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Dial cord slipping

Discussion in 'Troubleshooting and Repair' started by (*steve*), Dec 16, 2017.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,401
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    Jan 21, 2010
    I have a piece of equipment that has a dial cord. The cord is wrapped twice around a 1/4 in diameter brass rod which is the shaft for the knob on the front panel.

    In one position the dial cord slips leading me to turn the knob more until the resistance is overcome and the main control starts to move again.

    I really don't want to have to disassemble the thing and replace the dial cord. I don't think there's any grease on the shaft or the cord (but the cord is slightly discoloured).

    Are there any tricks I can employ to resolve this? I've though of spraying the area with isopropyl alcohol to remove any stray grease, but I don't know if there's any issue this might cause with the cord.

    Any advice?

    I read this. Isopropyl has been used by people, and violin rosin may also be a good idea (and weirdly enough, I have some of that).

    I have another issue to fix. The instrument was delivered knob side sown from a height :-( In the image below you can see a pot that has been pushed apart.

    IMG_20171216_121240.jpg

    And how one of those knobs survived (or didn't)

    IMG_20171213_170855.jpg
     
  2. dorke

    dorke

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    Jun 20, 2015
    Well,

    1st thing , verify that the string works freely without entangling (the turns should be parallel to each other and remain so while you move the tuning knob) in the tuning drum and the shaft.

    2nd thing, verify that all the plastic wheels turn freely.

    3rd thing, In most cases there are 3 turns needed.
    Try adding one turn ,do this gently(force is needed) with the cord as it is(don't disassemble it!).
    There should be a spring in the mechanism which will provide the necessary slack for this action.

    The pot may be salvageable,remove it for inspection,you may be lucky.;)
    Or else,replace it and the knob.

    Note:
    The voltage of that cap leads me to guess it is an old Tube radio/receiver.
     
  3. kellys_eye

    kellys_eye

    4,276
    1,146
    Jun 25, 2010
    More like some old test equipment to me - the knob is a giveaway.....

    If you mark the cord with a marker pen when it is on the turning shaft and at the 'slipping' position you can then wind on past (or before) that point and apply rosin to the marked areas on the cord.

    Cord replacement is the only proper method though.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I sprayed some isopropyl on a block of rosin and used a cotton bud to transfer that to the slippery part of the dial cord.

    Yes, I had checked that things move freely, although some lubrication of these may not go astray.

    I may post some pictures of the device later...

    And that damn pot is going to be pretty hard to get to. I *think* I can do it without needing to mess with the dial cord though.

    There are two electrolytic capacitors in this device. One is a dual electrolytic 32+23uF at 450V. If there's a problem with that then clearly the rectifier tube could be at risk (as well as potentially the transformer). I'm a bit wary of that...
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Here is the beasty...

    00001IMG_00001_BURST20171217121502.jpg
     
  6. dorke

    dorke

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    Jun 20, 2015
    What a beauty, with a magic eye too:)

    If you are lucky,you may be able to fix that pot.
    In case it is the kind that can be opened easily and if the carbon inside it is intact.
    I did in the past.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Currently the pot is open circuit. It also appears to have switch contacts, but any switching isn't mentioned in the manual, and not does it appear in the current diagram.
     
  8. dorke

    dorke

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    Jun 20, 2015
    As strange as it sounds,it dose look like there is no switch related to any pot in the schematics
    ,wouldn't expect this kind of slip from Philips.
    In this case you have to trace the wires and find out how and where it is connected.o_O
    Not very complicated,it's a relatively simple beast.;)

    And what is this pot at the bottom right?
    no writing on it on the front panel,strange :confused:
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That's the one with the dial cord attached. It controls the main dial.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I keep taking this thing apart to try to fix it, and then have to put it back together to take it to the post office to chase up the insurance claim...

    I have put the pot back together, but it is too far gone. I found only a single replacement. They also have some collet knobs that don't really match for another $10 or thereabouts for the pair. That's already almost half what I paid for the instrument, let's not add any value of time to that...

    Fortunately, to remove and replace the pot, all I need to do is:
    1. Remove all the knobs
    2. Desolder 2 wires to a pair of banana sockets on the faceplate
    3. Remove the connections to the three terminal posts on the faceplate
    4. Unscrew 5 bolts holding the front faceplate in place
    5. Remove 2 screws holding a bracket for a switch
    6. Remove a tube
    7. Unscrew 4 bolts holding a bracket that the tube socket is mounted on
    8. Remove 2 screws holding a transformer in place.
    9. Desolder 4 wires to the pot
    Easy!

    I also traced the wires from the switch on the pot They allow the removal of anode voltage from the magic eye tube. This may be used (I think) where you monitor the bridge voltage externally. I've confirmed this from a more detailed schematic for the instrument that is in another manual.

    Speaking of collet knobs... All the shafts are 6mm, however the new pot has a 6.35mm shaft. Do you think I'll be able to get a 6mm collet knob to fit on it?
     
  11. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    No, but you may be able to open the collet knob with a quarter inch drill. I had trouble some time ago and used too big a hammer and could not get the knob off again.
     
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