanks to all who answer questions here it is a great resource.

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Time for a new board? ST70

    Post by GP49 on Fri May 04, 2012 4:23 pm

    Problem could be caused by a bad tube in the channel where you can barely get it down to 1.5V.

    You can shift the range of the bias adjustment in the direction you want by substituting the 27KΩ for the 15KΩ, but both channels' adjustments will shift; this might shift the current "good" channel too far.

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: Time for a new board? ST70

    Post by tubes4hifi on Fri May 04, 2012 10:45 pm

    probably an unbalanced pair of tubes on one side, so the bias setting is different.
    you could try swapping one tube at a time from side to side to see if you can equalize better,
    otherwise, buy a new pair of matched tubes and keep the old ones for spares.

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