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LOUD buzz/hum in old Audiotronics 300-E Classroom Record Player

Discussion in 'Turntables' started by folksnake, Dec 5, 2018.

  1. folksnake

    folksnake New Member

    Messages:
    15
    Location:
    Central Maryland, USA
    Hi Folks-
    I'm working on restoring an old Audiotronics record player to listen to some Recordio 78rpm home recordings I came across, and I need some guidance. When I got it, it made barely any sound. I looked the components over and found two big electrolytic caps that were leaking, and replaced them. That helped a lot. As did replacing the ceramic cartridge and needle.

    I get decent sound out of it now, but it also has a loud, low hum that doesn't respond to the volume control--it's always the same (loud) volume.I have poked around looking for something obvious--a loose wire, bad ground, damaged component--but nothing is jumping out at me.

    What might be my next move? Any suggestions are welcome, as I do have a little experience with this sort of thing, but am no means an expert...

    Thank you!

    Audiotronics 300-E schematic.jpg Audiotronics 300-E.jpg
     

     

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  2. revox-b77

    revox-b77 AK Subscriber Subscriber

    Messages:
    3,037
    Location:
    Fingerlakes, NY
    Sounds like it is time to check all of the caps and transistors.
     
  3. folksnake

    folksnake New Member

    Messages:
    15
    Location:
    Central Maryland, USA
    Thanks! I guess I know what I'll be doing this weekend.:)

    I also assume that the resistors are less likely to have failed--is that a reasonable assumption?
     
  4. 63pontiacgp

    63pontiacgp "Tubes Smell Good" Subscriber

    Messages:
    2,227
    Location:
    Scottsville Kentucky
    Did you replace the ceramic cart with another ceramic? If you didn't that could be the cause of the noise.

    Mike
     
  5. folksnake

    folksnake New Member

    Messages:
    15
    Location:
    Central Maryland, USA
    I did, Mike. I'm pretty sure it must have been the original, so I got a direct replacement from VoM. The old one had just a nub on each side...
     
  6. tubeactive

    tubeactive AK Subscriber Subscriber

    Messages:
    237
    Location:
    NJ
    Nice machines, especially for mono discs... That lucite or plexiglass post lights up, right ? Cartridge was an Astatic 81T or 89T ? Or, some other turnover ceramic ?

    Buzz or low level hum can still mean the electros are leaking AC voltage, where the DC voltage is measured. If you read AC volts on that 34V DC rail, some cap is still electronically leaky. Or, you may have a grounding problem. Physical grounding and electrical grounds might have gotten "jostled" during your resto. Did you reconnect the new electro caps to the same exact points they came from ?

    The leads on any gain circuit near a mechanical motor, need "proper shielding" or re-routing to avoid hum fields. Even though the ceramic carts have higher output than magnetics, any phono stage is susceptible to grounding noises. Sometimes, even moving the AC wiring away from the amp reduces noise...I sure hope you find the trouble quickly...
     

     

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  7. folksnake

    folksnake New Member

    Messages:
    15
    Location:
    Central Maryland, USA
    Yes! That post is a pretty cool feature, and easily accomplished...a wheat bulb taped to the bottom of it :) The cartridge was an OEM replacement for one of those two--not sure right now, but that's them.

    The caps I replaced were placed in the exact positions as the old ones, but I think I'm going to have to go back over everything just to be sure. Thanks for the info and encouragement!
     

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