Help with PFC Yoke needed

[Rocky_53 1,566]

Hi guys, I recently purchased a used Vmax yoke, which is a few years old and I believe was made by PFC. I have been having some erratic behaviour with the elevator/pitch travel. On closer inspection it appears that the potentiometer (?) may be worn and needs replacing. I have been in touch with PFC and they say they no longer stock the parts or provide support for the unit. I am no expert in electronics, in fact a bit of a noob, but I am thinking it would be a question of a replacement part (?)

 

 I understand form PFC that I need a 100k ohm potentionmeter with a 100mm travel. Can anyone offer any advice on how I should proceed. Thanks fellas.

 

 

[MarkDH 234]

I would guess that thing on the side is a slider potentiometer that's the glitchy one? I would start by removing it and trying to spot any identifying marks. It would be nice to just buy a drop-in replacement!  If you can't identify it or find a direct replacement I guess it would be simple in principle to get another slide potentiometer with the same travel and figure out a way to put that in.  I can't really work out what the mechanism is from the photos but it I wouldn't have thought it could be too complicated. There is an alternative, which is to use a circular pot with rack and pinion arrangement to convert the linear movement to circular.

 

I don't think you will have any problems wiring it up - it should just have three connections - +ve, GND and a centre-tap (the sliding contact). You may need to solder/desolder those connections. Beware that everything is likely to be in imperial dimensions, so you may need to order overseas. Make sure you get a linear potentiometer (i.e. not log or antilog).

[fairwinds47 0]

It's hard to tell from the photo if there is any damage, but you may have some luck by just cleaning the pot strips using isopropyl alcohol and cotton swabs.  If there is a way to remove the slider, you should also clean its contacts.  If needed, the metal strips (not the carbon) and slider contacts may also be cleaned by gently using a clean pencil eraser, followed by the alcohol and cotton swabs. 

 

I hope this helps.

 

/Len

[Rocky_53 1,566]

I would guess that thing on the side is a slider potentiometer that's the glitchy one? I would start by removing it and trying to spot any identifying marks. It would be nice to just buy a drop-in replacement!  If you can't identify it or find a direct replacement I guess it would be simple in principle to get another slide potentiometer with the same travel and figure out a way to put that in.  I can't really work out what the mechanism is from the photos but it I wouldn't have thought it could be too complicated. There is an alternative, which is to use a circular pot with rack and pinion arrangement to convert the linear movement to circular.

 

I don't think you will have any problems wiring it up - it should just have three connections - +ve, GND and a centre-tap (the sliding contact). You may need to solder/desolder those connections. Beware that everything is likely to be in imperial dimensions, so you may need to order overseas. Make sure you get a linear potentiometer (i.e. not log or antilog).

 

It's hard to tell from the photo if there is any damage, but you may have some luck by just cleaning the pot strips using isopropyl alcohol and cotton swabs.  If there is a way to remove the slider, you should also clean its contacts.  If needed, the metal strips (not the carbon) and slider contacts may also be cleaned by gently using a clean pencil eraser, followed by the alcohol and cotton swabs. 

 

I hope this helps.

 

/Len

 

I have been unable to locate a similar replacement, so it seems I will have to follow your suggestion Mark. I did try cleaning the contacts Len, but maybe I'll try again with some alcohol.Thanks again for the advice fellas, very much appreciated.

[Bob Scott 7,716]

That looks like a custom linear pot...there's probably no off the shelf option that would be suitable.  It looks like the carbon resistive strip is pretty severely abraded about 15mm from the rear edge, which may be causing the problem.  If that's the issue, you could try filling in the abraded area by coloring it in (fairly heavily) with a #2 graphite pencil.

 

If I were doing it (I've converted several of the floor-mounted PFC yokes from pots to Hall Effect sensors), I'd attach a thin stainless cable to a screw or screw eye sunk into the outer side of the wiper assembly, and run it towards the back of the unit, where I'd mount a 30-40mm pulley to the shaft of a 180 degree (half turn) rotary pot mounted transversely through the slot in the side of the frame.  You'd need to anchor a spring and counterforce cable to pull from the other side to keep positive tension.  It doesn't look like there's enough vertical space in the cabinet to use a rack and pinion setup unless you use a fairly complex reduction gear arrangement or a multi-turn pot (which has the reduction gear internal to the transducer itself).

 

Good luck!

[Rocky_53 1,566]

That looks like a custom linear pot...there's probably no off the shelf option that would be suitable.  It looks like the carbon resistive strip is pretty severely abraded about 15mm from the rear edge, which may be causing the problem.  If that's the issue, you could try filling in the abraded area by coloring it in (fairly heavily) with a #2 graphite pencil.

 

If I were doing it (I've converted several of the floor-mounted PFC yokes from pots to Hall Effect sensors), I'd attach a thin stainless cable to a screw or screw eye sunk into the outer side of the wiper assembly, and run it towards the back of the unit, where I'd mount a 30-40mm pulley to the shaft of a 180 degree (half turn) rotary pot mounted transversely through the slot in the side of the frame.  You'd need to anchor a spring and counterforce cable to pull from the other side to keep positive tension.  It doesn't look like there's enough vertical space in the cabinet to use a rack and pinion setup unless you use a fairly complex reduction gear arrangement or a multi-turn pot (which has the reduction gear internal to the transducer itself).

 

Good luck!

 

That sounds all very complicated Bob. Will it be easy enough for me to replace the existing slider with something off the shelf?

[Bob Scott 7,716]

Very unlikely...from what I can see in the pic, that pot/slider assembly looks like a custom design.  Finding a linear pot with the right value and mechanical throw is a loooong shot, IMHO.

 

Regards

[MarkDH 234]

Very unlikely...from what I can see in the pic, that pot/slider assembly looks like a custom design.  Finding a linear pot with the right value and mechanical throw is a loooong shot, IMHO.

 

Really? Here's one. Okay, some tinkering will be required but it ought to work. (That's just the first one I found, it may not be the best option.) Can't speak for PFC but some controllers will auto-range the input so you can connect a wide range of pots.

[Bob Scott 7,716]

OK, the pot Mark linked to should be adaptable if you have some mechanical/electrical skills--it won't be anything like a drop-in replacement.  You'll have to fashion a bracket and some kind of linkage to the slider.  And I'd also consider buying some spares to keep around for later, too.  And also source a replacement for the rotary aileron axis pot...that looks like the ones PFC still uses in a lot of their flight controls, so maybe they can sell you a few of those.  Always good to have pots on hand, because they do wear out.

 

Regards

[Rocky_53 1,566]

Really? Here's one. Okay, some tinkering will be required but it ought to work. (That's just the first one I found, it may not be the best option.) Can't speak for PFC but some controllers will auto-range the input so you can connect a wide range of pots.

 

Hi Mark, I too found the one you have a link to. Unfortunately, it is too wide to use as a replacement. However, I have ordered this one from Germany. What do you think?

 

OK, the pot Mark linked to should be adaptable if you have some mechanical/electrical skills--it won't be anything like a drop-in replacement.  You'll have to fashion a bracket and some kind of linkage to the slider.  And I'd also consider buying some spares to keep around for later, too.  And also source a replacement for the rotary aileron axis pot...that looks like the ones PFC still uses in a lot of their flight controls, so maybe they can sell you a few of those.  Always good to have pots on hand, because they do wear out.

 

Regards

 Cheers Bob

[MarkDH 234]

I have ordered this one from Germany. What do you think?

 

Well it looks good to me! No rocket science here, you should be able to make that work with a drill and as many zip ties as it takes

[Rocky_53 1,566]

That's the kind of reply I was hoping for Mark :smile: Cheers.

[Rocky_53 1,566]

Well it looks good to me! No rocket science here, you should be able to make that work with a drill and as many zip ties as it takes

Hey Mark, I've received my replacement pot, which amazingly fits perfectly, right down to the positioning of the small screws! However, I am an ignoramous when electrics are concerned. Can you tell me, where do I solder the two wires from the yoke. There is one black and one blue, but three terminals on the pot. Help appreciated...

 

[MarkDH 234]

Hey Mark, I've received my replacement pot, which amazingly fits perfectly, right down to the positioning of the small screws! However, I am an ignoramous when electrics are concerned. Can you tell me, where do I solder the two wires from the yoke. There is one black and one blue, but three terminals on the pot. Help appreciated...

 

Well that's a result! If you have only two wires from the yoke I guess that means the original one may have had its ground connection through the chassis.

 

From your photos it looks like the blue wire connects to the centre tap (the carbon track in the middle of the original component in your picture). So this is the signal wire (terminal 2 in the new component). The black wire looks like it is connected to the the top track, which I would guess is the positive voltage (probably +5V). I am not sure of that since black doesn't strike me as a usual choice for a +ve connection. In the absence of any other connections this seems most likely. If the black is +5V, it seems likely that those two mounting screws (the vertical ones at the bottom) connect the component to the chassis, making the GND connection. Unless we can see the old component in more detail it's hard to tell. I would connect the old one back up and verify these with a multi-meter. You can also trace the two wires to that green mounting block and see if there are any annotations on it.

 

Basically you want +5V (or maybe 9V or 12V) on tag 1 and have tag 3 connected to GND, or the other way around. Then the two wires connected to the controller will be tag 3 (GND) and tag 2 (signal). This taps a variable voltage between 0 and 5V off the slider.

 

What you may find is that either tag 1 or 3 in the new component is connected to GND by the mounting screws. You can test this with a multi-meter. If so, you just connect to the other two tags and leave the other tag unconnected.

 

If that is all too confusing I will try again But it will definitely help if you have a multi-meter and can check things out before connecting anything to anything.

[Rocky_53 1,566]

Hi Mark, sorry pal, but that was just gobbledeegook to me!