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Thread: Need a replacement potentiometer for a Boss ME-25

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    Need a replacement potentiometer for a Boss ME-25

    The potentiometer for the expression pedal on my ME-25 wore out, so I opened it up and found it was an ALPs RK11K series, 10K ohms.
    My brother has the older ME-20, which still works OK, so I opened it up and checked the potentiometer in that one, also an ALPs RK11K (10K) and using an ohmmeter, it looked like a linear taper. I found the datasheet for the RK11K series and determined that the RK11K1150A3L part was the best match as far as I could tell. So I ordered a couple of new potentiometers. I installed one of them in my ME-25, now the volume function does not work, full volume all the time, Wah-wah still works but not like it used to. Has anyone successfully replaced one of these and if so, what part did you use?


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    Lifetime Member Enzo's Avatar
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    Does it install on a circuit board? Or did you put wires on the pins?

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    Quote Originally Posted by Enzo View Post
    Does it install on a circuit board? Or did you put wires on the pins?
    It's mounted on a board like this:


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    Lifetime Member Enzo's Avatar
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    OK, so unlikely to be miswired.

    You have a ribbon. use your ohm meter to verify continuity from each pin on the pot down to some solder joint on the board at the other end of the ribbon.

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    Quote Originally Posted by Enzo View Post
    OK, so unlikely to be miswired.

    You have a ribbon. use your ohm meter to verify continuity from each pin on the pot down to some solder joint on the board at the other end of the ribbon.
    I don't need a lesson in basic trouble shooting. I need an exact part number for the ALPs pot, do you have one?

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    Lifetime Member Enzo's Avatar
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    I was trying to help determine why your replacement did not work, sorry I wasted your time.

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    Quote Originally Posted by Enzo View Post
    I was trying to help determine why your replacement did not work, sorry I wasted your time.
    Apology accepted. I did some more work on it and I now know that the problem is the linear taper. However the other tapers are only available on special order with a minimum order of many thousands of pieces.

    So on to plan, B. This will be a Mc Giver type hack, I'll post more on this later.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by eaelec View Post
    Apology accepted. I did some more work on it and I now know that the problem is the linear taper. However the other tapers are only available on special order with a minimum order of many thousands of pieces.

    So on to plan, B. This will be a Mc Giver type hack, I'll post more on this later.
    The taper law simply changes the relationship between the rotation angle and the resistance. Therefore I can understand the linear taper giving you a problem in the wah-wah function in which a mechanical linkage is crucual, but it would not stop the volume from working. You might want to deep a little deeper into why that isn't working.

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    Quote Originally Posted by nickb View Post
    The taper law simply changes the relationship between the rotation angle and the resistance. Therefore I can understand the linear taper giving you a problem in the wah-wah function in which a mechanical linkage is crucual, but it would not stop the volume from working. You might want to deep a little deeper into why that isn't working.
    If you disconnect the pot from the foot pedal mechanism and rotate it with your fingers, the volume function works just fine. The problem is the foot pedal restricts the rotation to a small range of the pot's full sweep, that's why a logarithmic taper makes a huge difference in the output voltage on the wiper.

    This output voltage goes the a AtoD converter on the main DSP board, it's not like the wah-wah circuit on a Cry Baby where the pot is part of a RC filter circuit, this pot just supplies a DC control voltage. I'll post the voltage ranges later.

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    Last edited by eaelec; 02-14-2019 at 08:06 PM.

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    Quote Originally Posted by eaelec View Post
    If you disconnect the pot from the foot pedal mechanism and rotate it with your fingers, the volume function works just fine. The problem is the foot pedal restricts the rotation to a small range of the pot's full sweep, that's why a logarithmic taper makes a huge difference in the output voltage on the wiper.
    Ah! That explains it I didn't realize the volume was also a foot pedal.

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    Senior Member nevetslab's Avatar
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    Have you tried to open up the original pot to see if you can restore it to operational status? Of course if the wiper fingers are worn out, or the resistance track worn out/fractured, or leads broken off, it's no go. Just a thought...... I do that sort of thing a lot on pots that aren't going to be easy to find direct form/fit/function replacements on.

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    Don't forget the joker g1's Avatar
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    Yes, it's likely a custom pot that you either need to get from the manufacturer (Boss) or order a thousand (or whatever the minimum is).

    Quote Originally Posted by eaelec View Post
    I don't need a lesson in basic trouble shooting. I need an exact part number for the ALPs pot, do you have one?
    BTW, for a new member's very first interaction, a rude response to one of the most well-respected senior members (who also happens to be a moderator) is not a very smart move.
    All types of help is appreciated on this forum, and should be acknowledged as such.

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    Here's the key voltage levels at the wiper:
    .3 volts - "toe-down", the FX pedal light toggles when you drop below this level
    .5 volts - Volume reaches max, does not increase as voltage goes lower
    3 volts - max attenuation, no volume.

    With the pot attached to the pedal mechanism, the voltage range is limited from .16v up to 1.17v volts.
    So there is some effect on volume, but the way the human ear works it's not noticeable.

    Now if I were to feed the pot with a higher voltage i could fix this, the pedal runs on a 9V adapter so I could tap off of that.
    PS
    I made a real rookie mistake yesterday, voltmeter leads in the wrong way, toe-down voltage is actually 3V, vol. max is 2.8v, min vol. is .3V.

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    Last edited by eaelec; 02-15-2019 at 03:21 PM.

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    Quote Originally Posted by g1 View Post
    Yes, it's likely a custom pot that you either need to get from the manufacturer (Boss) or order a thousand (or whatever the minimum is).


    BTW, for a new member's very first interaction, a rude response to one of the most well-respected senior members (who also happens to be a moderator) is not a very smart move.
    All types of help is appreciated on this forum, and should be acknowledged as such.
    My bad, sorry to Enzo, was having a very bad morning.

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    Thank you and welcome to the forum.

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    Not sure how hard those are to take apart, but is it possible there is the proper taper available in a different format (other shaft for example) that you could cobble together one from the two?

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    Senior Member nevetslab's Avatar
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    [QUOTE=g1;522701]Not sure how hard those are to take apart, but is it possible there is the proper taper available in a different format (other shaft for example) that you could cobble together one from the two?[/QUOTE

    I'll find out in the morning, as the Blackstar Series One Model 200 is chock full of these pots. Is the part you're looking for 10k Log Taper?

    Click image for larger version. 

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    I'll also have the mechanical dimensions & disassembly photos from the surgery....this PCB is from my boneyard.

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    Last edited by nevetslab; 02-15-2019 at 04:37 AM.
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    You may be able to disassemble both pots and transplant the wafer or wafer and housing assembly onto the new shaft and wiper assembly. Itís tedious but it can be done.. even if you have to encase it in a block of epoxy. You have them already. Why not try?

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    Quote Originally Posted by g1 View Post
    Not sure how hard those are to take apart, but is it possible there is the proper taper available in a different format (other shaft for example) that you could cobble together one from the two?
    I'm very confident I can take one apart, putting it back together not so much.

    If the mod I'm thinking about works, there may be not need to do that.

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    Last edited by eaelec; 02-15-2019 at 03:17 PM. Reason: typo

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    Senior Member nevetslab's Avatar
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    Pot Dissambly & dimensions

    Quote Originally Posted by eaelec View Post
    I'm very confident I can take one apart, putting it back together not so much.

    If the mod I'm thinking about works, there may be not need to do that.
    What's on the Blackstar Model 200 front panel board are Alpha, 11mm size, mounting between the support tabs is 11mm, and between the center line of the support tabs to that of the four pot leads is also 11mm. 8mm dia threaded bushing, 5mm long, shaft length on these are 17mm, shaft dia is 6mm, D-shaft knob detail at the end. PCB Lead pitch is 2.5mm (between the four leads). Photos of the pot & disection:

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    Prying up the four fingers set down into the body's grooves and straightening them allows the resistance module to be removed. The shaft/rotor assembly slides out of the bushing/mounting bracket assembly. Leave the grease on the shaft/bushing insides, as that's your viscosity feel compound. Normally you would clean the resistance track with some alcohol, then apply some Caig DeOxit 5 for lubricant, and brush on a little bit onto the rotor assembly's wiper fingers. I didn't clean this one until after I had taken all the photos, and have more images, only I've used the max 9 images on this post. With the shaft/rotor assy back onto the body, I connected my Fluke 8060A in 200k Ohms mode, showing Min rotation between wiper and CCW terminal (0k), 50% rotation (15k) and full rotation (98k)...this being a 100k Audio Taper pot.

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    Oh well.

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    Quote Originally Posted by eaelec View Post
    Here's the key voltage levels at the wiper:
    .3 volts - "toe-down", the FX pedal light toggles when you drop below this level
    .5 volts - Volume reaches max, does not increase as voltage goes lower
    3 volts - max attenuation, no volume.

    With the pot attached to the pedal mechanism, the voltage range is limited from .16v up to 1.17v volts.
    So there is some effect on volume, but the way the human ear works it's not noticeable.

    Now if I were to feed the pot with a higher voltage i could fix this, the pedal runs on a 9V adapter so I could tap off of that.
    PS
    I made a real rookie mistake yesterday, voltmeter leads in the wrong way, toe-down voltage is actually 3V, vol. max is 2.8v, min vol. is .3V.
    Good idea. Alternatively, it looks like the PCB in the picture doesn't do anything other than connect the ribbon to the pot. So would it be possible to connect the ribbon directly to a common 16mm pot mounted in the plate. Is there room?

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    I got it working, but with a little more effort than I originally thought. After I found the mistake in post 13, i realized I needed to pull one end of the pot down below ground with a negative voltage.

    So, I tapped +9V off of the back of the adapter jack, then used a simple linear regulator to bring it down to +5V. The +5V feeds into a switched capacitor voltage inverter (ICL7660) which changes +5V to -5V. Then I cut a track to disconnect the end of the pot which was connected to ground and connected that to the -5V (that's the white wire in the picture). Now it would be prudent to add some kind of clamping circuit on the wiper. If the wafer in the pot breaks, it could send -5V into the main board, which would not be good. I tried a simple 3V zener between +3.3V and the wiper but it caused problems, I'll have to investigate that some more. Here's what it looks like:
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    Quote Originally Posted by nevetslab View Post
    What's on the Blackstar Model 200 front panel board are Alpha, 11mm size, mounting between the support tabs is 11mm, and between the center line of the support tabs to that of the four pot leads is also 11mm. 8mm dia threaded bushing, 5mm long, shaft length on these are 17mm, shaft dia is 6mm, D-shaft knob detail at the end. PCB Lead pitch is 2.5mm (between the four leads). Photos of the pot & disection:

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    Prying up the four fingers set down into the body's grooves and straightening them allows the resistance module to be removed. The shaft/rotor assembly slides out of the bushing/mounting bracket assembly. Leave the grease on the shaft/bushing insides, as that's your viscosity feel compound. Normally you would clean the resistance track with some alcohol, then apply some Caig DeOxit 5 for lubricant, and brush on a little bit onto the rotor assembly's wiper fingers. I didn't clean this one until after I had taken all the photos, and have more images, only I've used the max 9 images on this post. With the shaft/rotor assy back onto the body, I connected my Fluke 8060A in 200k Ohms mode, showing Min rotation between wiper and CCW terminal (0k), 50% rotation (15k) and full rotation (98k)...this being a 100k Audio Taper pot.
    The only RK11K series pot that you can buy off-the-shelf with a log taper is a 20K pot, not a 10K.

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