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unit hums loudly regardless of volume

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

  1. mark

    mark Guest

    What about the 3 or 4 section electrolytic usually at 150 volts that comes
    from the main rectifier usually being a tube.
     
  2. tempus fugit

    tempus fugit Guest

    Thanks again for your reply.

    I'll have to pop the chassis again to see about the power transformer thing.
    It is possible that the xformer I saw (there is 1 outside the chassis) is an
    output and not a power xformer. I don't remember if there was one inside the
    chassis or not. Bear in mind that I just took a quick look at the thing,
    checked the caps real quick (as I suspected them to be the problem) and put
    it back together. There is no PC board, just flying wires everywhere, which
    I didnt trace any of. I'll take a closer look to see exactly what is
    connected where and post back here with the results.


     
  3. Guest

    Guest 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


    I fixed an old record player with the same problem and I used to be a TV
    repair tech.

    I'd bet money on a bad filter capacitor. Just replace it with something
    of the same voltage rating and equal capacitance or slightly higher if the
    exact value isn't available.

    It's not something w/the tone arm because that would vary with volume.
    It's not a shorted diode because if it were, it'd be putting AC in
    to the capacitor and that would be an AC short and a destructive overcurrent
    situation would result. It's not an open diode because you'd get no hum
    if that were the case.

    Replace the filter capacitor. They're usually a tab mounted can and it'll
    be difficult to find one like it so instead, use an axial leaded capacitor
    and run insulated wires to it and glue it somewhere to keep it in place
    (silicone cauking/sealant would do the trick).

    b.
     
  4. Guest

    Guest 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.

    Sorry for another followup.

    The capacitor is a multi-section capacitor. Their values are usually
    stamped or imprinted on the side of the can. The leads will usually have
    a triangle, half circle and square markings/openings in the phenolic part
    of the capacitor (bottom). The "legend" of values will usually show that
    on the capacitor markings.

    If you can't get the replacement, you can wire up individual electrolytic
    capacitora and tie all of the minus sides of the wires together and ground this
    to what was previously the capacitor can ground and use the + lead of each
    capacitor to connect the the wires that previously attached to the
    capacitors.

    Here are a few sites relating to them:
    http://www.dialcover.com/caps.html
    http://www.partsexpress.com/webpage.cfm?&DID=7&WebPage_ID=3&CATID=41&ObjectGroup_ID=557
    http://www.kenselectronics.com/lists/caps.htm
    https://secure.tubesandmore.com/

    barry
     
  5. tempus fugit

    tempus fugit Guest

    thanks for the reply Barry;

    The cap is indeed multi section - I think there are 4 different caps.
    Unfortunately it is a cardboard container with no visible values. I'll look
    a little closer and see what I can see.

    Thanks
     
  6. Porky

    Porky Guest

    Hello tempus, are you still with us? Don't forget to pull the plug, eh!
    I suggest you search for a brand and model #, then search at
    rec.antiques.radio+phono, where there are expert advisors on how these
    units can be fixed. The transformer you saw was for the output to the
    speaker. You are dealing with a live chassis unit. the "death cap" may
    be shorted, and the hum you here is 120vac running thru the chassis.
    The filter caps may not be too bad, although they are always leaky at
    that age. You can use 50-80 uF for the first 2, then drop to 20-30 for
    the other 2. There is no diode tube to worry about. If the diode is a
    bunch of square plates with a central bolt, it is selenium. Replace it
    with a new silicon diode and a resistor. Cheers.

    John Kogel
     
  7. tempus fugit

    tempus fugit Guest

    Still alive and kicking John, lol.

    Thanks for the tips. The "death cap" I assume is the one that is connected
    from the live to the chassis? I'm not sure that there even is one in this
    unit, but as I mentioned earlier, I just took a quick peek inside the
    chassis and put it back together. I haven't had too much time to mess with
    it, but I'll definitely look for that when I do. If everyone would like, I
    could take a picture of it and post it on ABSE.

    What do youy think?

    Thanks again
     
  8. Porky

    Porky Guest

    tempus fugit
    Yes, there is often a .05 or so cap from live to chassis on
    transformerless tube gear. It's main purpose is to turn the chassis
    into a sheild to keep radio interference.from being detected by the
    device. It's secondary function is to shock the @@#$ out of you when it
    shorts, (although they are rated at 600v and don't carry much current,
    so failures are usually due to moisture and disuse).
    If you have small kids, keep them away from the thing. After it is
    fixed, install an isolation transformer for $30. Yuo can even build
    your own by connecting 2 12V trannys back-to-back. This isolates the
    voltages in the chassis from those in your house = no more shocks.
    Cheers.

    JK
     
  9. tempus fugit

    tempus fugit Guest

    Thanks JK.

    I got a chance to take a closer look at this thing and the neutral wire is
    connected to ground, but there appears to be about 200 ohms of resistance
    between the ground on the filter cap and the chassis. I didn't see anywhere
    were the live wire was connected to the chassis through a cap. Forgetting
    about the hum for a second (I'd like to make this whole thing a little safer
    1st) would a quick fix for the 120v I've seen on the chassis be to simply
    install a polarized plug on the power cord? This way I could ensure that it
    was never plugged in backwards thus connecting the chassis to the live 120v
    line.

    Also, I mentioned a small blue thing that looks like a bunch of thin squares
    stacked on top of one another, that someone suggested was a selenium
    rectifier. I hear they emit a poison gas when they burn. Would it emit any
    dangerous gases during normal operation? Should I make replacing this part a
    priority? I should also mention that there is 1 tab on this piece that has
    some solder on it but no wires or anything connected to it. Could this be
    the source of my hum problems?

    Thanks again
     
  10. sofie

    sofie Guest

    hum regardless of volume usually means bad or dried up electrolytic filter
    cap(s) and/or maybe a bad rectifier..... and no, during normal proper
    operation the selenium rectifiers do not emit poison gas. I would just
    simply bridge the filter cap with a replacement of the same or approximate
    value and the same or higher voltage to see if the hum goes away.... which
    it probably will given the symptoms you reported.
    - - - - - - - - - -
     
  11. tempus fugit

    tempus fugit Guest

    Thanks for your reply Sofie.

    I agree with your diagnosis - this reeks of a bad filter cap. The problem is
    that the filter cap is an old cardboard multisection with no marks of any
    kind on it. There are 4 tabs on the bottom, with the twistlocks acting as
    grounds (I think). Of those tabs, 1 shows a 300 or so ohm resistance to one
    of the twistlocks. Assuiming that that tab is a cap +, and not a ground,
    that cap definitely needs replacing. What I'd like to do is just replace the
    lot, but I have no way of knowing what the values of each of the 4 tabs are
    supposed to be. Would just replacing them all with, say, 22uF caps be OK?

    I'd like to replace the rectifier as well, but I need to figure how to set
    up a bridge or diodes properly in the place of the old one. I've never seen
    one of these before, but it has 2 tabs on it - + and (-). The (-) tab
    acually has nothing connected to it. You can see that it once did, but
    doesn't anymore. So..... I'll have to try to figure out where it should have
    been connected at one time. Oddly, there is about 150V DC across the filter
    caps.

    Thanks again
     
  12. sofie

    sofie Guest

    150vdc across the filter cap is exactly what you should expect.... as my
    previous reply suggested, you can quickly and easily bridge the first
    section right after the rectifier of that old multi-section cap with just
    about any thing you have laying around with a rating of at least 150 vdc and
    a capacitance of 20 to 60MFD... this is not very critical and will instantly
    confirm the diagnosis.... so stop guessing here and get a temporary cap
    bridged across the old one so you can go forward with the repair.
    More than likely all the sections are bad and can all be bridged (usually
    not very critical capacitance values) if you want to leave the old one in
    place... in fact, as long as it is not shorted or electrically leaking then
    you can leave all the connections in place and just neatly install new,
    usually smaller, replacement caps under the chassis.
    --
    Best Regards,
    Daniel Sofie
    Electronics Supply & Repair
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


    snipped:
     
  13. Porky

    Porky Guest

    Hello tempus. I've been away but so have you. From your last post, more
    clues are taking shape as to how to fix your phono. We need to know
    what power tube is in it. It will have either 8 pins, (25, 35 or 50L6)
    making it an early model, or a later model 9-pin 50C5 or 50EH5. Locate
    the pin for the plate, on the 50EH5 or 50C5 that is pin 7, looking at
    the bottom of the socket and counting clockwise from the gap. On the
    8-pin (octal) tube the plate is on pin 3, clockwise from the notch. The
    1st filter cap connects to this pin. It is likely a 40 uF rated for no
    less than 150 v. You can use up to 80 uF here and use a high voltage
    rating like the 400 v units for safety. The next filter cap connects to
    pin 5 or 6 I'm not entirely sure but you can trace the wire back to the
    multicap. It should also be a 40 uF (50 - 80 uF will work). For third
    cap going to pin 1 or 6 of the 12AX7, a lower value like 22 uF may be
    alright. The 4th cap should be 22 uFif it runs to the cathode, pin 1 on
    the 9-pin or pin 8 of the octal, or the 12AX7, pin 3 or 8. For these
    last 2 caps, 150 v rating is OK.

    It appears the selenium rectifier has been bypassed already. There is a
    solid state diode doing the job. It is likely black with a silver band
    but they do come in other colors, looks like a resistor but with only
    one band. Ideally, a resistor is included in series with the diode to
    bring the voltage down a little to the level of the original.

    The shock you recieved could very likey be due to the filter cap
    shorting to ground, and replacing all the caps in the multi-cap can
    will likely solve both shock and hum problems. A polarized cord is a
    very good idea. A 3-prong cord is even better, with the green wire
    going to a solid ground point on the chassis. If you ground the
    chassis, locate the death cap, (line to chassis cap @ +/- 0.05 uF), and
    clip it out of there to prevent future shocks.

    BTW, the tube heater supply is likely fed by a tap on the motor
    winding, eliminating the need for a separate tranformer. This ain't
    hi-fi. Take good care of that pickup cartridge.

    JK
     
  14. tempus fugit

    tempus fugit Guest

    Thanks again for the replies guys.

    Yet more information:

    The power tube is a 50L6. The selenium rectifier has been bypasssed by a
    single top hat diode (I guess I was looking for a bridge, or at least 2
    diodes).

    I tried to rewire the power cord, and I'm getting frustrated. I wired it
    correctly, so that the hot lead went to the switch. This put 120v on the
    chassis, though so I switched it around. Guess what?? Still 120v on the
    chassis. This is with the power off. Before I put the new cable in,
    switching the plug around took the 120v off the chassis. The weird thing is
    that I can't locate any place where either wire is connected to the chassis,
    and there doesn't seem to be a death cap.

    Sorry about the infrequency of the replies, but I don't get a lot of free
    time to work on it.
    Thanks
     
  15. Guest

    When in doubt, use 33uF or so, @450V (22uF is OK for a start. If the
    hum reduces but does not go away altogether, then go up). Otherwise,
    200uF @20V or so for LV stuff.

    Bridge: Voltage in X 1.4142 = voltage out.
    Single diode: Voltage in .7071 = voltage out.

    Keep that in mind.

    Peter Wieck
    Wyncote, PA
     
  16. Asimov

    Asimov Guest

    "tempus fugit" bravely wrote to "All" (13 Nov 05 22:07:08)
    --- on the heady topic of "Re: unit hums loudly regardless of volume"

    tf> From: "tempus fugit" <>
    tf> Xref: core-easynews sci.electronics.repair:348490

    tf> Thanks again for the replies guys.

    tf> Yet more information:

    tf> The power tube is a 50L6. The selenium rectifier has been bypasssed by
    tf> a single top hat diode (I guess I was looking for a bridge, or at least
    tf> 2 diodes).

    High voltage and low current allows using only halfwave rectification.


    tf> I tried to rewire the power cord, and I'm getting frustrated. I wired
    tf> it correctly, so that the hot lead went to the switch. This put 120v on
    tf> the chassis, though so I switched it around. Guess what?? Still 120v on
    tf> the chassis. This is with the power off. Before I put the new cable in,
    tf> switching the plug around took the 120v off the chassis. The weird
    tf> thing is that I can't locate any place where either wire is connected
    tf> to the chassis, and there doesn't seem to be a death cap.

    There is usually a 1meg or 470K resistor with a disc cap across it.

    BTW The dmm has 10M high-Z input and will read full 120V no matter
    where the line connects. It's safer connecting line to the rectifier
    circuit not the chassis. The 120V on the chassis should then be high
    impedance and not a widow maker. Load the dmm input with 10K to test,
    if it measures more than 50V then there is a serious problem.

    A*s*i*m*o*v

    .... Real techs don't lick nine-volt batteries!
     
  17. tempus fugit

    tempus fugit Guest

    Thanks again for the replies.

    I solved the 120V on the chassis mystery - the new cord I installed wasn't
    polarized after all. It has a white stripe on it (black cable) and one of
    the tips appeared bigger than the other, but it would fit in the socket
    either way. I installed an actual polarized cord and the chassis is AC free.

    Hopefully I'll get to the caps soon. I'll keep you posted.

    Thanks
     
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