Hideous Collett Knob

[thermionic]

Hi,

A client has brought a crossover to me for repair. The issue is either a busted pot, or more likely (considering the OEM has snipped the anti-rotation lugs on the pot...doh! They exist for a reason...) a damaged PCB track. This was easy to diagnose... However, all the screws on the unit are held in with this super strong thread-lock... I had the humiliation of calling the client to tell him that, although I know exactly where the problem lies, I can't get the knob off... I've had it in a vice...

Will I be able to get it off if I use the correct tool? What on earth is the tool? I'm thinking it must have 3 prongs that insert into the knob, with a hole drilled through the centre.

I'd be *very* grateful if someone could point me in the direction of the tool.

If the thread lock really is that strong, do you reckon the knob will come off it I drill the head off the screw?

Thanks in advance.

 

[MagnetoSound]

I would approach this by clamping the knob very firmly in a mole grip (you will scar the knob, so be prepared to replace it) and then, gently but very firmly and with downward pressure, try to undo the screw with a flat blade screwdriver that is a good snug fit for the slot in the top of the screw.

You will not likely get the screw loose with a cross-head screwdriver because it will just lift itself out, but the flat blade might just do it.

If you've already tried all of this, the drill might well be the next step

 

[sahib]

No.

Don't risk the knob. Risk the screw.

Use a side cutter to remove the screw as in the picture below. Because the screw is relatively soft the side cutter will bite into it and get a good grip.

You can still hold the knob using a mulgrip as Dan said. But wrap over a few layers of masking tape to protect it.

 

[thermionic]

It's either find the collett tool (are these knobs the same make as used on some Sontec / Masselec gear?) or drill  it out. Harming the knob, when you consider I have no idea where to get a replacement, who makes it - and that the client is touchy about the knobs (I offered them new knobs...), is not an option. The whole scenario is hugely embarrassing. I wish I'd not taken the job. Being self-employed, even though I'm lucky enough to be busy at the moment, means I'm highly reluctant to turn work away... I doubt the client has much sympathy for me - it can't look good when the engineer can't remove a knob...

 

[thermionic]

Thanks, Cemal. I will try the side cutter method, but I'm not optimistic. I think the only option, other than drilling the head off by using consecutively larger bits, will be to use a large flat-head driver  combined with the proper tool that has triangular pins.

 

[bruno2000]

I've had good luck with long-nosed "ViceGrip" pliers on the screw head.  Outside of that, I'd use a Dremmel Tool.
Best,
Bruno2000

 

[sahib]

Thanks, Cemal. I will try the side cutter method, but I'm not optimistic. I think the only option, other than drilling the head off by using consecutively larger bits, will be to use a large flat-head driver  combined with the proper tool that has triangular pins.

Justin,

Don't complicate things. Just do as I proposed. Trust me.

 

[thermionic]

Update: the issue is not gripping the screw. I can do this well with a decent flat-head driver. The problem is stopping the knob rotating. Had the OEM correctly used the anti-rotation lug, I could risk destroying the pot (I have spares) by locking it to its end stop and seeing if this gives enough support to undo the screw. But they snipped the lug off.. So, such a course of action will damage the PCB.

Next stage is try Dan's advice: use copious amounts of insulating tape and grip the fecker in a vice. I tried this before, but not with tape.

Hold tight: updates will be forthcoming. It's a 'gripping' story...

 

[sodderboy]

Here is another use for webbed shelf liner.  It is the webbed foamy friction sheets to put in a drawer to keep drawer items in their place.  Is there something like that outside of the states?  They make it for kitchen drawers and tool chests here in varying thicknesses.  You wrap a long strip around the knob and can prolly use your hand strength to hold it.  If you use a large plier, the matting will protect the knob and faceplate.
It is an indispensable item for techs.  I use it to hold a propped chassis from slipping on the bench, a large sheet to protect console knobs while working on an extended module, its a brilliant material.
Another must have is a pair of "sticky surface" work gloves.  They would also work here.  They magnify your hand strength with their high coefficient of friction. 
You can do it.  Get a super hold on that knob and OWN it.
Mike

 

[thermionic]

The plot thickens... As soon as you solve one problem, another emerges: got the knob off by drilling. However, because I couldn't find a vice wide enough, the drill went in at a slight angle, damaging the tightening collar...

Has anyone got any idea where to get these knobs?

If I admit defeat and tell the client to send this POS back to Japan it's going to look really bad for me. They have stickers all over this thing saying 'don't open'... Is this because within a minute of opening it I realised it was copied from a UREi 525, perhaps? Surely, the 525 isn't in copyright any more. Why would they be so touchy about other techs opening it? Revenue for servicing dept, maybe?

 

[bruno2000]

Several times I have swapped colletts on Sifam knobs.  Just used a large punch on top, and a socket on the bottom.
What size is the shaft?
Best,
Bruno2000

 

[thermionic]

The pot has a 6m shaft. I think it uses these knobs, but I've no idea where to get them: http://www.ebay.co.uk/itm/AUGAT-ALCO-COLLET-KNOB-RADIO-HAM-Control-NOS-1-4-Shaft-/200957073814

Maybe I'll see if a Sifam retainer will fit. This whole job's a nightmare - i'm only doing it to save face. The unit's a piece of junk, plagiarised from UREi. But they don't want anyone repairing it...

 

[MagnetoSound]

The pot has a 6m shaft.

Wow! I can see your problem ... 

 

[pucho812]

gear has a mind of it's own, I've noticed that when it comes to removing things if I think it will be difficult it tends to be and I strip a screw or what have you. but if I think it will be easy sure enough it is. Mind over matter. take a break and come at it later.

 

[helterbelter]

I think I have a bunch of those collet knobs. What's the diameter ?

edit : > The ones that I have are 35mm diameter/6mm shaft.