1. Rest in Peace Paul (Kegger) If you would like to help the family in this time of great sorrow and need, you may donate on their GoFundme page: https://www.gofundme.com/mckechnie-medical-and-funeral-fund?
    Dismiss Notice

One more Eico ST70 rebuild thread

Discussion in 'Eico Place' started by oldman55, Feb 17, 2017.

  1. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    Something not right.
    With the balance pot at 12clock; sound ok from L speaker.
    With balance pot full counterclockwise; high frequency only from L speaker
    With balance pot full clockwise; low frequency only on only from L speaker

    Did the loudness mod and added jumpers but left the existing one. From viewing NJ's photo, thats what it looked like to me.
     
  2. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    This is the mod I did on the loudness. While it indicates adding new jumpers, no mention is made of whether to leave in the two existing jumpers:
    http://web.archive.org/web/20050212191916/http://home.netcarrier.com/~rstevens/st-40.html

    Looking again at NJ's photo it appears that the mods were made with the exception of the new jumpers and his old jumpers remain.

    Guess I'll regretfully clip one set of jumpers and see how it sounds. Working in those tight spaces causes me a lot of frustration.
     
    Last edited: Apr 5, 2017
  3. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,480
    Location:
    New Jersey
    Honestly i don't remember adding jumpers.
    IIRC , The resistors and caps swapped position and that was it.
    (I'm not telling you what i remember is right!)
    Why don't you wait for someone else to chime in.
    If you get no help, I'll try and check later for you
     
  4. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    Here is the outline:
    EicoLoudnessMod.jpg
    Copyright © 1999-2004 by Roger W. Stevens. All rights reserved.

    No mention of existing jumpers. Hate just clipping the existing to test as working in those tight spots with my fumble fingers is a pain. But I think that it may be the solution...if my symptoms don't indicate otherwise.
     
  5. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,480
    Location:
    New Jersey
    You are right,
    Like i said i don't remember
    Going to eat dinner,
    When i come back i'll reread and see if i can shed some light if someone else hasn't already
     
  6. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,480
    Location:
    New Jersey
    It doesn't say anything about removing any existing jumpers. If that's the only change you made and now you you have a problem it would stand to reason something you did made it happen. Did you take before pictures to compare? Did you change values according to article? Double (triple check your work. Look for your jumpers touching other pins. If you have no luck i'll pull the cover off mine and post a better pic.
     
  7. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    Thanks but no need.
    Yours appears to have the original jumpers going from 2-4 and 8-10.
    eico st 70 loudness mod - other.jpg grid.JPG
    At least thats what it appears from that angle.
    I have those same original jumpers and added the 3-5 and 9-11 jumpers.

    So I now have two sets of jumpers in place. Next time I open it back up, I'll just clip the originals and see what is does with only the modified jumpers.
     
  8. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    Clipped original jumpers. Problem fixed. Sounds good on the bench speakers. Time to take it out for a ride.
     
  9. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,480
    Location:
    New Jersey
    I'm glad you found your problem
    I'll have to dig mine out and look.
    It would seem i made a mistake :eek:
    I thought it sounded fine:crazy:
    Hopefully i remember next time the bench is empty and i have free moment.
    Kind of shows the value of pics.
    I'll also have to look and see what i was doing with it in the current configuration.
    If i had to guess without looking I'd say defeated probably doesn't make a difference.
    Engaged probably isn't right
    I probably didn't run it engsged except to test and its been sitting idle.
    Thanks for finding my mistake:thumbsup:
     
  10. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    I would guess that it is in one mode or the other. Dont know if its worth disconnecting, lugging to the bench, hurting your back, taking out a half million screws and putting it all back in place.
    Though it is one of my favorite amps to listen to, I never look forward to having to work on it due to the above.
     
  11. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    Well, I only thought that I was done. The balance only works when the loudness switch is in. Otherwise it is back to one channel.
    Wish I could figure out that switch.
     
  12. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    Seemed to work fine on the bench once I tightened up the tubes, reset bias and reconnected. Though the hum is starting to become annoying. When running, it is in not audible enough to be a problem. It is most notable when idling, with or without a source plugged in. It lessens quite a bit when I touch the chassis. Any it gets angry and motorboats when my cell phone is within a few feet of it.

    Suggestions?
     
  13. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    31,371
    Location:
    Southern NJ
    Does flipping the plug over change anything? Do the hum pots do anything? If they do not do anything, are those pots good? I've had a number of hum pots turn up dead over the years, don't honestly know why. My Pilot tuner/pre had a hum, one of those pots was dead. I was able to fake it with some fixed resistors and it helped quite a bit.

    Other possible, try running a temporary wire from the chassis to the screw holding the outlet cover plate on. If it stops humming, put a 3 wire cord on it and ground the chassis.
     
  14. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    Flipped the plug already. Hum pots adust the hum and are quietest at near center.
    Have old wiring without ground but will see if center screw does anything.
    thx
     
  15. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    Eico ST70 is back on the bench to hopefully find my hum problem once and for all. I am thinking of replacing the cord with a grounded one.
    Since I have the original diode in place, I am going to replace it with a 1N540.

    I am convinced that I have grounding issues. Anything else that I could maybe do?
     
  16. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,480
    Location:
    New Jersey
    While you may in fact have an issue, itd been my experience that these have inherent noise even when working perfectly. I have a low but audible hum/hiss with 91db sensitive speakers. It'd only be worse if your speakers are more efficient.
     
  17. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,480
    Location:
    New Jersey
    I missed the part about you touching chassis and noise being different when you touch it. It suggests some kind of grounding issue.
     
  18. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    31,371
    Location:
    Southern NJ
    try clipping a lead from chassis to a grounded outlet first. If it changes for the better, give the grounded cord a go.

    Also, did you confirm its not a source related issue? I tend to use my ipod or my phone a lot for testing, no chance of ground loops or anything goofy since it has no power cord or other connections to the outside world.
     
  19. oldman55

    oldman55 Well-Known Member

    Messages:
    515
    Swapped diode-no difference. Grounded the chassis to center screw of outlet and hum all but evaporated.

    But Im running the wall outlet thru a two prong variac. ?
     
  20. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,480
    Location:
    New Jersey
    Yeah chassis grounded acts like a giant noise shield.
    Variacs can add noise too.
     

Share This Page