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Buzz In Portable Cassette Player

Discussion in 'Troubleshooting and Repair' started by Jesuit24, Nov 28, 2014.

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  1. Jesuit24

    Jesuit24

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    Sep 3, 2014
    So I've been tinkering with an old Walkman style cassette player, but I've noticed it's starting buzzing/humming. This was after moving the 3xAA battery pack, so could that be the issue? I know it could potentially be a grounding issue if the mains plug was concerned, but the buzz problem is when using batteries. Could a damaged battery pack be the problem?
     
  2. Gryd3

    Gryd3

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    what the pitch of the 'buzz'?
    Could be picking up external noise, or internal noise... they have a motor in them, and motors are horrible at creating noise.
    Show a pic?
    Explain the detail the change you made?
     
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  3. Jesuit24

    Jesuit24

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    Sep 3, 2014
    The pitch is in the mid range. When I remove the drive belt from the motor, and let the motor spin on it's own, the buzz is extremely quiet, so perhaps it's the drive belts making noise?
     
  4. Gryd3

    Gryd3

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    the belt drive would create a load on the motor making it work harder causing a higher current draw. Makes sense, but we can't say for sure.
    The modifications are still unknown aside from 'moving the battery pack'.

    Where did you run the wires?
     
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  5. Jesuit24

    Jesuit24

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    Sep 3, 2014
    I was making some cosmetic mods in a bid to make the walkman into an operational film prop, running a headphone extension through the interior and out the other side as a very rudimentary method to flip the headphone jack to the bottom of the walkman. The female jack was wedged pretty good in between the motor and the battery housing and I may have damaged the battery pack in the process. I removed the extension to return it to normal functionality and that's where I left it.
     
  6. Gryd3

    Gryd3

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    K . Here are some things to look out for when considering noise.

    Devices like DC motors cause a lot of noise. They are usually (but not always) dampened by the use of small capacitors which are usually connected right to the motor. There can be anywhere from 1-3 of these to help reduce the noise.
    Other than that, any 'high power' lines can induce noise in adjacent wires. High power does not always have to be 100s of volts or a few amps, it can simply be the main current path... ie, directly from the batteries, or the branch that feeds the motor circuitry.
    That would be common sources of noise in that device, but you need to consider the other half too... You have somewhere it has come from.

    Now when you are talking about noise, there are certain parts of the circuit which are immune, and parts that are very very sensitive to it.
    In particular, we need to take special care of any wires that are carrying a very low voltage, such as the wires from the mic, or the wires that run to the magnetic tape heads. Because they deal with such low voltages to begin with, the smallest amount of noise will be much more noticeable... It's always good to keep noisy wires like power wires and motor wires etc. away from these types of wires. If you cannot physically keep them away, you can use some other techniques to help mitigate how much noise is picked up.

    Use shielded wire. This will help prevent noise from actually reaching the sensitive signal wires.
    Use twisted pair. This is a bit odd to explain... but a signal from a mic, or signal from/to a tape head is usually carried with 2 wires. A signal, and a ground (or return). By twisting these wires, they both pick up equal noise. This essentially allows the noise to cancel itself out.
    Use filter components. This is harder to implement, but the use of capacitors and inductors can help eliminate noise... The reason this is harder to implement is because these filters are quite dumb and will filter anything in the frequency range they are built for... so you may end up with a song or speech that sounds like crap because the 'noise filter' chopped out certain notes.

    Anyway. If I were you, I'd pop it apart again and see if changing some wire positioning helps.
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Gryd3 asked you for some photos
     
  8. Jesuit24

    Jesuit24

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    Sep 3, 2014
    I rearranged the wires, although there is very little room for movement since they're all tightly packed with specific holes in the board for them to pass through. When I first opened it, the red wires in the bottom left were grouped up in that hole and the motor wires on the right were tucked alongside the battery compartment /the motor but there doesn't seem to be any change in the buzz. In fact, manuvereing the wires caused no change in the buzz whatsoever even when operation, just the occasional louder buzz when touching some of the contacts.

    The black wire in the top left is very tight; I can't move that at all.

    Before, the negative battery wire came off, which I replaced with regular copper wire, but given the thickness of the original wire, I don't think the original was shielded. I don't think any of them are.

    The other two red wires next to the positive battery wire belong to the copper contacts of the play button 'switch' if that helps and the tight black wire in the top left deals with stereo seeing as I get a loud buzz in each ear when I ground them.

    Here are pictures, one annotated:

    http://i60.tinypic.com/2v8he7q.jpg
    http://i62.tinypic.com/qq4c5d.jpg
     
  9. Gryd3

    Gryd3

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    I am a little unsure on the location of the original battery pack, or if you've simply made a replacement.
    The black wire you noted at the top is a signal wire and is very sensitive to noise. Take a look and help ensure that this wire has not been moved, or that another wire has not been moved near it.
    Noise can be transferred from one wire to another when they are run side-by-side.
    The tricky part about this though, is that the noise could wither be radiated from the motor, or from any of the power wires connected directly/indirectly from it. If you are certain this is only after the modification, then it would make sense that it is a wire placement issue.
    I can't give you a magic fix though, but I can recommend adding some 0.1uF capacitors to the motor. These capacitors will help suppress the noise generated directly from the motor. You can use 1-3 of them, they are quite small, and need to be directly connected to the motor.

    1-Cap : Solder directly to the power tabs on the motor.
    2-Cap : Solder each of the 2 capacitors to one power tab, and to the metal case of the motor.
    3-Cap : Do both of the above.

    Here is a sample.
    http://www.pololu.com/docs/0J15/9

    If the noise is indeed caused by the motor, this should help.
    (The noise from a motor is caused by the brushes sliding along the armature contacts. Dirty, corroded, or badly warn contacts/brushes will increase the amount of noise generated.)
     
  10. Jesuit24

    Jesuit24

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    Sep 3, 2014
    The pictured battery pack is the original one in it's original place. I managed to put a crack through the board and along a track which I thought may be the culprit but after hooking up a replacement battery holder today, I still get the same problem (it's a miracle the original still works but they were built to last I guess). There aren't any wires near the black wire either on or underneath the board except for the taped down blue and yellow wires (which are tight enough I can't move them either.) I see what you mean though. I've taken the motor housing apart and now have the motor running separately in my hand, moving it closer to the black wire does increase the amount of noise, but further away, it maintains a steady buzz without getting quieter or louder. Even as far away as the wires will allow, it maintains the same level of buzz. I twisted the motor wires together, and maybe I'm hearing things but it seems to be a slight improvement. I'll try the capacitors. I have a spare 10nF capacitor and a few 1uF electrolytic capacitors on hand if they'll do (either the one regular capacitor or three electrolytic capacitors). Otherwise I'll go out and get some. Is there any disadvantage to attaching capacitors to the motor? I'm guessing three is better than one, but are there any downsides to using three (like reduced motor speed?)
     
  11. Bluejets

    Bluejets

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    Oct 5, 2014
    Pic of what you have now would be helpful I would imagine.
    e.g. you mention 3 x AA but i only see 2 (unless one is tucked underneath somewhere)
    What happened to the cracked track?
    Looks like you made a new housing out of wood or similar.
    Certainly gives new meaning to the word "hack".
    Sounds an awful lot like shielding problem to me.
     
  12. Gryd3

    Gryd3

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    There are no disadvantages that I can think of to using capacitors on a DC motor other than increased cost, and a slight size increase.
    Keeping the values small will make any delay negligible. This is a DC motor, so the addition of capacitors that are too large will create a very slight delay in the motor turning on, but once on, the motor will run at it's regular speed.
    Good call on twisting the wires together, that is another common technique that I had forgotten to mention.

    Have any other wires been altered or replaced other than the negative wire on the battery pack?
    You also mentioned part of the board cracked, it is entirely possible that it isolated or disconnected a non-integral portion of the circuit... That black wire at the top has three conductors in it. There is a left and right signal wire, and a ground (or neutral) wire. Any unintentional alteration to the signal or ground path on the board from those wires could certainly create the buzzing you are experiencing.

    It is also a possibility that the buzzing you hear was always there to begin with, and perhaps you are noticing it more now?
    I can't say for certain, but if the wiring is not the culprit, that leaves the cracked board or any other unintentional or unknown damage.

    You are more than welcome to try the capacitors you have on hand. They are not the sizes advertised on the link I gave you,, but they should work.
     
  13. Jesuit24

    Jesuit24

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    Sep 3, 2014
    The battery housing is the original one. It's just an old fashioned brown board, with the third battery underneath the two on top.

    After some handling, the positive wire came loose at the battery end, which I resoldered back in place. And in order to twist the motor wires, I unsoldered the red wire temporarily, but other than that, no.


    Something like that is what I thought might be the problem, although I bought a couple of battery packs today (a 2xAA and a 1xAA that I wired together) and temporarily ran it off the new housing to find the same hum as before (I didn't unsolder the original battery housing, I just laid the new wires over the old ones). Using a mulitmeter over the positive and negative contacts on the board, I get 4.29V when the unit is off, 4.24V when it's on but without the drive belt attached, and 4.19V with the drive belt on. It does seem the drive belt is tugging more volts out of it.

    This is the underside of the motor.
    http://i59.tinypic.com/35n0ryg.jpg
    Am I supposed to attach the capacitors to the wires? There's also a black plastic wrapping which presumably needs removing to attach the capacitors to the casing.
     
  14. Gryd3

    Gryd3

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    Jun 25, 2014
    I'm willing to bet that it's not a problem with the battery housing. It does not make sense.
    We are either missing something, or your use of modern digital tech has you hearing things we used to tune out as kids ;)

    I'm not sure where to direct you from here.
     
  15. Jesuit24

    Jesuit24

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    Sep 3, 2014
    I reckon it's the motor, seeing how it runs buzz free without the drive belts. It's just the motor wires run inside the motor casing rather than attaching to external terminals. Could I strip a small piece of each motor wire close to the casing and solder the capacitors to them instead of the motor terminals which I assume are concealed inside the motor?
     
  16. Bluejets

    Bluejets

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    Oct 5, 2014
    You would not have inadvertantly tapped into the input with your so-called cosmetic mod in any way?
    Those voltages seem off a bit but would not cause hum , just 2 bob batteries I suspect.
    The "cosmetic mods" would be worth a look though.
     
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Do you still have continuity from the metal chassis to the 0V rail of the circuit?
     
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