EICO ST-70 Problems Consolidated Thread

Discussion in 'Eico Place' started by 71R/T, Jan 27, 2018.

  1. 71R/T

    71R/T AK Subscriber Subscriber

    Messages:
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    Location:
    Triangle Area, NC
    c42_old.jpg c42_new.jpg Ok so I have stayed up way too late too many nights lately with this piece of gear and upon Bob's advice I have decided to start over with a single thread. I am going to ask about a single issue first before I even discuss any of the other issues as this single issue may be the causative factor for the others. Okay so the power suppl;y voltages are as follows:

    S- 456v
    T- 406v
    U- 376v
    V- 225v
    W- 256v
    Y- 295v
    Z- 179v

    I first thought that I had high ripple on the supplies but i have since verified that all of these supplies are clean and not full of ripple. I had a grounding issue with the scope probe. Supply Y is the first DC supply after the first two 10K ohm dropping resistors and by the math for a voltage divider circuit this voltage looks correct for the voltage divider circuit with no load. However, we know that is not correct as the two 12AX7 tubes in the phono circuit should be conducting and dropping this voltage but we also know that this is not happening. The big question is why not? Looking at the schematic for VI and V2 I have verified that each resistor is good. There just isn't that much in this circuit. Please tell me what I am missing here. Is it a grounding issue?
     

     

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  2. Bob

    Bob AK Subscriber Subscriber

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    I thought we were going to consolidate - this seems to be a new thread and ignoring all the input
    from earlier ones. we can do this your way but it will take a lot longer.

    post a good schematic of just the PS with all the letters from above, marked.

    1. what did you change/mod/unsolder-than-solder-back - list it all. by component, by location
    2. how do you know the two resistors are good?
    3. if you're NOT going to post a schematic then walk us through those
    dropping resistors, before you get to Y.
    4. that "S" voltage is too much. what is the cap rating tied to that point.
    5. is that "S" voltage proportional to the increase in AC line voltage from
    design spec to wall measurement. eg original-AC:design B+ is proportional to measured-AC: 465?
    6. take close up pictures - you may simply have an undersized resistor that accounts for the higher
    than design spec voltage. pull all changed resistors at one end and take clear picture of the resistance
    measurement.

    again if you don't post a schematic, then describe joint by joint from tube rectifier (voltage1) to first
    component (name the component, cap or resistor, its value and the voltage immediately after), and so
    on and so forth. two resistors before doesn't tell us anything and may actually be the problem. it can
    also be where we decide to make changes to get ALL those points back in line.

    on dynaco pas, I've had to up the resistance values to get the various stages voltages down back to
    design center values. sometimes 2x. each and every PAS I've had required these changes, NONE
    had spec-defined voltages.

    Yours has already exhibited this symptom.

    I'll wait for the complete set of responses and the schematic.
     
  3. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
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    Location:
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    I just did a quick glance at the Digital docs (i think is the sams) and the one on Hifi engine which is the eico manual
    BTW you might find that useful (also you may want to blow up sections of drawing)
    All voltages seem reasonable except for Y
    What are the other voltages on the tube?
    squinting at drawing i think i'd very carefully test and check all components and connections at V1, V2
     
  4. 71R/T

    71R/T AK Subscriber Subscriber

    Messages:
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    Location:
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    I discussed with Dave Gillespie and he said that with the addition of the 12ax7 in the line stage replacing the 12dw7 along with the Tronola mods that higher supply voltages are normal. Now if I can figure out the idle hiss issue I will have it licked once and for all. I have replaced all of the resisters with metal film but I still have some idle hiss.
     
  5. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
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    Location:
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    I think i said before, these don't really get silent (that's straight from Dave). It a nature of the beast design flaw. If you're using 90 db speakers its there but quiet. if you're using cornwalls it might not be a great match.
     
  6. Bob

    Bob AK Subscriber Subscriber

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    congrats! seems the hum(s) has been fixed, all the over voltages are now explained,
    and I wouldn't worry about the hiss,

    now enjoy the music.
     

     

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  7. 71R/T

    71R/T AK Subscriber Subscriber

    Messages:
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    Location:
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    Thanks for all of your help!!
     

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