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Thread: Resistor behavior

  1. #1
    Forum Veteran 62vauxhall's Avatar
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    No such things as bad days, just bad moments

    Resistor behavior

    This is not PL related but I've been trying to figure out why a 1 watt, 56 ohm wire wound resistor is heating up. It would be too hot to touch if I kept power applied.

    I pulled it from the circuit and it does measure about 56 ohms. There is the correct 24V going in to one end and around 13 volts at the other but it's supposed to be 20.5.

    As I bring the variac up, the resistor heats up fairly quickly once it starts receiving 24 volts. Since it's dropping more voltage than it should, it seems logical that would account for the high temperature.

    But I don't get why the resistance checks out. That being so, it cannot be defective, can it?

    Any thoughts?

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    Halfbiass...Electron Herder and Backass Woof
    Your problem is current related, not resistance related. Something on one side is drawing too much current...
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  3. #3
    Forum Veteran 62vauxhall's Avatar
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    Dang. And the schematic only has those two voltages figures printed on it. 24 volts going into the resistor and 20.5 on the other side - which I physically measured at 13 volts.

    So high current draw does not sound like a short or is that an incorrect assumption?

    If so, then I guess it's not a bad trace or solder connection but a faulty component of some kind? Maybe a variable resistor?
    Last edited by 62vauxhall; 09-12-2017 at 07:52 PM.

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    Administrator Gepetto's Avatar
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    Old 'Arn Enthusiast
    Quote Originally Posted by 62vauxhall View Post
    Dang. And the schematic only has those two voltages figures printed on it. 24 volts going into the resistor and 20.5 on the other side - which I physically measured at 13 volts.

    So high current draw does not sound like a short or is that an incorrect assumption?

    If so, then I guess it's not a bad trace or solder connection but a faulty component of some kind? Maybe a variable resistor?
    You should share the schematic so we can offer some suggestions

  5. #5
    Forum Veteran 62vauxhall's Avatar
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    I'd love to do that but all I can access are PDF's of the service manual and if it's possible to isolate the page(s) containing the relevant schematic(s), I do not know how to do that.

    The only thing I seem to be able to do is copy and save the entire manual as a PDF document and I doubt a "print screen" would be much if any good.

  6. #6
    Administrator Gepetto's Avatar
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    Quote Originally Posted by 62vauxhall View Post
    I'd love to do that but all I can access are PDF's of the service manual and if it's possible to isolate the page(s) containing the relevant schematic(s), I do not know how to do that.

    The only thing I seem to be able to do is copy and save the entire manual as a PDF document and I doubt a "print screen" would be much if any good.
    use the Windows snipping tool

  7. #7
    Forum Veteran 62vauxhall's Avatar
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    No such things as bad days, just bad moments
    OK. Seems I have next to zero understanding of Windows. I had no idea that feature existed so had to Google it to see how to use it. I saved the images as jpeg's thinking that was as good as any.

    There is only one board I'm dealing with, from a Teac A2300SD R2R. The manual refers to it as the Bias Oscillator but I believe it also contains the power supply or what was I was told - AC Regulator.

    The overheating resistor is R27 which is fed from pad 37 and hopefully this makes sense.

    The entire board is disconnected with the exception of a black (presumably) ground going to hole 9 and 2 blue wires going to holes 26 & 27. This arrangement does produce 24V (closer to 25V) at pad 37 which is correct. However, when I re-connect a wire from pad 37 to hole 4 thereby feeding 24V to R27, it heats up.

    There is a voltage figure of 20.5 volts on the opposite side of VR1 which to my understanding, should be coming directly from R27 and be no less than that. Yet, what I read coming out of R27 is about 13 volts.

    Click image for larger version. 

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    EDIT: FWIW, There are only 4 transistors on this board which I removed and checked and they are OK as are their emitter resistors. Since it was suggested to me that capacitors C14, C15 & C22 might be suspect and I had equivalents on hand, I replaced them.

    Must get to sleep now. Spent 17 hours on duty and I've got to get up in 5 hours.
    Last edited by 62vauxhall; 09-14-2017 at 02:54 AM.

  8. #8
    Administrator Gepetto's Avatar
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    Quote Originally Posted by 62vauxhall View Post
    OK. Seems I have next to zero understanding of Windows. I had no idea that feature existed so had to Google it to see how to use it. I saved the images as jpeg's thinking that was as good as any.

    There is only one board I'm dealing with, from a Teac A2300SD R2R. The manual refers to it as the Bias Oscillator but I believe it also contains the power supply or what was I was told - AC Regulator.

    The overheating resistor is R27 which is fed from pad 37 and hopefully this makes sense.

    The entire board is disconnected with the exception of a black (presumably) ground going to hole 9 and 2 blue wires going to holes 26 & 27. This arrangement does produce 24V (closer to 25V) at pad 37 which is correct. However, when I re-connect a wire from pad 37 to hole 4 thereby feeding 24V to R27, it heats up.

    There is a voltage figure of 20.5 volts on the opposite side of VR1 which to my understanding, should be coming directly from R27 and be no less than that. Yet, what I read coming out of R27 is about 13 volts.

    Click image for larger version. 

Name:	Teac A2300SD bias oscillator.jpg 
Views:	21 
Size:	87.3 KB 
ID:	30223Click image for larger version. 

Name:	Teac A2300SD power suppy.jpg 
Views:	19 
Size:	99.1 KB 
ID:	30224

    EDIT: FWIW, There are only 4 transistors on this board which I removed and checked and they are OK as are their emitter resistors. Since it was suggested to me that capacitors C14, C15 & C22 might be suspect and I had equivalents on hand, I replaced them.

    Must get to sleep now. Spent 17 hours on duty and I've got to get up in 5 hours.
    Is the bias oscillator formed by Q20 and Q21 running? If it is stalled, you will have potentially high current running through your 56 ohm resistor causing it to heat up. It is meant to be running all the time and dumps either into the record head or into the dummy inductor depending on the position of the record relay.
    Last edited by Gepetto; 09-14-2017 at 10:58 PM.

  9. #9
    Forum Veteran 62vauxhall's Avatar
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    No such things as bad days, just bad moments
    Negative - not running. Nothing currently connected to that board at all except wires from the transformer.

    Would connecting only the shielded leads constitute running - or should I go ahead and connect everything?

    I was considering doing the latter anyway but with the exception of the shielded leads. They are still attached to original board and I did not want to apply heat to them unnecessarily until (totally optimistic thinking) the replacement board could be re-installed. But by the sounds of things, that would have been a waste of time.


    As an aside: Hypothetically, if the center conductor of one of those leads was disconnected from the original board (as it currently is and may have been for quite some time) could that have been responsible for resistors (more than one) burning up on the original board?

  10. #10
    Forum Veteran rtp_burnsville's Avatar
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    If the switch is in the position shown on the schematic I would check the capacitor C22 and C29. It looks to me as if those caps go to ground. If one or both are shorted the current would be higher as Lee had mentioned. I assumed that the +24V is from the power supply rail and that the small cap is also not shorted (very unlikely that it would be).

    Robert

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