Fitting fader knobs

For most mixers I build I use ALPS K faders with T-bar fixings for the knobs. For the poor man's tube mixer I have been looking for cheaper alternatives and have settled on the Bourns PTF series because they are lower cost, have a good feel and a guaranteed 1 ohm or less off resistance. So I bought a few and some knobs to go with them. They do not have a T-bar with pips like the ALPS but instead they have a plain upright lever that the knob fits onto (I think this is called Type A).

The problem is, the fit of the knobs to the lever seems to be very variable. Most knobs slip on all to easily and to my view are rather sloppy (but they feel OK). Also, out of a batch of ten, three will not fit at all. I am reluctant to push them on with much force for fear of damaging the fader.

Is this normal with this type of lever knob combination or do I just need to buy better made knobs?

Cheers

Ian

ruffrecords said:

The problem is, the fit of the knobs to the lever seems to be very variable. Most knobs slip on all to easily and to my view are rather sloppy (but they feel OK).

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In that case, the fix is easy. Put a drop of liquid neoprene glue, press the knob on the fader, then disassemble them, let dry a couple of minutes and you can reinsert it.

Also, out of a batch of ten, three will not fit at all. I am reluctant to push them on with much force for fear of damaging the fader.

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Could you identify where it's too tight? Is it the length or the width (or both). I think you'll have to resort to making a tool...

abbey road d enfer said:

In that case, the fix is easy. Put a drop of liquid neoprene glue, press the knob on the fader, then disassemble them, let dry a couple of minutes and you can reinsert it.

Click to expand...

Do  you mean something like this?

https://www.amazon.co.uk/Black-Witch-Neoprene-adhesive/dp/B001PAGOYK

So you are suggesting I put a drop in the hole in the knob, place the knob on the fader and then quickly remove it? Let both parts dry and then reassemble.

Could you identify where it's too tight? Is it the length or the width (or both). I think you'll have to resort to making a tool...

Click to expand...

Difficult to tell - it only goes on a mm or so. I might try applying a sliver of emery paper.

Cheers

Ian

ruffrecords said:

Do  you mean something like this?

https://www.amazon.co.uk/Black-Witch-Neoprene-adhesive/dp/B001PAGOYK

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Well it looks this one is specifically for repairing frogsuits and may leave dark residues.
I use this.
https://www.amazon.fr/Pattex-1563695-contact-liquide-Blister/dp/B006FUWPCU/ref=sr_1_9?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=colle+contact+liquide&qid=1616448895&s=hi&sr=1-9

So you are suggesting I put a drop in the hole in the knob, place the knob on the fader and then quickly remove it? Let both parts dry and then reassemble.

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Exactly.

Pattex seem to have hundreds of different adhesives but 1563695 does not appear on the uk amazon web site. How about this one instead?

https://www.amazon.co.uk/McNett-Black-Witch-Neoprene-Cement/dp/B01KU3LFE6/ref=sr_1_2_sspa?dchild=1&keywords=neoprene+glue&qid=1616493481&sr=8-2-spons&psc=1&smid=A27D7XORWF8L03&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFYTldXUDJWSVI2NlYmZW5jcnlwdGVkSWQ9QTA5NDgwNTUyOUJLMU83Tko3Vk4mZW5jcnlwdGVkQWRJZD1BMDMyMjcxMTFZUElaUkdHSVJINEomd2lkZ2V0TmFtZT1zcF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

Cheers

Ian

ruffrecords said:

Pattex seem to have hundreds of different adhesives but 1563695 does not appear on the uk amazon web site. How about this one instead?

https://www.amazon.co.uk/McNett-Black-Witch-Neoprene-Cement/dp/B01KU3LFE6/ref=sr_1_2_sspa?dchild=1&keywords=neoprene+glue&qid=1616493481&sr=8-2-spons&psc=1&smid=A27D7XORWF8L03&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFYTldXUDJWSVI2NlYmZW5jcnlwdGVkSWQ9QTA5NDgwNTUyOUJLMU83Tko3Vk4mZW5jcnlwdGVkQWRJZD1BMDMyMjcxMTFZUElaUkdHSVJINEomd2lkZ2V0TmFtZT1zcF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

Cheers

Ian

Click to expand...

I'm not familiar with this one. Is it black or yellow-ish transparent? I just would be concerned about black glue leaving streaks.
The most important parameter IMO is that it was a liquid formula, not gel, because it results in better adhesion to metal and capability of making thin layers.

abbey road d enfer said:

I'm not familiar with this one. Is it black or yellow-ish transparent? I just would be concerned about black glue leaving streaks.
The most important parameter IMO is that it was a liquid formula, not gel, because it results in better adhesion to metal and capability of making thin layers.

Click to expand...

I will keep looking.

Cheers

Ian

I just built a bunch of fader modules for my modular synth using the low cost Bourns PTA series, and the various fader caps vary widely in how well they fit. I've destroyed several faders by applying too much force while sliding on the caps, bending the contacts towards the carbon track. They are very easily damaged, even when I quickly switched over to holding the actuator with pliers while applying the cap as to not apply pressure to the contacts, but still managed to do so. I'm surprised how little pressure they can take!
Unfortunately I haven't found any suitable replacement fader that fit the pcb I'm using them for. However, some fader caps intended for the Roland Aira series i bought locally just to try, slide on much easier but don't look as good as the other ones (Akai APC40 black type). I third set, similar to the Akai ones, also are super tight and could again easily damage the faders.

I tried to use my new hot air station to warm up the plastic inside the knob, and it did loosen the hole in the upper part, but the lower part didn't get hot enough, and when I tried to increase temp/flow/time I just ended up melting too much and destroying the knob instead

fragletrollet said:

I tried to use my new hot air station to warm up the plastic inside the knob, and it did loosen the hole in the upper part, but the lower part didn't get hot enough, and when I tried to increase temp/flow/time I just ended up melting too much and destroying the knob instead

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You may try warming the metal shaft instead of the cap itself. In any case, I suspect it's not doable in one go.

abbey road d enfer said:

Well it looks this one is specifically for repairing frogsuits and may leave dark residues.

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Well spotted! I work as an industrial diver and this exact product is in every divers toolkit (if he uses neoprene suits), pretty much force fed since school. It works great on neoprene, but it sort of melts the material so I would be very careful applying it to plastics as it might easily dissolve it.

+1 to heating the metal shaft but carefully since it is no doubt connected to plastic on the other end. Perhaps grip it with some pliers to wick heat away from the shaft so it doesn't melt the other end.

JR

fragletrollet said:

Unfortunately I haven't found any suitable replacement fader that fit the pcb I'm using them for.

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I do not mean to go off topic but I have some faders for sale which might be of interest.

https://groupdiy.com/threads/f-s-faders-alps-k-and-other-types.70485/
Ian,

How about filing the fader actuator gently using needle files? Again you can hold the actuator using long nose pliers. This will not put any stress on it. Hold it upside down so that the metal chips do not fall into the fader.

I am more inclined to try to make adjustments to the cheaper component, but whatever works.

JR

sahib said:

Ian,

How about filing the fader actuator gently using needle files? Again you can hold the actuator using long nose pliers. This will not put any stress on it. Hold it upside down so that the metal chips do not fall into the fader.

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I played around with filing the inside of the caps and also the fader actuator. Then I went a bought a bunch more caps and found which ones would fit OK. The tolerance problem is definitely with the caps so filing the actuator could be problematic if you later need to change the cap. I suspect the 'better' fader manufacturers already know this which may well explain the small bumps they put on the actuator.

I wonder if you sand papered an actuator you could then thicken it slightly with a thin layer of solder??? Might give that a try.

Cheers

ian

Hi Ian,

I do not think filing the inside of the caps would be feasible. It is bound to go all uneven.

I would think that normally there should not be a tolerance issue with the caps but you obviously have measured them all and there is. This would probably be due to using multiple (injection moulding) tools, hence the difference in tolerances between the tools. Bad practice.

The other reason I can think of is that to save from time they might be releasing the caps (moulds) way too quickly, before they had sufficient time to cool down. As a result some would shrink more due to stress. I have spent a life time working with plastic materials and I can not tell you how many machine shops do not think of this. I once subcontracted the plastic (acrylic) parts for one of my products (robot arm) to a machine shop. They rattled them on the CNC in one go. None of them passed the quality test. The dimensions varied upto 0.5mm. Whereas we would machine the parts rough first. Then leave them for 24 hours to release the stress. Then bring them to size with about an hour between the passes.

I would file the actuators. Later if the replacement cap is too slack then one round of masking tape around the actuator would build it back up.

You can sand the actuators too. I am sure you know this but I'll mention just in case. For sanding a surface evenly stick the sand paper onto a bit of rectangular flat surface wooden block, plastic, perspex or whatever material you get your hands onto, using double sided tape. Part of my business used to be in modelmaking and prototyping (closed down 10 years ago). We would have sanding blocks made with varying grits. For what you will be doing I would use something like 280-320 grit.

Edit: I still get confused with the interactivity of the new forum format. I thought this was a PM. So, edited the holiday bit.

Hi Cermal, thank you for the inside story on plastic moulding. I am sure it is this level of knowledge that sets groupDIY above all the others. I will definitely try sanding the actuators as you suggest.

(Right now it is probably hotter here than in Menorca )

Cheers

Ian

Can you designate one fader as sacrificial and use it to force on the tighter caps to loosen them up before putting them on the actual faders? Or pull the metal part out, clamp it, heat it and use that to widen the holes a little? (perhaps clamp it directly into some kind of heated thing to keep it at the right temp?

ruffrecords​

good day! I printed knobs on a 3D printer for my mixer. There was a danger of not getting into the landing zones, but everything turned out perfectly.

NOON said:

Can you designate one fader as sacrificial and use it to force on the tighter caps to loosen them up before putting them on the actual faders? Or pull the metal part out, clamp it, heat it and use that to widen the holes a little? (perhaps clamp it directly into some kind of heated thing to keep it at the right temp?

Click to expand...

I don't know. There are lots of options to explore. The trouble with plastic is - it's plastic, which means if you force it it will undergo plastic deformation but when you remove the forcing it may well return to its former size.

The odd thing is I have no problem at all with any caps for T-bar actuators which have small pips on them which I suspect grip the plastic but don't need lots of force to do it. The Bourns faders I am using have a smaller rectangular actuator with no pips.

Cheers

Ian

I've had this problem before, I just pushed a little blue tack down the knob- well and then fitted to fader, worked 99% in all cases!
Oh, forgot to mention - a bit of needle filing the knob-well for the ones that are tight!