Yam FC7 pot

[Reezekeys]

Apropos the recent FC7 thread, I dug mine out to make sure I wasn't talking out my a** when I mentioned the weird taper. And... the pot inside is kaput; I need a new one (I took the pedal apart and used my VOM on the pot's terminals to verify, also hooked it up to my KX88 and looked at the midi stream - it's definitely broken). A fair amount of searches turned up posts claiming it's a 50K log taper pot - and then I found an old thread here saying it's a linear taper, but has a "custom actuation range":

 

https://forums.musicplayer.com/ubbthreads.php/topics/2996521/re-equip-a-yamaha-fc7-with-second-pot#Post2996521

 

Great, a hard-to-source pot. There's one on Ebay right now: $23 shipped, from China. Not my tempo, considering I rarely use my KX88 (no it won't be for sale ever, sorry!). I do want to fix this pedal, however I'd like to try to find a less expensive solution if possible. I think I'd be fine with a plain linear taper pot (actually I think I'd prefer it), but I also wonder if the KX will have an issue with that. It looks like the housing has plenty of space, so I'm not limited to using a particular size pot - however the D-shaft has to fit the sleeve attached to the pedal platform.

 

I'm curious if anyone has been down the road of replacing this pot and what they wound up with. A part number would also be amazingly helpful, although I'm ready to break out my Harbor Freight $10 digital caliper, make some measurements, and download spec sheets from Mouser. Thanks!

[kenheeter]

Have you tried a Yamaha repair depot? I am one ( I'm in Canada ) and I have them in stock.

Ken

[Reezekeys]

Thanks but Google does not return any hits on "Yamaha repair depot." I see a Yamaha site where I need to set up a "consumer account", after which they'll presumably let me in to search for this pot. I may need to provide a Yam part # but can't find any online resource for that. I thought there might be a service manual here but nothing came up searching for "FC7", "pot", or "potentiometer." If it's gonna be this much trouble to find a part that I'm sure will be 3-5x more expensive than a generic pot from Mouser, I'm probably going to pass. I can live without a working FC7 pedal for now, although it would be nice to have if I start using my KX88 in my studio. I see brand new FC7s for less than $40 â I think it's probably worth the few extra bucks I'd pay for a new one to avoid spending the time to find this part on its own â which is exactly what Yamaha wants, I'm sure. Thanks for trying to help!

[Reezekeys]

Here's a montage of the part in question, should this visual be of any assistance.

 

[Delaware Dave]

Mine went kaput as well. i was using it with my Gem Equinox and it stopped working. I did the search and found a replacement part offshore but for about $15 more I could just buy a new pedal. It was dismantled for a few years. About 5 months ago I tossed it out.

[Reezekeys]

I'm gonna break out the caliper, measure this guy, and try to find a replacement at Mouser. I'm certainly in no rush to fix it. In the interim, if I need an expression pedal for my KX I'll try using my M-Audio EXP â since it has the switch to flip the tip & ring connection. I know the pot's value is different, and the taper too. I'll try it tomorrow to see how it does.

[MIDI2XS]

Thanks but Google does not return any hits on "Yamaha repair depot." I see a Yamaha site where I need to set up a "consumer account", after which they'll presumably let me in to search for this pot. I may need to provide a Yam part # but can't find any online resource for that. [...]

I suspect the "Yamaha site" you mentioned is yamaha24x7.com

 

This is the pot info...

WF876300 VR ROTARY B 50.0K RK1631110G FC-7

 

WF876300 is the Yamaha part number

RK1631110G is likely the Alps number, which may be a custom device

[jyrkik]

Hi,

 

50k B hints that the pot is 50kohm (50000ohm) and has linear taper. Letter A would indicate logarithmic taper. I can`t see that Yamaha would bother using some custom/extraordinary type of potentiometer for a bulk product like the expression pedal anyway.

 

Jyrki

[MIDI2XS]

Hi,

 

50k B hints that the pot is 50kohm (50000ohm) and has linear taper. Letter A would indicate logarithmic taper. I can`t see that Yamaha would bother using some custom/extraordinary type of potentiometer for a bulk product like the expression pedal anyway.

 

Jyrki

Alps RK16 pots can be configured in various ways...

https://tech.alpsalpine.com/prod/e/pdf/potentiometer/metalshaft/rk16/rk16_varietyother.pdf

 

By "custom", I mean a variation that is not normally stocked/off-the-shelf.

 

For the RK1631110G, I don't know specifically what the G suffix indicates.

[MIDI2XS]

Note that the RK163 "Product Line" stock units are all indicated as 10k resistance...

https://tech.alpsalpine.com/prod/e/pdf/potentiometer/rotarypotentiometers/rk16/rk16.pdf

 

Perhaps Alps can offer some insight into the RK1631110G specifics...

https://tech.alpsalpine.com/inquiry/catalog?lang=en

[JimboKeys]

From what i recall from previous discussions on this topic, the FC7 pot is a special short-throw version. Whereas a standard pot takes 300 degrees to go through its full range, the pot in the FC7 only turns (let's say) 100 degrees from pedal up to pedal down, with the remainder of the pot's rotation being a static value. So it requires a particular custom pot - a generic replacement won't work.

 

- Jimbo

[jyrkik]

From what i recall from previous discussions on this topic, the FC7 pot is a special short-throw version. Whereas a standard pot takes 300 degrees to go through its full range, the pot in the FC7 only turns (let's say) 100 degrees from pedal up to pedal down, with the remainder of the pot's rotation being a static value. So it requires a particular custom pot - a generic replacement won't work.

 

- Jimbo

 

That may well be the case - 50kB sure doesn`t yet tell anything about the throw length..

 

Jyrki

[Reezekeys]

You guys are great. Thanks MIDI2XS, yamaha24x7.com was the site I saw, and since you gave me the part # I'll get an account on there to see what they want for this pot. I understand the economics of selling a single piece to a schlub keyboard player. Let's see how bad it's gonna be. Of course it's probably worth paying the big bucks vs. spending time researching Alps part numbers & specs.

 

I'm still a little surprised by the confusion that seems to reign over the taper â log or linear, custom-or-not, etc. One thing I can do is try to measure the throw to see if it indeed doesn't do the full travel. Can't look at the taper since it's busted.

[Reezekeys]

One quick bit of news - my M-Audio EXP with its 10K linear-taper pot works perfectly on my KX88, sending a smooth range of midi values from 00 to 7F. I've read in earlier posts that resistance values for pots used in these applications aren't critical, as they're part of a voltage divider circuit. I had a question as to whether Yamaha was doing anything funky that might affect how it reads a pot that's not 50K, but I guess that question is answered. I suppose there might be more current flowing through a 10K pot than a 50K - and I only had this connected for a a minute. Maybe it does matter?

[Reezekeys]

Posting replies to my replies now! I got an account at yamaha24x7.com, then entered the part # MIDI2XS gave me. What a clunky website - the only one I've had to type my billing address in addition to my mailing address - they're the same, and every other website has a checkbox to quickly mark that. Another thing is that, were it not MIDI2XS giving me the part #, I wouldn't have found this pot at all. Try searching the model # field on "FC7" or "FC-7", or just entering "FC" by itself - you get a list of nine "FC-x"s, none of them being the FC7. Then they wait until the last step before telling you the shipping charge. Long story short, it's $15 for me to get this pot. Of course I understand why these small parts orders usually wind up with the shipping being more than the part itself; most of that cost is the labor to find it in a warehouse and pack it into an envelope or box. I get that, but seeing as my FC7 gets zero use and my EXP works fine on my KX88, I'm going to leave things as they are. I'm not gonna toss the FC7 though; I plan to keep my eyes open for a pot that will go into it. I might even have some old gear somewhere I can cannibalize. Knowing that the value isn't critical is helpful â it should just have a linear taper, and the correct shaft type and length so it'll fit into the pedal assembly.

 

Thanks everyone for your input! Throughout my time here I've gotten some great guidance from many of you and I really appreciate it.

[Mike Warren]

The "B" signifies it's a linear taper, but many pedals need short throw pots. You'll be able to tell this by looking at how much of the 270 degree range is actually used.

[Reezekeys]

I'm curious, so I'll see if I can determine the range the FC7's pot turns - but as I mentioned, my M-Audio pedal works perfectly with my KX88. Different company, different value pot, both with the same exact short throw? Seems like that would be quite a coincidence, unless there's something about how this circuitry works that I'm not getting.

[Mike Warren]

The actual value of the pot is not the most important thing. Typically, the keyboard send out 5V on the top end of the pot. The bottom end is connected to ground and the wiper forms a voltage divider. For most keyboards, I'd expect anything between 20K and 100K to work, but the range is the killer.

[Reezekeys]

Ready for a nerd explosion? I think I may have solved the mystery of this pot - or maybe not!

 

I took some pics. First, with the pedal "actuator" (or whatever that plastic piece attached to the pot's shaft is called). I took pics at the extreme pedal up & down position. Then I removed this plastic piece, and took pics of the bare shaft at its full CCW and CW position.

 

Using Photoshop's measuring tools I came up with the degrees of travel. The D-shaft made this pretty easy. This is what I found (I did round values a bit):

 

The full travel of the pot is 300 degrees. The pedal uses only 90 of those. However, the unused portions are equal on both ends of travel: approximately 105 degrees of travel at each end are not used. 105+90+105 = 300. So I'm guessing this is a linear pot, not one with a custom taper. If anyone thinks my methodology is wrong, please correct me! PS, I have to get a life! (Actually it's my first weekend with no gig in a month, so I do have some time to kill right now).

[Mike Warren]

For the volume to go to 0 the wiper has to be connected with almost no resistance to one end of the pot, so that pot would have to be 0 ohms for a large part of its travel, so it's a special one.

 

(BTW, I'm an electronics engineer)

[MIDI2XS]

OB Dave determined the special resistance track several years ago...

https://forums.musicplayer.com/ubbthreads.php/topics/2887601/re-expression-pedal-summary#Post2887601

 

I'm sure Alps has no problem accommodating Yamaha's needs.

[Reezekeys]

Thanks guys for straightening me out! I thought that total resistance wasn't critical in a voltage divider circuit, and that by having a straight linear pot be in the middle of its resistance range when the pedal was halfway down (which would be the case if my uneducated assumption was correct), that's all that mattered. Now I know - the resistance has to be zero at the low end of the pedal's range - wherever the pedal might put that pot.

 

So, presuming the FC7 has been around as long as the KX88 (since 1984) and that it works with any Yamaha keyboard made since then that accepts an expression pedal, Yamaha had to be certain that it could get this custom pot made for it - for as long as they make midi keyboards. I sure don't know the potentiometer manufacturing business, but I assume that this is no big deal for any of them? To my uneducated brain it would seem to drive up the cost of the pedal, compared to other pedals that use more off-the-shelf pots.

[rogs]

I've just measured my FC7 using a simple DMM....(Tenma 72-7930)

 

⢠The pot measures 44KΩ, measuring between sleeve and tip (2MΩ max. scale)

 

⢠Pedal minimum:

Tip and ring: 44KΩ (2MΩ scale)

Ring and sleeve: 0Ω (2MΩ scale) -- c.330Ω (2kΩ scale) (less than 1% of pot value, and typical of the minimum pot resistance of a 50KΩ pot.)

 

 

⢠Pedal maximum:

Tip and ring: 44KΩ --

Ring and sleeve:0Ω (2MΩ scale) -- c.350Ω (2kΩ scale)

 

⢠Pedal mid position:

Tip and ring: 22KΩ (2MΩ scale)

Ring and sleeve: 23KΩ (2MΩ scale)

 

So on this particular FC7 (straight out of the box - no mods) I have a linear taper pot that uses the entire range.

 

(The 'residual' minimum resistances are largely academic, and are a function of the pot track limits)

[Reezekeys]

So on this particular FC7 (straight out of the box - no mods) I have a linear taper pot that uses the entire range.

Are you referring to the entire 300 degree range of the potentiometer itself, or only its range as determined by the pedal, from full heel down to toe down? I suspect the latter, since you'd have to disassemble the pedal and remove the cam to look at the taper throughout the pot's entire range.

 

Unless I'm misunderstanding, you're corroborating what MIDI2XS, Mike, Jimbo and a few others have said - the pot is linear from 0 to ~50K for its travel within the 90 degree range controlled by the pedal's movement - outside of that range (which my post with pics above detail), it's probably at 0 ohms for the first 100 degrees of travel â so a custom item. Thanks for checking this - I wanted to, but of course my pot is broken so I couldn't!

[rogs]

So on this particular FC7 (straight out of the box - no mods) I have a linear taper pot that uses the entire range.

Are you referring to the entire 300 degree range of the potentiometer itself, or only its range as determined by the pedal, from full heel down to toe down? I suspect the latter, since you'd have to disassemble the pedal and remove the cam to look at the taper throughout the pot's entire range.

 

As you suspect -- the latter. I simply measured the resistance across the TRS connector at max - min - and half way. The manual suggests that the pedal only moves through between 20 and 30 degrees, depending on the 'standing up' or 'sitting down' setting.

I measured it as it is out of the box, so 'sitting' setting (around 20 degrees).

 

It certainly measures as if the pot is covering the full range -- it must be a very special taper that does nothing for most of its (unused) rotation...

[Reezekeys]

The manual suggests that the pedal only moves through between 20 and 30 degrees, depending on the 'standing up' or 'sitting down' setting.

I measured it as it is out of the box, so 'sitting' setting (around 20 degrees).

I see that reference to 20-30 degrees of travel in the manual, however I measured the pot moving 90 degrees through it's travel from heel down to toe down. I'm sure this is on a physics exam somewhere when converting straight to circular motion - and it's way above my pay grade! The pot definitely rotates 90 degrees (out of its total of 300 degrees) during the travel of the pedal platform, unless I totally screwed up on my methodology when measuring. I suspect it's a function of the radius of the cam attached to the pot's shaft.

It certainly measures as if the pot is covering the full range -- it must be a very special taper that does nothing for most of its (unused) rotation...

Yes, that's the point being hammered home to me on this thread - this is a special pot. See the link five posts up from this, to an older thread where OB Dave looked at the taper of the pot. Summary: "It is a linear taper pot, but the range of the taper has been restricted to only a subset of its full rotation, and they've engineered this subset to correspond to the motion of the cam."

 

Which means that if I want to have a working FC7 I'll need to buy the replacement from Yamaha. It'll cost me $15 â that's a lot for one pot but I'll probably bite the bullet somewhere down the line. Even though I have no use for the FC7 right now, that might change. I'm a bit of a packrat and can't see tossing this pedal into the garbage.

[Gourockian]

The purpose of this post is to detail how I managed to finally repair my FC-7 expression pedal.  It also shows what the inside of the Yamaha FC-3 potentiometer looks like and hopefully, this will clarify how the pedal works.  First, a little bit of history about mine.

I've owned an FC-7 from the late 1980's when I also bought a Yamaha DX21 and it initially gave me great service.  However, over the years it began to get crackly and I occasionally opened it up and sprayed the pot with contact cleaner.  Later on when spraying no longer seemed to help, I would open up the pot a few times to adjust the tension of the wiper half-ring, which had a small black cone-shaped tip making contact with the track.  I'm not sure what this was made of, possibly a hard carbon but my attempts to adjust the tension eventually caused the tip to be badly mis-alligned so, back around 2012, I changed the pot for a different linear 50K one but never could get it to work as well.  I kept the original Yamaha pot for a future repair project.  

 

Recently I took up the challenge to try and fix the existing pot, as a new Yamaha one would have cost me around $30 with shipping and tax added on (a brand-new pedal can be bought for $39). This meant having to fully dismantle it so after (yet again) prying back the 4 tabs of the metal case, I separated the wiper assembly from the track, by carefully drilling the flared-out end of the shaft and prying off the nylon disk. 

 

 

Here is a picture of the track surface of the original Alps pot from the FC-7 on the right, compared to another 'standard' Alps 50k linear pot.  As has been previously suggested by others, the carbon track does not cover the full extent of travel of the wiper.  Mine is in quite a poor state with a broken solder tab at one end and if you look closely, you can see the remnants of a pink and yellow wire on the other.  They are for a polarity switch which I wired up but that's another story.

 

 

The next picture shows the differences in the wiper arm arrangement and the hole in the one on the right is where the cone-shaped tip had been attached.

 

 

After separation of the original wiper, the contact tip eventually became dislodged, fell on the floor of my garage and was nowhere to be found.  However, I managed to replace it by inserting a 1/16 inch ball bearing, after ensuring the wiper arm was well tensioned.  This had always been difficult to do earlier, before separation and although using a pair of ex-British Telecom specialised tension adjusting pliers, my previous attempts probably contributed to the damaged of the original tip.

 

 

After spraying the track with Deoxit D5 and F5, I re-assembled the pot by carefully using a punch on the end of the shaft and installed it in the pedal.  It's now working as good as new and hopefully, will continue like that for a good few years.  Perhaps this will encourage others to at least attempt a repair if you are having serious issues with the potentiometer in your FC-7.

[Al Coda]

Excellent,- thx a lot for sharing !

 

☺️

 

A.C.

[Reezekeys]

This is very cool, thanks for taking the time to put that post together. Seeing this, I'm motivated to take my FC7 pot apart to see what's doing. The ball bearing is very ingenious!

 

Seeing that picture of the carbon track - where in the range it's located - makes perfect sense now, and completely confirms what my makeshift "science experiment" above found. It corresponds exactly to the start & end points, and amount of travel, the FC7's pedal mechanism turns the pot. Of course OBDave posted this info here over four years ago – I'm certainly not the first with this revelation!