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    Dynaco Stereo 70 Bias Adjust Issue

    misterc57
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    • Post n°1

    Dynaco Stereo 70 Bias Adjust Issue Empty Dynaco Stereo 70 Bias Adjust Issue

    Post by misterc57 Thu Jan 13, 2022 3:12 pm

    Hey Folks

    Have a ST70 bias issue and questions.  

    Left channel will only reach 1.3 VDC and bias pot is maxed out.  Center tap of bias pot to ground is 9.8K ohms.  
    Right channel is good at 1.56 and pot is not maxed out.   Center tap of bias pot to ground is 12.1K ohms.  

    Components in the bias checked good.  

    Swapped the left and right channel tube pairs.  The problem followed the tubes to the right channel, left channel bias ok to 1.56 VDC

    So now the problem is in the right channel.  Tried a completely different pair of tubes in the right channel and maxing out around 1.33 VDC.

    Any suggestions?

    I did notice that the 2 x .02 disc cap is bubbling but did not measure any shorts there.  Would this effect the bias adjustments?

    I will attempt to include two images in this post.

    Thank you.  MC

    ps.  I also noticed a lot of wax on the bottom cover that must have dripped from the choke.  Is this something to be concerned about?

    Dynaco Stereo 70 Bias Adjust Issue JtqBLkHSmCca2kWPj4ElVhLmldQzT-Z7SZHJQurNPGX5bs-cUiuodKTrt0wRPBMbP6S5ZeSjYxAv_ouPBWoPXMZSlNNG3H3ufCce5zQwh2dl6LO-SUdN9wZlvjibc8eqK6e-baKVr5Du8fGZVRyNi1uWIxaDuD7Oy-hSKUQWx3luwoLJV-aC7xrOvThMCYLVwzcDxPTBHrqM-pc0kJosjeQY2D9BhZEad3BDzbWGg9EMyuwLSY_94b4sIR05775kBXul0n1wLVx0kCz-akaTa8RHhFDuizM_UEoGBOA5DLRmswZgo5us1YBdcoG4bcG3oP4MotuLnFe2_z9lc8uALFaK8D6_-v1gQA-4uCBMtwjeJCioNYq29ymHjduoG_d4qw4m96PpTPab85XsBxm_HcOIF1VYTzWYDdCZeRlCJ1OY5fDaTjPizqNkSZLfwF6_YP7BjoqcjJFts1BVrmB6JTTY8HnYKnzLswhb1BnMjmSG4DXaYZg-3BvSUUA71MkEkICTf4clKqYpWAnMc12oVDDEO0iyoLSTb2ahMTS7EM3MI3XUs36QNkhUFbZGhHfCQPnXwXRsSryukQ8NhiBgCAZdPqHC-HfNxkk6gEsw8f1vLeaon6kQoIiZVbf-APtkqsPjhb18_wf1tAJ1HluiS1jfYlZMQ-NOOThFBM0OTfiDk7YoBuId5dHLYk27iRarsmfQH7v4A1bmUOOl3UjtBB9Q=w684-h912-no?authuser=0

    Dynaco Stereo 70 Bias Adjust Issue S22r565LeHTq38G-W_pUkPeYOKpq-rDySfEYZ9WJ6cFmEaJ0qKeVkrpo56Wh_Zh-Qdzzn9hWIt2JfCOdeG8x_1WWyipIqSZjyNIFnN6X0I8ArQcOcrmmYk8zOAPu8IEFBQaYBFiVvaTFWAAx92IsRHsf3ZVFghF8yatucXhDKam_FydYZRlw_Np2rmNbfhaUBHJdTzXT65WbPUsCI1584wyjt7qNymOmsAPql2s-rWG0I35sFN2hx-92qgnNgo3xPIW6Mcr97hwY6Air5nsQ6HzbikuhXuGIM1xSWtkh4R9bYyMDZlHumZ3_iKpyJMy14yiQQp1Yn6YOXCLjhvOmmHkB0sG42SjmaPwwt2JZATW9-d7fwLvRGPwPASNrcSlKki3cdt77XOnzhXTJfF843KrDfn7SN_ziPgc3nsGQoPiBYLEfiVAMWOEhTLQzqU1AitM9SdXhnHhOc58G5XRhiaC9lkiP0K86uZekHyO24BgaNB-VtJs3LQqImAV3-LjQfSUjYW_cGsS_rcDWs4QXkw66ilwGOCIief3Yg4BMndPNwP6isGAaHUwnKmE7M1zq8I3PRHI6-2MMqvC9C2AIXvfaD7KJ7nrLrfaHQ7Z4fFt-p_8tuCcfTrGVp48fpZJM7WhlMfCYC5ydiPspTaiTz5GcZBTRGOeXsIU3HSxdB45w1Ef54o_QhXC2q7CM3pfTib9kqwoi5b7SDZJmvubhTqV3=w1216-h912-no?authuser=0
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    Bob Latino
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    • Post n°2

    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by Bob Latino Thu Jan 13, 2022 3:46 pm

    Replace the selenium rectifier with a diode. Also replace that possibly damaged double .022 capacitor @ 50 volt with TWO .022 uF @ 50 or 100 volts caps and replace your output tubes.

    If swapping output tubes from channel to channel causes the problem to move to the other channel, then use new output tubes on BOTH channels.

    Bob
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    • Post n°3

    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by misterc57 Thu Jan 13, 2022 5:06 pm

    Bob Latino wrote:Replace the selenium rectifier with a diode. Also replace that possibly damaged double .022 capacitor @ 50 volt with TWO .022 uF @ 50 or 100 volts caps and replace your output tubes.

    If swapping output tubes from channel to channel causes the problem to move to the other channel, then use new output tubes on BOTH channels.

    Bob

    Hi Bob

    Is the selenium replaced by one 1n4007?

    Will a pair of .01 caps work in this scenario or should I order .022?

    Thank you! MC
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    • Post n°4

    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by Bob Latino Thu Jan 13, 2022 5:32 pm

    Yes - You can use a 1N4007 silicon diode to replace the selenium rectifier.

    A pair of .01 uF caps may work ? I can't say for sure.  If you already have some .01 caps, use two pairs of .01 uF caps in parallel and you have two .02 uF caps.

    Note - Those caps only look at 1/2 the 6.3 volt AC voltage (about 3.15 VAC) on the two filament lines so either a 50 or a 100 volt cap is plenty large enough to handle the voltage.

    Bob

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    • Post n°5

    Dynaco Stereo 70 Bias Adjust Issue Empty rectifier diode

    Post by misterc57 Fri Jan 14, 2022 10:28 am

    It looks like the selenium rectifier has already been bypassed with a diode.  See pic.  The top lug of the selenium appears clipped off and the lower lug is being used as a connection lug.

    Perhaps I need to change the resistor values at the 7 lug terminal strip to obtain the correct bias at the output tubes.

    Dynaco Stereo 70 Bias Adjust Issue SDM_oO-54VmWYNauypdIbGVpceKL7OuD_JNt_pwQVGi98r2ILpzv5Fe6U27wxiCVNMYGlOZT57m19fYua8ihxHmxEB1yb4DxooInCapRzmexixKUlqQoQdXHYOG2BJ9INJ0VL3u3nOwws6BgExArHtPTiu2toPCjZPzKHV9G73pip5kfp0jpZMI9CI-wIx2e8jeGPFbvWoEkCQLoboUbPris_Em7rsYhjgRP7UYxFuFNB1zAiMGX-NxUXUiVfsrSeCqMi0fSIbSIIRf2YOcY4KHFdCvY-gjCSfOCibS1dNtFTnxpQGZ1feBWhO3f_QONn_VkRj42c7Sp5DL6Z1BYI4l8b26rkCklndfyskHZ30o1B1EqnnhEyvYAsGAZdr6GeEjz5oJ-_pu3swLayAuXrmiuW4m57sW09WZEpBf2FMRmcR60lm1xdSZiEVYH1S3aEp3c4Nra4m_qIqRqkOizQlX3-cstlGc5lIM9GbTz7CWfpam5Y4OnUnpUD_2eQg0Fhyfnqaa9waq9ylBrkf6roBN4V9sWkwB2lCjzybIEOTAk3p13zNFDLALAfV8HCMpdIqOZprHy_hYbnnTaR0gMTYZ_vUv5Fdn9HM5e2Rsj5TQlLYbW-5lXoT2A52W3SObwADxZccggKi1L0gUE7-6oE6LuPAyK20kUYATrkrHrduErkc2OzDLrEh5-2826HJ0YuVh0YyYHrwO-LDKvw9At7pcf=w1216-h912-no?authuser=0
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by Bob Latino Fri Jan 14, 2022 10:56 am

    Yes - I believe the selenium rectifier has been bypassed BUT I would still remove it. I had a friend years ago tell me that his selenium rectifier cracked and leaked selenium (a poisonous substance) on to the inside of the chassis.

    If you are using a diode to replace the selenium rectifier, you should lower the value of the two resistors on the 7 lug terminal strip. They are probably the original 10K resistors. The should be replaced with two 5K resistors. This will raise the bias that can be applied to the output tubes. A diode has more "forward voltage" than the original selenium rectifier so a resistor change is necessary to bring the bias on both channels back up into the proper range.

    Bob
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by misterc57 Fri Jan 14, 2022 11:17 am

    Ok, I will remove it.

    What should the wattage be on the replacement resistors? 1/2 watt okay?

    Thank you!
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by Bob Latino Fri Jan 14, 2022 11:23 am

    misterc57 wrote:Ok, I will remove it.

    What should the wattage be on the replacement resistors?  1/2 watt okay?

    Thank you!

    I would use two 5K 1 watt resistors

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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by misterc57 Fri Jan 14, 2022 4:08 pm

    Since I am going to have to order the resistors, is there a preference to carbon film vs metal film?

    Also, any preference on the .022 cap type?

    Thank you
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by Bob Latino Fri Jan 14, 2022 4:25 pm

    misterc57 wrote:Since I am going to have to order the resistors, is there a preference to carbon film vs metal film?

    Also, any preference on the .022 cap type?

    Thank you

    Not really .. Any kind of cap will do .. These two caps are not in the signal path ...

    Bob
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    Roy Mottram
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by Roy Mottram Thu Jan 27, 2022 9:36 pm

    bias is also dependent on tube quality and B+ voltage.
    As the tubes wear out, they require more bias, eventually you will not be able to bias high enough,
    that is when you need to replace the tubes.
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by ltusler Thu Jan 27, 2022 9:41 pm

    ^ That is a great piece of information to know!
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by Seamus Fri Jan 28, 2022 2:09 pm

    Bob Latino wrote:A diode has more "forward voltage" than the original selenium rectifier so a resistor change is necessary to bring the bias on both channels back up into the proper range.

    A selenium rectifier has a forward voltage of about 1v / section, so a few volts for the ST-70 stack. A silicon diode with a forward voltage of 0.7v will increase the negative bias by a few volts.

    This doc REPLACING SELENIUM RECTIFIERS gives more detail.

    Dynaco ST-70 power supply schematic:

    Dynaco Stereo 70 Bias Adjust Issue St-70_10

    Changing just the 10k resistor between the 50µF caps will add about 10v to the bias range with the silicon diode replacement.
    Changing the 10k resistor to ground lowers the minimum voltage and reduces the maximum value.

    The total power dissipated in the resistors and pot is under 0.1w or about 0.033x per 10k, so ½w is fine.

    Don't use a polarized electrolytic for the 22nF. Any film or ceramic is fine.
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by misterc57 Tue Feb 01, 2022 8:35 am

    Thank you everyone. Changes made and all good.

    New question. A friends ST-70 bias will drop from 1.55 to 1.45 after playing several hours. At this time I do not know if the bias supply has been updated. Would this issue be indicative of an old bias supply?

    Thanks again.
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by Seamus Tue Feb 01, 2022 10:41 am

    It could be any of several issues from a drop in line voltage at the wall socket [unlikely] to failing transformer[s] to failing diode to leaky caps to shot pots.

    Measure voltages at each point in the bias circuit when cold and when hot.
    What every is driving that point is bad.
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by peterh Tue Feb 01, 2022 10:48 am

    misterc57 wrote:Thank you everyone.  Changes made and all good.

    New question.  A friends ST-70 bias will drop from 1.55 to 1.45 after playing several hours.  At this time I do not know if the bias supply has been updated.  Would this issue be indicative of an old bias supply?

    Thanks again.
    It could be old power tubes. In any case i would not worry as long as bias is back next time.
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by misterc57 Tue Feb 01, 2022 2:52 pm

    It is not in my possession. I do have a pic and I see updated bias circuit and rectifier diode.

    Dynaco Stereo 70 Bias Adjust Issue St70-c10
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    Dynaco Stereo 70 Bias Adjust Issue Empty Re: Dynaco Stereo 70 Bias Adjust Issue

    Post by peterh Tue Feb 01, 2022 3:06 pm

    Again, i would'nt worry as long as bias is not increasing during use.
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