Increasing potentiometer value via resistor?

Discussion in 'The Projects Forum' started by Videodrome, Dec 21, 2011.

  1. Videodrome

    Thread Starter Member

    Nov 12, 2009
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    I'm experiencing a problem with the gain potentiometer on a guitar overdrive circuit i've just wired up. When I turn the gain up, the gain increases as it should until around 3-4 oclock, where the gain then disappears as if the throw is starting over again but at the end of the turn. The pot used is a dual gang 100K linear, and i've been told that both pots need to be measured on or very near 100K or in the least have both pots close in value if not. The best pots I had out of my batch to replace the troubled one used measured at 93K at the top and 100.2K at the bottom, and another pot measuring at 93.6 K at the top and 96.8K at the bottom, I used the latter. When I wired it up, it was still giving me trouble and I now want to try the 93K/100.2K pot instead and try to bring up the value on the 93K pot up to 100K to see if that'll do the trick. I have very little experience in electronics but figured it'd be as easy as putting a 7K resistor across the lugs, but when measuring it seemed to bring down the value drastically. Does anyone know of what value of resistor I should use to get that top pot up to snuff?
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    If I understand your question, I think the answer is that you need to put the 7k resistor in series with the pot, not in parallel with it.
     
  3. Videodrome

    Thread Starter Member

    Nov 12, 2009
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    Could you explain what each mean? I'm pretty green but i'm assuming parallel is when the resistor is connected via the 2 and 3rd lugs?
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    It would be a little surprising if such small errors in resistance and matching had such a big effect. Since pots are subject to wear, a circuit requiring such accuracy is likely to be most unreliable.

    On the other hand, it may be that there is some other issue that could be addressed. Please post a schematic of your circuit, and somebody may be able to help you further.
     
  5. Videodrome

    Thread Starter Member

    Nov 12, 2009
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    well i've been told that the values for the gain pots being in sync is rather important to the circuit, to the point that the original designer had to order special potentiometers for the task. heres the vero layout i'm working off of, though it is devoid of the wiring diagram.

    [​IMG]
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Without a schematic, or circuit diagram it will be hard to help. I would doubt that anyone will have the dedication to work one out from the physical layout.

    If the degree of matching you describe is really necessary, you might be better to provide for readjustment as the pots wear in use.
    With the sort of radical malfunction you describe, I would still think it more likely that you have some kind of construction error, possibly in the connections to the pots themselves, or even a faulty part.

    Constructing a circuit of this type from scratch on strip-board gives many opportunities for making errors, even for somebody with a fair level of experience.
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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  8. tracecom

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    Apr 16, 2010
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    That part is a mystery to me. A 100k linear taper pot should change resistance from 0 Ω to 100,000 Ω as it is rotated. That is measured from the wiper (center connector) to either side connector, and of course must be measured out of the circuit with nothing connected to it but the meter. Are you saying that the full range of one of the pots you have is only from 93k to 100.2k? Can you post a picture of the pot?
     
  9. Videodrome

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    Nov 12, 2009
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  10. tracecom

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    That seems to be a standard pot. What about the measurements?

    Rotate the shaft fully CW and measure the resistance from the center lug to the right lug on pot A. Then rotate the shaft fully CCW and measure the resistance from the center lug to the right lug again. Repeat for the other pot on the same shaft (pot B.) Post the readings.
     
  11. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If you have connected the ends of either of the two sections in the wrong order, the sense of variation of the two sections will not work together in the way the designer had planned.

    This is a separate problem to the matching of the sections to each other.
     
  12. Adjuster

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    Dec 26, 2010
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    From the picture he has posted, I think he is talking about the matching between the two sections of a dual pot.

    The connections to the sections need to be the right order, or the variations created by the two sections will be out of step.
     
  13. Videodrome

    Thread Starter Member

    Nov 12, 2009
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    thanks a ton for the input folks. So there was one glaring detail I forgot to mention in my original post. When the pedal is first plugged in, the gain knob works just fine, with the pot going all the way to the end of its physical turn with the sonic raise in gain corresponding to it as it should. No problems of any kind. After about 10 minutes of using the pedal though, you can hear the background noise and the gain disappear from the signal as the end of turn then SONICALLY moves up to 3 oclock, leaving everything after that physically and sonically vacant when turning. I plugged my guitar in unplayed and dimed all the controls (gain treble, volume) and just let the background hiss go for about 10 minutes until you could literally hear the background hiss and gain quell and quite down till the control near the end of its turn is useless. Now, if I leave the pedal sitting unplugged from its power source for another 10 minutes and plug it back in, it works just fine for another 10 minutes before doing the same thing, as if it needs to charge or reset to work for the small time that it does. After explaining this on another forum, I was told that it may not be an issue with the gain potentiometer at all but more of a thermal issue, possibly attributed to the charge pump. I was told to go in and make sure I didn't fry the charge pump and to then measure my voltages to see if they match the readings taken from a fully functional unit, as seen below:

    Supply 8.98V

    IC1
    1.3.37 5.3.78
    2.3.37 6.3.78
    3.3.22 7.3.78
    4.0 8.8.98

    IC2
    1.3.68 5.3.00
    2.3.79 6.3.81
    3.3.78 7.3.91
    4.-8.72 8.16.19

    IC3
    1.8.98 5.-8.72
    2.4.47 6.4.40
    3.0 7.6.62
    4.-4.47 8.8.98

    My troubleshooting experience is very slim and hasn't gone as far as replacing diodes that were installed with the incorrect polarity so I am green as far as measuring values in circuit. Does anyone know where I start in checking the charge pump to see if it indeed got baked? I don't see any physical evidence of anything amiss(burning smell, heat, etc) but i'm sure theres a more technical approach to being sure. And how I should go about measuring the IC's to achieve the readings seen above? Do I measure for from the pin of the IC to the first component that is connected to that pin and repeat for the rest of the pins or is my assumption incorrect?




    i'll go ahead and pull the pot once I can get it in front of me later this evening or early tomorrow morning and take some measurements as instructed to see what may be the matter.

    I wired mine up using this diagram, specifically for use with a 2pdt switch (the jack out lead that is devoid in the diagram is wired in mine so thats not a culprit), and there is a bridge that connects lug 3 on the top pot to lug 2 of the bottom pot, and I wired a lead to enter into the top pot of lug 3 on the top which then bridges to lug 2 on the bottom. Im pretty sure i'm reading you wrong but just to be sure, are you saying that I may need to wire up that lead opposite to what I did by going through the bottom 2nd lug, through the bridge and onto the top 3rd lug?
     
    Last edited: Dec 23, 2011
  14. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I am referring to the wiring of the two sections of the gain pot. If you have the end connections of either of the two sections the wrong way around, one section may effectively be turning up, while the other turns down.

    It is by no means certain that this is your problem, but the symptom of the control only seeming to go so far then reversing makes me suspect such an issue.
     
  15. Videodrome

    Thread Starter Member

    Nov 12, 2009
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    ahh i see. naw it doesn't look like thats the case but thanks for the help.

    I finally learned to troubleshoot by measuring the voltage within a circuit but I have a couple questions I forgot to ask the fellow at the electrical depot this morning. Where do I need to set the control knobs (in this case, GAIN, treble, volume) to get the most accurate readings? all the way left, at 12 oclock or all the way right(i'm assuming its the latter).
     
  16. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Indeed, having read your last long post thoroughly, I'm pretty sure that it's not the issue too.
    Yes, that qualifies as a glaring detail, because not mentioning it can lead people on false trails like the one I went down, wasting everybody's time (including yours).

    The fact that the behaviour changes over time rules out sort of the problem I first thought of. It seems more likely that there is a voltage drifting off somewhere that needs to be found.

    There are lots of things that could cause this, for instance one general thing to look for is whether any electrolytic capacitors are connected the wrong way round.

    As for checking the charge pump, what voltages appear on the +V, +V, +V2 and Vb lines? (You might like to begin your electrolytic polarity checks on the capacitors in this part of the circuit, C17 to C22.)

    Edit: I would not expect the treble or output pots to change the DCs much, maybe leave them set half-way.
    There may be some interest in seeing if varying the gain pot has a large effect - like making an output go to rail for instance.
     
    Last edited: Dec 24, 2011
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