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Power transformer getting too hot,low plate voltage

Discussion in 'Amp Tech Center' started by J-Turn, Mar 25, 2020.

  1. D'tar

    D'tar Friend of Leo's

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    1N4007 would work.... Buy them by the dozen or two.

    Do you have any diodes at all? There are lots of options
     
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  2. J-Turn

    J-Turn TDPRI Member

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    I do, I’ll look for them
     
  3. J-Turn

    J-Turn TDPRI Member

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    Put in some new 1N4007’s
    +39 and -21mv (bouncing up to -30mv)
     
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  4. D'tar

    D'tar Friend of Leo's

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    Did you check that secondary winding for output on both halves of the 25-0-25 coils. What is vac on either side and across both outer legs? You would check this just like you did the HT winding earlier. On the center tap of that winding... What is the resistance reading to ground/chassis. If you have less than say 140r(r702) which i believe you will find, you have a short to ground.

    Just in case... Where are your probes connected while checking voltage? black to chassis and red to each test point?
     
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  5. J-Turn

    J-Turn TDPRI Member

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    With all tubes in
    Rectifier pin1 269ac
    Pin7 269ac
    Pin1-7 536ac
    Pin1 to ground 209ohm
    Pin7 to ground 197ohm
     
  6. J-Turn

    J-Turn TDPRI Member

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    Yes black to chassis and red to test point
     
  7. dan40

    dan40 Tele-Afflicted

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    Never rely on a meter to tell you that an old capacitor is good or bad. It may read correctly with 9v from your meter, but in a circuit with several hundred volts it could read and perform much differently. Not suggesting that they are the cause of the problem though. Keep following the advice of the fellas above.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    “With all tubes in
    Rectifier pin1 269ac
    Pin7 269ac
    Pin1-7 536ac
    Pin1 to ground 209ohm
    Pin7 to ground 197ohm”



    J-Turn, aren’t these measurements taken on the HT secndary winding to the rectifier? D’Tar was suggesting that you measure the
    25-0-25 secondary that is feeding SD701 & SD702 to provide the bias voltage.
     
  9. J-Turn

    J-Turn TDPRI Member

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    Maybe I’m barking up the wrong tree, but the voltage going out of the plastic connector to the organ(schematic)goes through the tone generator etc. and feeds back in to MY power amp via the yellow wire That feeds directly into the negative side of C712 which goes directly to Pin8 of the 12at7.
    The voltage is too high what would be sent out to the Organ, and now is too low what is being sent back from the Organ.
     

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  10. elpico

    elpico Tele-Afflicted

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    The 25V-0-25V winding is the B+ supply for every solid state device in the organ. Do you have all those tone generator boards running? If they're not drawing current then no voltage is developed across R702, so no negative bias voltage is available for the EL84 tubes. (which frankly is kind of a dumb design)

    Some options to make it work without the mountain of obsolete solid state boards:

    - add a second 140ohm 5watt resistor to balance R702. You'd connect this from the positive end of C702 to ground. (1 new part)

    - get rid of this bias circuit altogether and convert it to cathode bias. (2 new parts)

    - get rid of this bias circuit altogether and build a better negative bias supply off of the 25-0-25 winding. (several new parts)

    side note: don't turn it on with the power tubes in again until you deal with the bias voltage.
     
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  11. J-Turn

    J-Turn TDPRI Member

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    Thanks for the input! I don’t have anything belonging to the Organ outside of the power amp unit in my pics.
     
  12. J-Turn

    J-Turn TDPRI Member

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    I added a 139ohm resistor and here are my new measurements:
    El84 (1) pin9 347v. Pin7 350v
    El84 (2) pin9 347v. Pin7 447v

    6ca4 pin3 355v

    12at7 pin1 237v pin3 109v pin6 325v pin8 (bouncing maybe 25v)

    12ax7 pin1 193v pin6 162v

    Should I add a little more resistance to bring those numbers down slightly to schematic, or is this close enough to not harm anything? PT is now running cool
     

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  13. D'tar

    D'tar Friend of Leo's

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    Yep, I was thinking of this on the way home from work today.


    JT... what do you have for bias voltage on power tube pin 2
     
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  14. Wally

    Wally Telefied Ad Free Member

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    +1 on wanting to know what that negative bias voltage on pins 2 in the EL84s is.
     
  15. J-Turn

    J-Turn TDPRI Member

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    -14.3v for both
     
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  16. elpico

    elpico Tele-Afflicted

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    Rocking. Next you'd check how many watts that -14.3 idles the el84 at. Then you get to stop measuring and have fun. Does the trem work? Sounds like your oscillator is running.
     
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  17. Wally

    Wally Telefied Ad Free Member

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    Okay....now are you ready for the test that will be administered tomorrow focusing on this exercise.....what tube elements can you identify, what is bias voltage, etc???? (;^)
    What does the amp sound like? It will have lost some of that heat in the Sonics along with losing the heat from the excessive current, but it should be operating properly. next exercise would be learning how to measure the current draw and how to figure the plate dissipation. Do some research????
     
    Last edited: Mar 31, 2020
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  18. D'tar

    D'tar Friend of Leo's

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    Bravo Sir! I was assuming the rectified positive half of wave available after the diodes would have the negative half of the wave on the center tap. As you pointed out it was an incomplete circuit on the + side. Thank you

    Possibly able to do this with the components at hand with some creativity!?
     
    Last edited: Apr 1, 2020
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  19. D'tar

    D'tar Friend of Leo's

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    Ha! I bet it sounded pretty radical running wide open!!!!
    Surprised the el84s are still usable!!!
     
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  20. D'tar

    D'tar Friend of Leo's

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    Good job JT!!!
     
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