Help me adjust the taper of this pot for EH HOG exp pedal

[ianmgull]

The plastic M-Audio expression pedal that came with my HOG broke. took the guts out of it and put it in an old Ernie Ball volume pedal case. The Ernie Ball case has more travel but aside from that things work ok. Here is what I want to do:

Since there is more travel, half of the sweep has no effect on the HOG. If I start in toe up position, there is no effect until about half way through. I want to spread out the effect through the range of the new sweep.

The which I am using from the old expression pedal is a 12k. There is also a 1k resistor in series with the toe down position ( I guess the HOG wants to see some sort of minimum resistance).

I measured the resistance through the sweep and found that in the portion of the sweep which is having no effect, the resistance is only changing a couple ohms. That is at toe up position (full CCW) the resistance is 12.23k. By the time i get about half way though the treadles travel, it has only changed a few ohms and then drops sharply. This makes me wonder if this is a custom taper. At the full toe down postion (full CW) the resistance is 1k.

Would I be better off starting with a larger pot and putting taper resistors across the outer lugs? Or do I need to put just one resistor across the outer at toe up position to wiper to only change the part that has little effect? I've read the article on pots at geofex, just not sure if what I'm trying to do is possible.

Thanks.

[ianmgull]

 

[markeebee]

I thin it's just a reverse audio taper pot.  That would reduce the resistance by less than 10% for the first 50% of travel, and then drop away sharply after that.

I think you could do a fair simulation of the way it's supposed to work using a larger value linear pot and a taper resistor.  Problem is that there are still a bunch of variables that you need to estimate before we can do any of the sums:

~  Percentage of total travel of pot possible in old pedal
~  Where the travel occurs (i.e. if it's, say, 50% of total travel is it at the centre of the pot's track, or at either end)
~  Same as above two, but relating to new pedal

If you can conjure these up, I think it's possible to plonk the figures into RG's calcs and at least come up with a few values to experiment around.  Without them, I'm stuffed and will bow out and hope somebody who actually knows what they're talking about can help you.

[ianmgull]

Thanks for the reply Marky, I've been pulling my hair out trying to wrap my head around this one. Now that you mention it though it's starting to make sense. Since the travel in the old pedal is significantly less, it would make sense to do exactly what they did and use a reverse audio taper. This allows the useful part of the pots rotation to be set over the shorter range of travel. Here is my idea:

I'll take a 25k linear pot and put a resistor from each outer lug to the wiper. The value should be around 12.5k to effectively cut the resistance in half while maintaining a reasonably linear sweep (by reasonable i mean not reverse log).

This is just a hypothesis. I'll start by measuring the exact resistances where the effect occurs on the current pot.


Thanks for the help!

[ianmgull]

Ok I'm stumped here. to cut the resistance of a 25k pot in half shouldn't I just be able to put a 12.5k from each of the outer lugs to the wiper? I'm getting some strange readings here.

[ianmgull]

I figured out why I was getting the strange readings. I instead of putting two resistors from each of the outer lugs to the wiper, I just need to one resistor (the sum of the two smaller in series: about 25k) from one outer lug to the other. That being said, here is a schem of the original expression pedal for clarification:



The "minimum pitch" pot is just a small pot on the side which (when in oct up mode on the HOG) sets the minimum pitch when the pedal is toe up. Any further advice would really be helpful.