Need a replacement potentiometer for a Boss ME-25

[eaelec]

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?

[Enzo]

Does it install on a circuit board? Or did you put wires on the pins?

[eaelec]

Does it install on a circuit board? Or did you put wires on the pins?

It's mounted on a board like this:

[Enzo]

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.

[eaelec]

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?

[Enzo]

I was trying to help determine why your replacement did not work, sorry I wasted your time.

[eaelec]

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.

[nickb]

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.

[eaelec]

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.

[nickb]

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.

[nevetslab]

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.

[g1]

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).

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.

[eaelec]

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.

[eaelec]

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.

[g1]

Thank you and welcome to the forum.