Replacing potentiometer inside Brent pedal

[Brandon B]

I have a model B from the 70s, and it seems to the potentiometer replaced. I was thinking I’d buy the assembly from Amaco, but they said it wouldn’t fit in my pedal housing.

Slide pot ID number is ZM4509 137/539, I can't find anything that is even close to this number online. Does anyone know what the modern equivalent is?
 

wouldn’t fit how? Could I modify the assembly to fit the old pedal?
 

I also figured I could remove and solder in a new potentiometer. I have looked everywhere online and called a dozen people but can’t find the $1.50 potentiometer for sale. Does anyone know where I could get one?

$250 seems about right for the whole pedal I guess, but not for $5 worth of parts. I must be able to get these simple electronic components somewhere, right?

thanks!

Edited December 29, 2020 by Brandon B

[neilestrick]

1 hour ago, Brandon B said:

wouldn’t fit how? Could I modify it? 

Usually the pedal casting has specific forms cast into it to hold the parts in position. It's not just a rectangle with stuff screwed to the inside. It could also be a totally different type of potentiometer.  Some have slider arms, some have gears, and they aren't all the same size. The potentiometer in a Skutt pedal won't fit in a Brent pedal, for instance, even though the pedals look about the same from the outside. Could you modify it? Anything's possible with enough effort, but it might not be worth the time or effort or be as durable as the real deal. If your old pedal has the gear-and-arm type system, the current slider system would be totally different.

[Brandon B]

I've familiar with the pedal construction. I was hoping someone who has experience with Brent pedals could tell me what the difference between the B pedals from the 70's and the new replacement part is. It looks VERY similar on the parts sales page. The new potentiometer is very similar to the old one in looks. but not sure if the dimensions are the same. the old one is 2'x7/16'. I measured the resistance at 0-0.75M Ohms. Tech support won't reveal any of the specifics, since they are, understandably, trying to cover their profit margins.

The real idea here is to find the potentiometer that Brent uses, either currently or "old, new stock" and swap it out. I have looked all over the web and haven't been able to find a straight part for part swap. (~$5)

If it is truly not possible to swap out the pot, then I was going to buy the new replacement pot assembly and modify or remove and replace the slider pot, if its the same dimensions. (~$90)

[Brandon B]

18 hours ago, Rockhopper said:

If you know the spec's for the pot, you might be able to find it at one of the following...

 

I'll look at those websites. I checked parts express already, and didn't find anything that matched. Thanks for the info!

 

Here are pictures of the old and new potentiometers. As far as google and every online store I have tried, the numbers on the side mean nothing.

Edited December 29, 2020 by Brandon B

[JohnnyK]

Can you send pix of the old pot in place on the foot pedal. As for the resistance...I don't think it is that critical as long as the starting point is zero. Kinda like a gas pedal in a car. The more you press, the faster you go...

[Hulk]

A wiring diagram might help - am not finding one with any specifics other than "Petal Pot .7M" not sure if that's helpful.

For future reference:

Brent part number 22143G for "Replacement speed control and cord for foot pedals that connect to silver face plate control boxes made before 2000..." ($123);

Brent part number 22858R depicted above right, "A replacement sub assembly for Brent Potters Wheels. This part replaces the speed control sub assembly in Brent foot pedals." ($81). 

Bill wrote (in an earlier thread): "Probably best just to measure them. The schematic above may not apply to your version and the trim pots are unlabeled in the diagram.  The pedal pot says .7 meg but I would measure to be sure. Measure across the two non wiper leads. Often they will be stamped with a value on them as well. The problem with measuring is you might have to disconnect them from the circuit else risk getting a false reading."

[Mark C.]

The deal with the older  brent foot petals (I have owned two) is If you can find the part let us know (I never could) Brent stopped selling them decades ago and I think the manufacture stopped making them. The new ones do not fit in the old foot pedals . These as you noted are cheap parts and finding a supplier would be great.I had to swap out to the new style foot pedal a few decades ago (maybe 3 decades) when I could not find a replacement.There is a slight chance I may now have an older one (I'll look as I do not have the older foot pedal anymore)If I do you can have it.I'll look today.

As you can see from the images the slider has the part number . It fits into the base which is a separate part so if you can find the slider part you can modify the part to fit your base.

Just look for the part number and send a photo to compare to suppliers-I did that about 20 years ago with zero luck

It would be a godsend if you find the supplier for us all as that part is under 10$ I'm sure and just snaps in the plastic base . The old Brent footpedl has a different shape than the new one. 

As you know the old potentiometers are all white the new ones  (late 70s onward are all black)If you can find the newer ones we all will benifit as Brent stuff is way overpriced.

[Mark C.]

I looked and all I have is the new potentiometer (new spare) and a new style broken one. I think I gave all my old Brent stuff to the last person who I sold an older wheel to from the early 70s.

let us know if you find a source 

another thought is those new ones as seen in photo have the same slider so look for that one and fit it to old body mount.I think they attach the same way.I have the number on the side but you can read that number in above photo on right (black unit)

If you find them please let us know the source.

[Brandon B]

Thanks for all the replies.

I haven't had a chance to open up the pedal again and take pictures, but there are pictures online that show what it looks like. I was posting about this on a electronics forum, and some people didn't seem to think the resistance mattered, just as JohnnyK had stated. They said that any resistance will work, but that the power will dissipate differently for different resistances. They had some fancy equations to describe the relationship...

I can find some slide pots that are the same size as the old one, so I'm thinking that I am going to try buying one and putting swapping it. Worst case, I'm out $2. They said it wouldn't have any effect on the motor, somehow...

I was able to clean it thoroughly and that seems to have helped significantly, however, I was told that cleaning is usually a temporary fix with diminishing rewards over time.

Seeing as this is mid century technology, I figured it would be as easy to fix as working on my stereo. Frankly, I'm amazed at how hard it is to find this part. I was tipped off that manufactures sometimes buy parts from other countries (from companies that don't sell in the US)  in order to hide their source, so people can't buy component parts. Who knows whats going on here, but it sure is annoying.

[Brandon B]

Here is that explanation about potentiometer resistance:

The pot makes a voltage divider that feeds the analog input. Vout = Vin * R2/(R1+R2)

where R1 is the resistance between Vin and Vout, and R2 is the resistance between Vout and Ground.

If the knob is halfway, then R1 = R2, so the equation simplifies:

Vout = Vin * R1/(R1+R1)

so with 5K:

Vout = 5V * 2500/(2500+2500) = 2.5V. 

and with10K 

Vout = 5V * 5000/(5000+5000) = 2.5V.

 

So the overall value of the pot doesn't really matter.

Where the value does come into play is how power it dissipates.

Power = Current x Voltage, or P=IV.

Current = Voltage/Resistance, so a 5K pot with 5V will have 5V/5000ohm = .001A, 1mA going thru it.

Power = .001A * 5V = 5mW, which is not much, but might be an issue for battery powered operation where long ontime is a concern. In which case a 10K pot might be better.

[Brandon B]

I'm going to buy the one on this page https://www.surplussales.com/cgi-bin/cart.pl

 

labeled as: (RCV) BX8195-0

We will see what happens!

 

[Mark C.]

That link does not show anything 

Can you repost a better one?

[Mark C.]

How about  getting that 10k ohm one as well for 4$ more you have options-did Brent give you an ohm spec on their potentiometer ?Shipping will cost the same-whats another $4?

Edited February 28 by Mark C.

[PeterH]

10 hours ago, Mark C. said:

That link does not show anything 

Can you repost a better one?

Look like it's Malatchi CTS Slider Pot 10k Ohm on https://www.surplussales.com//Potentiometers/Slider/Slider-1.html

[Bill Kielb]

12 hours ago, Brandon B said:

I was able to clean it thoroughly and that seems to have helped significantly, however, I was told that cleaning is usually a temporary fix with diminishing rewards over time.

I think you have the answer If you are able to clean it then you should be able to measure it. Pots are rated end to end. To check linear or audio taper set the slide to midpoint and measure wiper to any side. If approximately half, then linear taper. If significantly different than half then audio taper. Very likely linear though. Post and tag the value here for folks in the future.

[Mark C.]

I just measured a new spare I have (its the same model number as photo of black one #22126K)

the top end in ohms is 170 to zero with midrange about 130

The  two models that are on that page are

1M ohms

and 10k ohms

Bill what model is best?

 

Edited December 31, 2020 by Mark C.

[Bill Kielb]

I would check your measurement of the existing. Especially if it’s in circuit, might have to disconnect two of the wires to make sure you are measuring the pot alone. I say this because low potentiometer ranges are not as common and when used they tend to (Not always) be a high current device. Having such a small change in resistance say zero to 200 ohms  often leads to difficulty in precision.

So double check first with nothing else connected and if it is truly prox 0-200 ohms neither the 10k nor the 1meg will likely do. Just need a 0-200. With that part number I would feel better if you would have said 26k ohms. Any chance your reading above is really in thousands?

[Mark C.]

2 minutes ago, Bill Kielb said:

I would check your measurement of the existing. Especially if it’s in circuit, might have to disconnect two of the wires to make sure you are measuring the pot alone. I say this because low potentiometer ranges are not as common and when used they tend to (Not always) be a high current device. Having such a small change in resistance say zero to 200 ohms  often leads to difficulty in precision.

So double check first with nothing else connected and if it is truly prox 0-200 ohms neither the 10k nor the 1meg will likely do. Just need a 0-200. With that part number I would feel better if you would have said 26k ohms. Any chance your reading above is really in thousands?

I'll recheck in am as I'm doing some electric kiln work tomorrow  

The one i checked is a brand new unit-I have a few used ones as well so I do not have to diassemble anything.

Edited January 1, 2021 by Mark C.

[Bill Kielb]

Not sure if this helps but at some point I got a Brent schematic from someone here which clearly shows .7 m  double shafted pot for the pedal and a mysterious trim pot  going into a voltage divider. Definitely not your pot but .7 M  the value is interesting if they stuck with the diac control.

[Brandon B]

sorry here it is if you still need it

https://www.surplussales.com/Potentiometers/Slider/Slider-1.html

 

[Brandon B]

On 12/31/2020 at 8:41 AM, Bill Kielb said:

I think you have the answer If you are able to clean it then you should be able to measure it. Pots are rated end to end. To check linear or audio taper set the slide to midpoint and measure wiper to any side. If approximately half, then linear taper. If significantly different than half then audio taper. Very likely linear though. Post and tag the value here for folks in the future.

I measured it at 0 - 0.75M ohms from top to bottom. Its definitely linear.

For anyone who is checking the resistance, ohm meter must be set to M (I have a blue point meter and is says 2M). There are three prongs, on the pot, two next to each other and one at the other end. measure from one of the two that are next to each other, to the prong at the other end (that isn't connected to anything). It should give you a pretty accurate reading.

Edited January 1, 2021 by Brandon B

[Brandon B]

12 hours ago, Bill Kielb said:

Not sure if this helps but at some point I got a Brent schematic from someone here which clearly shows .7 m  double shafted pot for the pedal and a mysterious trim pot  going into a voltage divider. Definitely not your pot but .7 M  the value is interesting if they stuck with the diac control.

I'm pretty sure that the drawing you have  posted (not totally positive) is for the very first generation of wheels. I think they were 220V. Mine is from mid to late 70s which I think was the second generation, it runs on 120V. The trim pots are connected, one in series and one in parallel, to adjust the stop position and the high end speed of the pedal. I was told that a 1M ohm  potentiometer should work for this application, If I can't find a 750k ohm. Finding a 750k with the correct shape, and PC connection has remained elusive. If anyone finds one, let me know.

My grandfather was an electrical engineer back in the 50s - 80s and I remember just about any motor with enough torque to spin 75lbs while someone is pushing against it was 220V. He had shelves and shelves of electric motors he would let me play with. I wonder if Brent would have used a 220V motor originally, as it was the "go-to" back then. with some more playing around, maybe Brent found that they could use a 120V successfully... I don't know, just a thought. 

[Mark C.]

Brent makes  and did  make 220 wheels they where mostly for export market.

The pot slider also has to have the end connection to work well to attach to base.

I have moused around for hours looking for the Pot sliders with the right lever and mounts-never found one yet.

Edited January 1, 2021 by Mark C.

[Dick White]

Not so much about the initial question (replacing the pedal pot) but regarding the later comments about motors and voltage - dunno about what Brent was doing back when the earth was still flat, but I believe Brent motors now  are 90v DC. The controller rectifies whatever design input of local AC voltage and the pedal pot instructs the controller to increase/decrease the DC voltage to the motor to make it go faster or slower.

[Mark C.]

1 hour ago, Dick White said:

Not so much about the initial question (replacing the pedal pot) but regarding the later comments about motors and voltage - dunno about what Brent was doing back when the earth was still flat, but I believe Brent motors now  are 90v DC. The controller rectifies whatever design input of local AC voltage and the pedal pot instructs the controller to increase/decrease the DC voltage to the motor to make it go faster or slower.

Dick whats the amps on that 90v ?