Home › Forums › Help/Technical Questions › EHX POG LPF Expression Control?

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  • January 12, 2009 at 6:59 pm #77527
    TweedBassman
    Member

    does anyone have any info on how to do this mod? i opened up my POG and to put it simply, it’s not a matter of replacing or adding to the existing LPF slider… trying to do so shuts the pedal off. i really don’t want to fry the pedal by doing anything dumb so if anyone has any ideas please let me know.

    oh, and HI!

    January 17, 2009 at 2:18 am #90971
    jonphipps
    Member

    Hi there!

    I’ve recently purchased a faulty POG that needed repair: it’s fixed now and sounds great! I think your idea for the expression pedal control of the LP filter is a great idea, i’d like to try it myself. The slide pots have three solder terminals just like a normal rotary pot. My idea:

    Remove the slider pot for the LP filter

    Install a stereo jack into the POG

    connect the pot input terminal to the jack tip terminal , the pot wiper terminal to the jack ring terminal and the pot ground terminal to the jack ground terminal.

    I’ve opened up my bespeco expression pedal (nothing special). By the looks of things i think this minor modification should do the job for you, with the slider pot terminals corresponding to the wiring of expression pedal pot via the stereo jack. I’m going to try it and let you know how i get on!

    Some one please correct me if my idea is wrong, but i’ll try it and let you know if it works or not otherwise!

    Thanks for the inspiration, i think this will be most effective in LP filter mode 1 so you can just fade the harmonics in over the guitar sound at will.

    Another thing you could try is to install a rotary pot and use an Option Knob (http://www.optionknob.com/) to enable foot control of the LP filter without the need for an expression pedal cluttering up your pedal board :)

    I hope this has been helpful..!

    January 17, 2009 at 3:09 am #90976
    jonphipps
    Member

    I was just browsing through some other threads and found this:

    https://www.ehx.com/forums/viewthread/264/

    he’s done a similar thing to what i suggested but he has used a ‘dpdt’ jack socket instead of the usual spdt. This connects three pairs of contacts to the tip, ring and sleeve of the jack plug that connects to the expression pedal. When the plug is inserted this disengages the sliders that control the frequency sweep of the micro synth. I think this is a better idea than my intial thought because you don’t have to remove the original slider for the expression pedal to work!

    I hope this is helpful :)

    January 17, 2009 at 3:48 am #90977
    TweedBassman
    Member

    that was my original thought too. problem is this: usually, when you replace a pot, you can ‘test out’ the idea by using alligator clips and connecting them temporarily to the pins and connect the other end to the appropriate sized pot; turn the fader to most resistance and the knob should replace it. when i tried this, the entire pedal shut off. as in, electronically switched off. the bottom lug of the LPF fader has a marking next to the solder joint that says “switch’ or something i forget, i’ll pop it open and look.

    in other words, i basically tried what you said, but it was a little scary when my POG ‘died’ for a second!

    January 17, 2009 at 3:51 am #90978
    jonphipps
    Member

    bugger! As i said, that micro synth mod seems to have been a good solution, maybe it’s worth contacting ‘shotgunn’ regarding how he did the expression pedal mod to his pedal :)

    January 17, 2009 at 4:32 am #90979
    julian
    Moderator

    It’s a really simple mod to do. But it’s hard to explain.

    Basically you want to wire a switching jack in series with the fader. But it can be a little different from pedal to pedal, or potentiometer to potentiometer. If it uses all 3 lugs, it’s a bit less simple I believe.

    January 17, 2009 at 9:30 pm #91025
    Ned Flanders
    Moderator

    You wire a stereo jack to the 3 lugs on the pot, one jack terminal for each pot terminal. You must wire it in the correct order for it to work so check your expression pedal and see which wire from the Ex pedal stereo cable goes to which lug in the ex pedal pot. Once you have the pinmapping down you can wire in the stereo jack to the POG.
    Note: A lot of effects require you to bypass the original pot in the pedal to use an expression pedal so it may also require you to install a 3PDT toggle switch BEFORE the slider pot to bypass it to the exp jack. The signal will always take the path of least resistance and since you will have two pots in together the sweep on the exp pedal will not work as if you’ve bypassed the slider pot completely,you will only have a very minor sweep (rotation of the pot)

    A lot of people think its as simple as wiring in a jack but in most cases its not and in the POGs case I wouldn’t even bother with it because if your asking how to do it its likely too complicated for you to do. Providing you know what your doing its pretty simple to do and only takes about 20 minutes but I wouldn’t recommend it to a beginner personally. I’d hate to see you destroy your pedal as cool as it is!!!
    visit DIYSTOMPBOXES.COM and do a search for “expression pedals” etc for more info if you’re still interested.

    January 19, 2009 at 5:06 pm #91112
    TweedBassman
    Member

    thanks for the replies guys.

    but, i think you’re missing the most important part. when i try to ‘add’ an external pot, either 10k or 100k, to the existing fader connections with temporary clips attached to the fader points, the pedal SHUTS OFF. as in, bypasses. yes… i have the right spots.

    there’s absolutely no problem with me adding in a 3PDT to switch the 3 fader lugs to an external pot, i’ve worked on numerous pedals and added many expression jacks. but, this seems to be different, and i’d like to err on the side of caution given the price of the pedal.

    usually before i add an external expression jack (be it switched to replace, or in series with the original pot) i test out the effectiveness by putting the original pot at it’s maximum resistance and adding the secondary pot temporarily to test it out. the signal will, as Ned said, take the least resistive path through the aux pot, given the pinouts and impedance are similar.

    this has worked just fine on my microsynth and other pedals.

    so, BEFORE i modify the pedal by adding in a 3PDT to route the LPF signal to either the fader or an external jack, i want to make sure the same result with my tests (turning off) will not happen. this leads to the fader… obviously, if it is a standard taper/impedance fader there should be no problem, read it with a meter or just read the value and go from there. oh, no… not that easy. the only marking on the fader is:

    “442NA4H103CDN” … google leads to nothing.

    meter? sure. measuring the pot value with no power to the unit, it seems to be 420 ohms. at 50% the fader offers up a 2.6K reading. at 75% up, the fader reads 1.1k. every fader on the unit is like this, absolutely identical readings from pin 1 to 3 (total impedance), and mid and 3/4 points. now, given the problem of the unit switching off electronically when testing those pot/fader points, i’m not going to power it up again and start probing, although that might get the answers i need.

    in summation, i guess i need a 100% accurate description of the fader pot size and taper, because both the printed number on the side and meter readings don’t mesh with anything i’ve encountered before… or an explanation as to why my unit shuts off when i try to add a secondary pot.

    phew!

    January 19, 2009 at 5:52 pm #91116
    TweedBassman
    Member

    OK i couldn’t wait for you guys. :)

    took out the fader, it’s 10K linear.

    alright. here’s the deal.

    January 19, 2009 at 6:16 pm #91118
    electro-melx
    Moderator
    Quote:
    OK i couldn’t wait for you guys. :)

    took out the fader, it’s 10K linear. to be continued…

    hahaha, this is way over my head!!

    good luck! :thumb:

    January 19, 2009 at 6:26 pm #91119
    TweedBassman
    Member

    alright. here’s the deal.

    i removed the 10K linear fader. there are two lugs on the right, one on the left; the left one is the wiper.

    replacing the fader with a 10k linear pot worked perfectly, and it seems the bottom lug on the fader (closest to the footswitch) is ground; aka, wired up very much like an analog LPF or tone control on a guitar.

    or, so i thought.

    in a standard analog LPF, you can set the pot to full resistance (in this case, 10k) and add a second pot in parallel ‘tacked’ onto the 3 lugs. the second pot can offer a less resistive path to ground and will take over when turned down.

    unfortunately, this is not the case on the POG. the fader must be controlling a digital message, because wired in this fashion the replacement pot does nothing until the very end of the travel, then it shuts the pedal off.

    so, the correct answer is: the fader must be completely bypassed, and switched out with a 3PDT switch wired to a stereo jack. luckily, the 3rd lug is ground so a non-isolated stereo jack ill work fine.

    now, it’s just a matter of mechanics. the easiest thing to do would be to replace the fader with a knob and rotary pot. without the lugs of the fader soldered to the circuit board (remember they have to be isolated for the exp out to work), the fader would just flop around inside. no good.

    so, is the mod worth adding a secondary knob (and removing the fader) and drilling two holes? hmmmm/

    January 19, 2009 at 7:26 pm #91126
    Ned Flanders
    Moderator

    Just like I suspected, you need to totally bypass the original pot for it to work. Also, the reason the slider was giving incorrect readings IN the unit is because the other parts create resistance and lead to false readings, the only real way of testing resistors and variable resistors is to remove them entirely…as you’ve found out.

    BTW, are you tweedbassman from pennypedals?

    January 19, 2009 at 7:43 pm #91131
    TweedBassman
    Member
    Quote:
    Just like I suspected, you need to totally bypass the original pot for it to work. Also, the reason the slider was giving incorrect readings IN the unit is because the other parts create resistance and lead to false readings, the only real way of testing resistors and variable resistors is to remove them entirely…as you’ve found out.

    BTW, are you tweedbassman from pennypedals?

    thanks man… yeah that’s me. i’ve never encountered weird readings and such with analog circuitry, digital stuff scares me.. :) you should have seen my face the first time the POG turned off while hacking it.

    the final piece to the puzzle is a picture of Joe Perry’s pedalboard. the LPF exp out was always rumored to have been done on his POG but no other information was around. knowing what i do now, you can see in his old board pics the LPF fader is gone from his POG; completely replaced by an expression pedal.

    there sure are a lot of unused connections inside a POG… CV… MIDI…

    January 19, 2009 at 7:48 pm #91135
    Ned Flanders
    Moderator

    I thought it was you because I see you at offsetguitars sometimes. I’m, 2N3904 over there.

    So are you going to do the mod? Its always reversible I suppose.

    I don’t mess with digital stuff TBH, just analog because I know I can more than likely fix it if i break it.LOL

    January 19, 2009 at 7:54 pm #91139
    TweedBassman
    Member

    tough call. i would need to drill 3 additional holes… EXP out, replacement pot, and the 3PDT switch. the 3PDT could just be a footswitch, and the EXP out, well that’s the point; but a big goofy knob replacing the fader would look ridiculous.

    unless i could find a panel mount fader that fit perfectly and had a low enough profile to not touch the board… i doubt it. i guess i could hot glue the fader in place.

    if i end up doing it i’ll post pics and how-to here. :)

    ahhh screw it i’m gonna do it. i think i figured out the best way. stay tuned.

    oh btw admins/mods… can we post modifications here?

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