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low hum on Realistic receiver

Discussion in 'Electronic Repair' started by buddy, Jan 15, 2006.

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  1. buddy

    buddy Guest

    I just acquired an old Realistic STA-45B stereo receiver. It works
    fine except that it produces a low hum to the speakers as soon as it is
    turned on. It does not get louder as the volume is turned up. It is
    also in the headphones.

    The hum occurs even if no inputs are connected, just the speakers or
    headphones. I took the cover off and sprayed everything with
    electrical contact cleaner. All screws from boards to chassis were
    tight.

    It is annoying but if I can't fix it I will use it like this if I must.
    I only want to use it to go from my computer's soundcard to the Aux
    inputs, to use stereo speakers instead of PC speakers. I do home
    recording and need better sound to mix/master.

    I do not have any experience to diagnose/repair audio equipment, but a
    little long-ago experience testing components on computer boards. I
    have a multimeter.

    Can someone describe what to do to track down the problem?
     
  2. Arfa Daily

    Arfa Daily Guest

    The problem here is almost certainly the main smoothing capacitor. This will
    be the biggest electrolytic capacitor in the unit. Value will be 1000 or
    2200 or 4700uF or something similar. Replace with similar value and voltage
    rating, and there's a 95% chance that the hum will go away.

    Arfa
     
  3. Ken

    Ken Guest

    As Arfa said, it is almost certainly an electrolytic cap. This is a
    situation where an ESR meter would be helpful. In fact, I had a
    receiver that developed the same type of problem and found it in minutes
    by using an ESR meter. Just one problem found could pay for the meter
    in time saved.
     
  4. Asimov

    Asimov Guest

    "buddy" bravely wrote to "All" (14 Jan 06 23:10:07)
    --- on the heady topic of "low hum on Realistic receiver"

    bu> From: "buddy" <>
    bu> Xref: core-easynews sci.electronics.repair:355180

    bu> I just acquired an old Realistic STA-45B stereo receiver. It works
    bu> fine except that it produces a low hum to the speakers as soon as it
    bu> is turned on. It does not get louder as the volume is turned up. It is
    bu> also in the headphones.

    bu> The hum occurs even if no inputs are connected, just the speakers or
    bu> headphones. I took the cover off and sprayed everything with
    bu> electrical contact cleaner. All screws from boards to chassis were
    bu> tight.

    bu> It is annoying but if I can't fix it I will use it like this if I
    bu> must. I only want to use it to go from my computer's soundcard to the
    bu> Aux inputs, to use stereo speakers instead of PC speakers. I do home
    bu> recording and need better sound to mix/master.

    bu> I do not have any experience to diagnose/repair audio equipment, but a
    bu> little long-ago experience testing components on computer boards. I
    bu> have a multimeter.

    bu> Can someone describe what to do to track down the problem?


    There might be an open rectifier but if the rectifier isn't discrete
    then test the bridge rectifier. However, given the age of the
    equipment, it is more likely to be a problem with dry electrolytic
    supply filters. Test all electros with an ESR meter. (beg, borrow, but
    don't steal!)

    A*s*i*m*o*v

    .... A stereo system is the altar to the god of music.
     
  5. Guest

    Ac hum in an audio device is very easy to find with an o'scope. It is
    obvious which filter capacitor is not doing its job when looking at it
    with the o'scope. It is also possible that a power supply bypass
    filter near the audio circuit is also weak and will also be easy to
    find in this way.
     
  6. In addition to the main large filter capacitors, also look for any
    other power supplies that might have bad caps. I found a small
    capacitor that was high-ESR in a Reaslitic STA-20.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
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    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  7. JANA

    JANA Guest

    Change the main filter capacitors, and most likely the hum will be fixed.

    --

    JANA
    _____


    I just acquired an old Realistic STA-45B stereo receiver. It works
    fine except that it produces a low hum to the speakers as soon as it is
    turned on. It does not get louder as the volume is turned up. It is
    also in the headphones.

    The hum occurs even if no inputs are connected, just the speakers or
    headphones. I took the cover off and sprayed everything with
    electrical contact cleaner. All screws from boards to chassis were
    tight.

    It is annoying but if I can't fix it I will use it like this if I must.
    I only want to use it to go from my computer's soundcard to the Aux
    inputs, to use stereo speakers instead of PC speakers. I do home
    recording and need better sound to mix/master.

    I do not have any experience to diagnose/repair audio equipment, but a
    little long-ago experience testing components on computer boards. I
    have a multimeter.

    Can someone describe what to do to track down the problem?
     
  8. buddy

    buddy Guest

    Thanks to all who replied, and "southmetro" who emailed. Sounds like
    the first thing I should try is:

    Replace main "smoothing" "large filter" capacitors.

    So I googled to get an idea what they look like. I saw photos of the
    small "M&M" ones, and the cans. I assume I am looking for the cans.

    Could someone help me identify the ones to replace?

    Please download these 2 photos:

    www.idcomm.com/personal/buddyboy/realistic-sta-45b-top.jpg
    www.idcomm.com/personal/buddyboy/realistic-sta-45b-bottom.jpg

    In the "top" photo, there are 2 large cans next to each other, in the
    middle of the board at the top of the photo (which is rear of the
    receiver). They are lavendar color, and say

    1000 uF 25 V
    CE
    1 9.

    In the "bottom" photo, there is a small board in the top of the photo,
    just left of center. The board says "AC Input". The cans are:

    (lavendar one)

    CE (W)
    1000 uF 35 V

    looks like it has some kind of glue on the bottom in addition to being
    soldered

    (silver one)

    + Nippon Chemi-Com
    + CE W
    + 35 V 470 uF

    which are the one(s) to replace?

    Questions:

    -To replace, should I :

    1. clip leads, leaving about 1/8 " nub to solder new one to (seems to
    me this is easiest way) or
    2. unscrew the board and unsolder where the lead comes through the
    board on the other side

    - do I need to do anything to "discharge" them first? leave unplugged
    for some amount of time, or something else?

    - unrelated question - there is no light for the FM scale and
    indicator. Is this the way the receiver was made? In the "top" photo
    you can see the bulbs for the two meters, but I don't see anything to
    light the FM scale and indicator. Hard to believe it was made this
    way. But no biggie, as I said in my OP I only want to use the Aux
    input.

    Thanks,
    Rod
     
  9. Arfa Daily

    Arfa Daily Guest

    I would go for the big grey one in the middle of the underside photo first.
    This would appear to be the main smoothing block. The board at the back left
    on the topside photo, looks like it is part of the output stages. The pair
    of axial caps in the middle of that board may be supply rail decouplers or
    output caps. Likewise the pair of radials at either end. These caps are much
    less likely to be the cause of the problem than the grey one.

    If you can get access to a 'scope, or have a buddy with one, I would feel
    inclined to have a look at the ripple levels on the grey cap. It looks as
    though it may be a ' dual ' type having 3 tags. One is usually common
    negative, whilst the other two are the positives of two different caps in
    the one can. They may or may not be both the same value, but are usually the
    same working voltage - ie could be 2200uF + 2200uF @ 35v, but could also be
    2200uF + 1000uF @ 35v.

    If you do finish up replacing the wire ended caps, they should be replaced '
    properly ' that is by removing the old one from the board, clearing the
    holes, and resoldering the new caps' wires directly back to the print.
    Leaving a cut end from the old one, and trying to tack solder the new one is
    going to give you problems with the original solder crystallising as it
    melts when you try to solder the new wire. As far as I can see, both sides
    of all the boards are get-at-able, without having to unscrew them. Or am I
    missing something in what you're asking ?

    Arfa
     
  10. buddy

    buddy Guest

    No, that is what I was asking. But I realized that it's fairly easy to
    do the "right" way. I replaced the two caps in the Input board in the
    "bottom" photo before I saw your post, but no change.

    After I saw your post I removed the big gray one - had to drill out the
    screw $%& for the clamp - it was just a regular one, 2000 uF and 50 V.
    I replaced it, but still no change.

    I think I'm at the limit of what I can do with no test equipment so
    I'll have to use it as-is for now.

    I really like the look of the old Realistic with machined Alum knobs
    and brushed Alum faceplate and solid wood case. The sound is very
    "clean".

    Reminds me of a separate tuner/amp combo from Technics I had in the
    70's.

    Thanks for trying,
    Rod
     
  11. Asimov

    Asimov Guest

    "buddy" bravely wrote to "All" (17 Jan 06 13:55:16)
    --- on the heady topic of "Re: low hum on Realistic receiver"

    bu> From: "buddy" <>
    bu> Xref: core-easynews sci.electronics.repair:355552

    bu> I really like the look of the old Realistic with machined Alum knobs
    bu> and brushed Alum faceplate and solid wood case. The sound is very
    bu> "clean".

    bu> Reminds me of a separate tuner/amp combo from Technics I had in the
    bu> 70's.
    bu> Thanks for trying,
    bu> Rod


    IIRC Realistic were likely re-branded Sharp. You will find that
    styling and look in Sharp components of the same period.

    A*s*i*m*o*v

    .... When I was your age, we carved transistors out of wood.
     
  12. Arfa Daily

    Arfa Daily Guest

    Transistors !! Wood !! ??

    We had to make tubes out of old jamjars and paper clips, all whilst we lived
    in a cardboard box in the middle of the highway, watching the snow come in
    through the open top. Sounds to me like you had it easy ...

    Arfa
     
  13. Michael Ware

    Michael Ware Guest

    IIRC Realistic were
    likely re-branded Sharp. You will find that
    Tape decks, yes. Receivers were made to RS specs by unknown factories.
     
  14. jakdedert

    jakdedert Guest

    Some of the mid-70's ones were identical to Hitachi models....

    jak
     
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