Connect with us

unit hums loudly regardless of volume

Discussion in 'Electronic Repair' started by tempus fugit, Oct 27, 2005.

  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I'm trying to repair an old tube portable record player. When it is on, it
    hums really loud, regardless of the volume. I assumed that this must be a
    problem with filter caps, but they test OK for ESR and shorts. 1 of the
    problems is that there are 3 or 4 in a can, so I don't know what the values
    are supposed to be.

    Is there anything else that may cause this symptom? I suppose I could
    parallel some caps across the existing ones, but I don't even know what the
    values are supposed to be.

    Thanks
     
  2. Mike Berger

    Mike Berger Guest

    You could be getting hum through the tonearm wires to the pickup,
    but I'd bet on filter capacitors.
     
  3. CJT

    CJT Guest

    One or more diodes in the power supply (assuming it uses them) is
    shorted. If it has a rectifier tube instead of semiconductor diodes,
    then either the tube is shorted or, if it has its own filament supply
    (e.g. 5U4), then that supply could be shorted inside or outside of the
    power transformer (assuming it has a transformer), although there's a
    pretty good chance the tube would be glowing bright red if that were
    the case. If it's all tube and has no power transformer, then you've
    got a potentially lethal hot chassis and you'd better be d*mned careful.
    Check for shorted line bypass capacitors if it's this last case.

    Or it could be one or more capacitors, as earlier suggested, or a break
    in some shielding.

    As you can surmise, additional information about the circuit involved
    could narrow the possibilities (and possibly suggest others).
     
  4. tempus fugit

    tempus fugit Guest

    Thanks for your reply.

    Sorry, here's some more info:

    There is a power transformer.
    Only 1 diode that I could see.
    Measured voltages at cap in question was ~150v.
    Only 2 tubes - 12AX7 and can't remember the power tube offhand.

    There was also some small device which I assume was a transformer that hadd
    100mA marked on it. It looked like a trasformer with cooling fins.
     
  5. Asimov

    Asimov Guest

    "tempus fugit" bravely wrote to "All" (27 Oct 05 11:17:42)
    --- on the heady topic of "unit hums loudly regardless of volume"

    tf> From: "tempus fugit" <>
    tf> Xref: core-easynews sci.electronics.repair:346437

    tf> Hey all;

    tf> I'm trying to repair an old tube portable record player. When it is
    tf> on, it hums really loud, regardless of the volume.

    Have you tried turning the ac plug around?

    Which way the plug is inserted matters with these. Let the record
    player warm up, then insert the plug one way or the other, and then
    paint a mark for which was the quietest way. One way: lots of hum, the
    other: quiet hiss.

    Those old tube units used to get B+ directly from the powerline (or
    sometimes a voltage doubler) and used a large value resistor bypassed
    by a 0.01uF to RF ground the tone arm shielding to neutral. The power
    for the filaments often came from a secondary winding in the motor
    coil. Don't use this type of record player near the bathtub. Lethal!

    Another possibility is a broken wire at the cartridge.

    A*s*i*m*o*v

    .... Just a little force field zap.
     
  6. CJT

    CJT Guest

    If the cap had DC on it, the diode is probably ok. The capacitor(s)
    could be dried out (although I think you said you checked them, so
    that's not too likely). From there, you move toward shielding type
    problems as already described elsewhere in the thread.
    If one of the tubes had an internal heater-to-cathode short (or for
    that matter, a short at the tube socket), that could cause a lot of hum.
    Such shorts are not uncommon.

    At any rate, now you have plenty of ideas to consider.
     
  7. TimPerry

    TimPerry Guest

    possibly a selenium rectifier. back in the old days they hadn't figured out
    to make high voltage silicon diodes yet.

    the usual warnings about selenium: emits poison gas when it burns

    replace with modern diodes is recommended.
     
  8. tempus fugit

    tempus fugit Guest

    Thanks for the suggestions guys. I'll try some of these things out.


    "tempus fugit" bravely wrote to "All" (27 Oct 05 11:17:42)
    --- on the heady topic of "unit hums loudly regardless of volume"

    tf> From: "tempus fugit" <>
    tf> Xref: core-easynews sci.electronics.repair:346437

    tf> Hey all;

    tf> I'm trying to repair an old tube portable record player. When it is
    tf> on, it hums really loud, regardless of the volume.

    Have you tried turning the ac plug around?

    Which way the plug is inserted matters with these. Let the record
    player warm up, then insert the plug one way or the other, and then
    paint a mark for which was the quietest way. One way: lots of hum, the
    other: quiet hiss.

    Those old tube units used to get B+ directly from the powerline (or
    sometimes a voltage doubler) and used a large value resistor bypassed
    by a 0.01uF to RF ground the tone arm shielding to neutral. The power
    for the filaments often came from a secondary winding in the motor
    coil. Don't use this type of record player near the bathtub. Lethal!

    Another possibility is a broken wire at the cartridge.

    A*s*i*m*o*v

    .... Just a little force field zap.
     
  9. mark

    mark Guest

    Change the A.C. filter capacitor that should cure usually in the 10's of
    microfarids at 150V
     
  10. Mike Berger

    Mike Berger Guest

    That sounds more like a selenium rectifier -- if it were bad
    you'd smell it!
     
  11. CJT

    CJT Guest

    If the voltage on it measures 150V, then a 250V cap might be a better
    choice for a replacement.
     
  12. tempus fugit

    tempus fugit Guest

    Tried the turining around the plug trick, but it made no difference. I also
    disconnected the phono plug that connects the tone arm to the amp unit,
    which also made no difference.

    Here is something curious though. I accidentally touched my scope while my
    hand was on the record player chassis and got a fairly good shock. The scope
    has a 3 prong AC plug (the record player doesn't). I should've measured the
    voltage difference between the sope chassis and the record player chassis,
    but I didn't think to. I'll have to do that. Perhaps he chassis is live. I
    didn't think this would be the case though, since there is a cap with the
    negative connected to the case, as well as a few other green wires.

    Thanks


    "tempus fugit" bravely wrote to "All" (27 Oct 05 11:17:42)
    --- on the heady topic of "unit hums loudly regardless of volume"

    tf> From: "tempus fugit" <>
    tf> Xref: core-easynews sci.electronics.repair:346437

    tf> Hey all;

    tf> I'm trying to repair an old tube portable record player. When it is
    tf> on, it hums really loud, regardless of the volume.

    Have you tried turning the ac plug around?

    Which way the plug is inserted matters with these. Let the record
    player warm up, then insert the plug one way or the other, and then
    paint a mark for which was the quietest way. One way: lots of hum, the
    other: quiet hiss.

    Those old tube units used to get B+ directly from the powerline (or
    sometimes a voltage doubler) and used a large value resistor bypassed
    by a 0.01uF to RF ground the tone arm shielding to neutral. The power
    for the filaments often came from a secondary winding in the motor
    coil. Don't use this type of record player near the bathtub. Lethal!

    Another possibility is a broken wire at the cartridge.

    A*s*i*m*o*v

    .... Just a little force field zap.
     
  13. CJT

    CJT Guest

    That capacitor might be leaking or (worse) shorted. It can be a lethal
    failure (I read just this week about a minister in Waco who was
    electrocuted during a baptism when he grabbed a microphone that was
    "hot."). Be careful.
     
  14. CJT

    CJT Guest

    Here's a cite to that story:
    http://www.cbsnews.com/stories/2005/10/31/national/main995829.shtml
     
  15. tempus fugit

    tempus fugit Guest

    I tested the cap and it showed about a half meg of resistance. maybe
    something is shorted somehwere else. Oh, and I checked the voltage between
    the record player chassis and the scope chassis - 120VAC (I thought that's
    what it felt like). Odd though, I would think with 120v on the chassis that
    it would pop a fuse at the breaker box.

    Thanks
     
  16. CJT

    CJT Guest

    We're talking about a capacitor between the chassis and one side
    of the line, right?

    It should be infinite (assuming you tested it out of the circuit).

    When in doubt, replace it (or just snip it out and don't replace it).
    Google "death cap" if you'd like some further opinions, typically from
    the world of guitar amps.

    maybe
    Reversing the plug in the wall socket should change that (but don't
    count on it).

    (I thought that's
    Not until your hair catches fire.

    You really need to be careful with this device. I can't stress that
    enough. Death is not to be trifled with.
     
  17. tempus fugit

    tempus fugit Guest

    I havent reopened the chassis, but it seems to be connected between the
    ground of the speaker and the chassis.
    Reversing the plug did correct that. Is this a design shortcoming (if so,
    there's gotta be a safer way to set that up) or should I be looking for a
    fault somewhere?

    I am being very careful, not to worry, but I appreciate your concern and
    helpful advice (including those on personal safety).

    Thanks again
     
  18. CJT

    CJT Guest

    OK, that's something totally different.
    That's normal for a "hot chassis" device. But such a device won't have
    a power transformer, and I thought you said this one did. Could you be
    confusing an output transformer with a power transformer?

    If you are (and I suspect you are) then you are such a newbie that you
    had better stop now and enlist the help of somebody who knows what
    they're doing before you kill yourself. No kidding. I don't want to
    have anything more to do with this pursuit.

    tempus may fugit, but not for the dead
     
  19. Ken Weitzel

    Ken Weitzel Guest

    Hi...

    Sorry for the top posting; but the rest is getting a bit
    convulted with a confusing mixture :)

    Just wondering... has tempus or any of the rest of you other
    old timers considered the possibility that it might be a dc
    "choke" speaker?

    Take care.

    Ken


     
  20. CJT

    CJT Guest

    That's an interesting possibility I hadn't thought of.
    I haven't seen one of those in quite a few years. Did
    they even make portables before PM speakers became the norm?
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-