1969 with low plate voltage. Help needed

Discussion in 'Building the Classics' started by Chris4189, Nov 5, 2019.

  1. Chris4189

    Chris4189 Well-Known Member

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    Brief history of the amp:

    A local mom and pop music store in my little town is closing and is in the process of blowing out what little stock they have and what unclaimed repairs they have. Long story short they had a ‘69 1959 that was dropped off years ago for a repair and was never picked back up.

    OT is the Original C1998, choke appears to be original but the PT has been replaced with a T4145 that I’m guessing hails from the 70’s or 80’s. A three prong power cable has been added and the death cap was been removed. I assume this was the repair. The owner of the store passed away and his son is clearing the stock and stated that his father used to send the amps out to be repaired by a fellow that retired from NASA. Anyway, I scored this diamond in the ruff for $1000.00.

    Here is where I am stuck:

    I replaced the screen grid resistors with 1K 7watt Welwyn’s and two new BC 10uf Axial 160v bias caps. There were no power tubes present so I cleaned the sockets and then decided to take some readings before proceeding. Readings are as follows:

    V4: Pin 2&7 3.33 VAC both pins are lose
    V5: Pin 2&7 3.33 VAC pin 2 lose
    V6: Pin 2&7 3.33 VAC
    V7: Pin 2&7 3.33 VAC

    PT bias wire at board 100 VAC
    V4: Pin 5 0 VDC
    V5: Pin 5 0 VDC
    V6: Pin 5 -39.5 VDC
    V7: Pin 5 -39.5 VDC

    Pins 3 & 6 on V4-V7 all read 242 VDC

    Standby switch is reading 250 VDC

    I double checked my screen resistors and all are reading 989k, sockets all appear to be grounded and wired correctly following a diagram for a ‘69 via the metro forum.

    I’m ready to bang my head into the wall. Any help would be greatly appreciated. Amp is for 120v
     
  2. Chris4189

    Chris4189 Well-Known Member

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    So, after a little more investigation, I noticed that the 56k resistors on the filter caps were not running through all three lugs. I removed them and when I tried to straighten the legs for a proper install they about crumbled to dust. Have those heading this way from Valvestorm. Much to my surprise the filter caps while wired right were not bridged together on the positive lugs. Maybe this will cure my issues?
     
  3. neikeel

    neikeel Well-Known Member

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    Sounds like someone was deliberately running it with lower filtering, for a browner, softer more tactile amp (nearer to 68 values).
    Do you know how long it was sitting there for?
    When you say the ins were loose, the pins are often loose unless there is a valve inserted, unless you mean very loose and about to fallout?
    I doubt that your issue is cap related, you need to check the orange (or green) wire under the board that goes from the node where the 0.022uF couplers meet with a 220k resistor. Obviously one of them is intact (as you are getting -40vac on V6 and V7. The wire will go to pin 5 of V5 and then to pin 5 of V4, does yours have 5k6 swamps on each pin 5 or the earlier type with a 1k5 just to V4 and V7 each?

    You need to check that the HT secondary centre tap wire is hooked up correctly between the first two filter caps, if that is floating it will give just the voltages you describe.
     
  4. Chris4189

    Chris4189 Well-Known Member

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    Thanks so much for replying!
    Pins are just lose, not about to fall out.

    What exactly I’m I looking for in regards to the orange and green wire? Both are intact and go to the 1.5k resistors on V5 and V6

    Mine has the 1k5w on pin 4&7. I just replaced them.

    PT is wired as follows:


    Green wires: Both grounded by F6 filter cap

    Black wires: pin 2 & 7 on V7

    Purple and blue: taped off

    Yellow: taped off

    Orange: hooked to bottom Buss wire running thru standby/main/polarity switch

    Red: Hooked to the little board where the death cap was.

    White: bias at 27k resistor

    Blue and grey both going to standby switch


    Should the yellow wire be left taped off? Grounded? Most diagrams have the yellow wire going to either F4 or F5 filter cap?
     
    Last edited: Nov 5, 2019
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Location:
    Start the reactor... Free Mars!
    Output tubes out.
    power on, standby set to "operate."
    Set meter for AC volts and measure the the 2 wires that go to the standby switch from the PT.
    Black probe on 1 side of the standby switch, red probe on the other side of the standby switch.
    What is your AC volts reading there?

    1959power.png
     
  6. Chris4189

    Chris4189 Well-Known Member

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    360 VAC
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Location:
    Start the reactor... Free Mars!
    Can you find the Center tap of the HT secondary?
    It's hooked between 2 filter caps.
    Black probe on the center tap where it's hooked between the filter caps.
    Red probe on one side of the standby switch. What is the AC voltage?
    Red probe now on the other side of the standby switch, what is the AC volts there?

    1959power.png
     
  8. South Park

    South Park Well-Known Member

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    The filter cap that is circled in red is a voltage doubler it has a floating ground one of the caps might be bad. All caps should be replaced
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Nice of you to figure that out using no information.
    You must be a member of the psychic friends network ? :lol:
     
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  10. Chris4189

    Chris4189 Well-Known Member

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    I’m still learning so take it easy on me. My caps are wired just as in this diagram. Where do you want me to measure?
     

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  11. ampmadscientist

    ampmadscientist Well-Known Member

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    1959CT.png
     
  12. South Park

    South Park Well-Known Member

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    Check the data sheet for the floating ground I think it is the yellow wire on the drawing is the ground and voltage on all caps
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Location:
    Start the reactor... Free Mars!
    Connect the black probe to the place I'm showing (above).
    Power tubes out.
    Power on, standby set to operate.
    Red probe:
    check AC voltage on both sides of the standby switch. What is the AC voltage on one side, then what is the voltage on the other side of the standby switch?
     
  14. Chris4189

    Chris4189 Well-Known Member

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    Yes sir. So black probe where black circle is?
     

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  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    1959CTbrown wire.png
     
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Yes black probe to where black circle is.
    Set meter for AC volts.
    [​IMG]
     
  17. Chris4189

    Chris4189 Well-Known Member

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    The brown wire is actually a red wire and it comes of .22uf cap. That’s closest to the filter caps.
     
  18. Chris4189

    Chris4189 Well-Known Member

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    Okay 180 VAC on both sides of the switch. Now keep in mind I’m waiting on the 56k resistors for the appropriate caps.
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    Location:
    Start the reactor... Free Mars!
    Tubes out.
    Power on, standby set to "warm-up."
    Red probe to pin 7 of output tube socket.
    Black probe to pin 2 of output tube socket.
    What is the AC voltage?
     
  20. Chris4189

    Chris4189 Well-Known Member

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    Which one?
     

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