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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


    Stereo 70 Repair Diagnostics & Journal

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    misterc57

    Posts : 10
    Join date : 2017-03-10
    Location : Franklin MA

    Stereo 70 Repair Diagnostics & Journal Empty Stereo 70 Repair Diagnostics & Journal

    Post by misterc57 on Thu Aug 27, 2020 6:59 pm

    Thank you for any help and suggestions along this journey.  Its been three years since I last worked on a Stereo 70.  That one is still up and running!

    I picked one up last week and have started to walk through and discover its issues.

    The EL34s do not match.  The pairs on each side match, but there are 2 completely different brands.  One pair tests strong, the other pair just makes it into the good section of the tube tester.  None have shorts.

    The 2 12AX7 were a mess on the tube tester.  Each one of them glowed red and got very hot.  Never seen that before on a 12AX7 on a tube tester.  Both are no good.  Replaced with a pair of JJs.

    The rectifier tube tests fine.

    I noticed a cold solder joint at the preamp board eyelet #2 which I re-flowed.  The wire at the eyelet could be moved in and out.

    Brought the unit up on my variac/isolation transformer.  She hums.  Adjusted the bias on each side to 1.56 VDC.  One side hums louder than the other.  There is no DC at the outputs.  The filter can looks like it had been replaced, but who knows how many years ago.

    Connected my PC to the inputs to play some music.  I hear no music.  Noticed that the 12AX7 do not look lit and are not warm to the touch.

    I think I should focus on one problem at a time.  I would like to get the preamp working, then the hum.

    I did notice that the heaters had continuity from the PT all the way to the 12AX7 on both sides.  I have not taken readings at the 12AX7 yet, I will do that next and report back.
    MechEngVic
    MechEngVic

    Posts : 79
    Join date : 2019-01-16

    Stereo 70 Repair Diagnostics & Journal Empty Re: Stereo 70 Repair Diagnostics & Journal

    Post by MechEngVic on Fri Aug 28, 2020 1:53 am

    If it's an original ST-70, the driver tubes are 7199's, not 12AX7's. The 12AX7 is a dual triode tube and the 7199 is a triode-pentode. Many owners install adapters and use a 6GH8 tube, also a triode-pentode about identical to the 7199 except for the pin arrangement.

    rjpjnk likes this post

    peterh
    peterh

    Posts : 1403
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Stereo 70 Repair Diagnostics & Journal Empty Re: Stereo 70 Repair Diagnostics & Journal

    Post by peterh on Fri Aug 28, 2020 2:14 am

    misterc57 wrote:Thank you for any help and suggestions along this journey.  Its been three years since I last worked on a Stereo 70.  That one is still up and running!

    I picked one up last week and have started to walk through and discover its issues.

    The EL34s do not match.  The pairs on each side match, but there are 2 completely different brands.  One pair tests strong, the other pair just makes it into the good section of the tube tester.  None have shorts.

    The 2 12AX7 were a mess on the tube tester.  Each one of them glowed red and got very hot.  Never seen that before on a 12AX7 on a tube tester.  Both are no good.  Replaced with a pair of JJs.

    The rectifier tube tests fine.

    I noticed a cold solder joint at the preamp board eyelet #2 which I re-flowed.  The wire at the eyelet could be moved in and out.

    Brought the unit up on my variac/isolation transformer.  She hums.  Adjusted the bias on each side to 1.56 VDC.  One side hums louder than the other.  There is no DC at the outputs.  The filter can looks like it had been replaced, but who knows how many years ago.

    Connected my PC to the inputs to play some music.  I hear no music.  Noticed that the 12AX7 do not look lit and are not warm to the touch.

    I think I should focus on one problem at a time.  I would like to get the preamp working, then the hum.

    I did notice that the heaters had continuity from the PT all the way to the 12AX7 on both sides.  I have not taken readings at the 12AX7 yet, I will do that next and report back.

    If it has 12ax7 tubes it is not a dynaco st70. A couple of pictures could be informative.
    avatar
    misterc57

    Posts : 10
    Join date : 2017-03-10
    Location : Franklin MA

    Stereo 70 Repair Diagnostics & Journal Empty 7199

    Post by misterc57 on Fri Aug 28, 2020 8:25 am

    My (very) bad. Yes they are 7199. No wonder it looked wrong on the tube tester. 7199 are back in the amp. Audio is working and no more hum.

    MechEngVic likes this post

    peterh
    peterh

    Posts : 1403
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Stereo 70 Repair Diagnostics & Journal Empty Re: Stereo 70 Repair Diagnostics & Journal

    Post by peterh on Fri Aug 28, 2020 9:19 am

    misterc57 wrote:My (very) bad.  Yes they are 7199.  No wonder it looked wrong on the tube tester.  7199 are back in the amp.  Audio is working and no more hum.
    Good. Enjoy the music !

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