Remove botched screw from tuning dial knob - Page 1
Forum section: electronics / repair

author: michalism
author_rank: Regular Contributor
timestamp: February 11, 2019, 01:42:51 pm
content: Hi all.I am restoring an old Sanyo DCX-2000 radio receiver/ amplifier.I need to remove the faceplate but in order to do that, the tuning knob needs to come out.Unfortunately, it seems that the little screw that keeps it in place is botchedI tried using a flat head screwdriver, hex, philips, torx, triangle...everything failed. Sheding some light in the hole, revealed what I was afraid of.I am attaching a picture that might help, unfortunately, i can't get a better shot...My questions:- is there any way that I can remove the screw without damaging the dial?- is it possible that this may be some kind of special screw? I found no relevant info in the service manual.- where can I buy a replacement knob?I have been looking around for replacement parts, but so far I had no luck, so any hints are more than welcome!Thank you very much in advance!!Michalis

author: TheWelly888
author_rank: Frequent Contributor
timestamp: February 11, 2019, 01:47:30 pm
content: Only way I can think of is to very carefully drill out the screw.

author: michalism
author_rank: Regular Contributor
timestamp: February 11, 2019, 02:12:21 pm
content: Quote from: TheWelly888 on February 11, 2019, 01:47:30 pmyeah...tried that, but I need a long thin drill bit, and can't find any. The pieces I have are short, and the drill shaft is scratching the faceplate.To be honest, I would prefer to find a raplacement knob and get on with it...

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: February 11, 2019, 02:15:54 pm
content: A drop of creep oil, a sharp edged screw driver and some good amount of pressure while unscrewing might do the trick.Exact replacement knobs may be hard to find, though there should be a bunch of suitable options available via various electronics parts dealers. You could also drill out the screw carefully and then just cut a bigger diameter thread for a new set screw.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 11, 2019, 08:50:21 pm
content: One of the most effective penetrating oils is a 50/50 mix of acetone and automatic transmission fluid (the technical term for this concoction is "Weasel Piss" - google it!).I have used this successfully on automotive projects, but also electronic equipment, by dispensing it very carefully (drops!) on the screws that are resisting. The penetrant literally runs around, into, and through the threads, carrying lubricant with it.100% success rate so far when I have been smart enough not to mangle the screw before reaching for the weasel!

author: Gregg
author_rank: Super Contributor
timestamp: February 12, 2019, 06:15:13 am
content: After using a little penetrating oil like SilverSolder suggests, grind a screwdriver to just fit the hole and the slot of the set screw. The screwdriver should be of reasonable quality (hardness) so that it won’t easily bend and the shank long enough that the handle clears the face of the device. If you have another similar knob removed, use its set screw as a template for grinding your screwdriver.If you have a hot air station or other means of gently heating the knob, heat will help.Next, put a block of wood under the knob with the screw up and penetrant applied. Take the newly sharpened screwdriver and insert it so that the screw is engaged with the screwdriver. Give the screwdriver a sharp rap with a small hammer or even the plastic handle of a larger screwdriver so that the wood takes the force of the blow and not the shaft. This will help engagement of the screwdriver and shock the set screw allowing penetrant to do its job.If you have help holding things it may help to apply light loosening pressure (torsional force) and multiple taps with the hammer.If you resort to drilling it, a left hand twist drill bit is highly recommended.

author: vealmike
author_rank: Regular Contributor
timestamp: February 12, 2019, 07:06:08 am
content: The problem with drilling is that the screw is almost certainly harder then the knob (am I in actress to the bishop territory?)Something is probably going to have to become sacrificial.Can you get to the component on the back? Is it replaceable? Could the shaft of the component be cut from the back? Once the knob and fascia are separated, heavier duty equipment will become available.

author: michalism
author_rank: Regular Contributor
timestamp: February 12, 2019, 08:06:08 am
content: Quote from: Gregg on February 12, 2019, 06:15:13 amGreat! Thanks, I will give this a try...Quote from: vealmike on February 12, 2019, 07:06:08 amI have full access to the back but I am not sure how this will help. There's no long shaft. I can't describe the mechanism, I'm not familiar with the terminology; I will post a picture later today.What do you mean by the heavier equipment will become available?

author: Ian.M
author_rank: Super Contributor
timestamp: February 12, 2019, 10:33:15 am
content: If you have to drill it out, order a left hand drill bit (so any movement of the screw loosens it), and hard silver solder it onto the end of an extension rod with a small length of tubing at the joint for strength. Protect the panel with duct tape over masking tape. Cut a softwood block with a hole saw then split it to form a clamp for the knob, with a bracket holding a bushing to act as a drill guide. Mark the depth from the edge of the knob (or top of the bushing) to the shaft on the drill bit so you don't go too deep! Use a hand drill or a reversible battery drill with good speed control so you can run it dead slow. Expect to have to drill and re-tap the knob for a larger setscrew as FUBARing the existing thread is virtually unavoidable.If the setscrew is hardened you are in for a whole world of grief unless you have (or can build) an EDM 'tap burner'.

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: February 12, 2019, 11:49:23 am
content: Quote from: Ian.M on February 12, 2019, 10:33:15 amxD You may very well just buy a bunch of fully restored DCX-2000 for the price of that EDM approach.

author: vealmike
author_rank: Regular Contributor
timestamp: February 12, 2019, 12:15:07 pm
content: Quote from: michalism on February 12, 2019, 08:06:08 amDestroy the component from the rear (Dremel / hacksaw), then push what remains of the shaft out from the rear, complete with knob attached.When the knob is detached from the panel, you can use a vice / pillar drill / blowtorch / oxyacetalene (delete as aggression levels dictate), to separate the knob from the shaft.

author: amyk
author_rank: Super Contributor
timestamp: February 12, 2019, 12:49:30 pm
content: A bit of heat may help loosen the screw, or what remains of it.

author: tggzzz
author_rank: Super Contributor
timestamp: February 12, 2019, 12:51:32 pm
content: In addition to the other suggestions, sometimes thermal shock can help.Heat upnthe outer piece using whatever technique you feel comfortable with. Then apply cold (freezer spray, acetone) to the inner piece. Them twist(No, silly; not the dance).There are also bolt removing devices with a tapered left-hand thread, but they are probably too large in this case.

author: Bud
author_rank: Super Contributor
timestamp: February 12, 2019, 02:22:28 pm
content: Is the screw too small to try with a impact screwdriver? (after soaking in penetrating oil for a while)

author: michalism
author_rank: Regular Contributor
timestamp: February 12, 2019, 02:34:10 pm
content: Quote from: tggzzz on February 12, 2019, 12:51:32 pmLOL!!! I will place it on the floor and start dancing around it...Quote from: Bud on February 12, 2019, 02:22:28 pmYes it is small...unless there are small impact screw-drivers that I am not aware off...The thermal shock merhod is quite interesting...I will give it a try together with the grinded screwdriver...

author: Ian.M
author_rank: Super Contributor
timestamp: February 12, 2019, 03:11:36 pm
content: Another approach to heating it for thermal shock would be to pass a large current through a heavy copper or brass wire (more of a rod actually) with the end drilled for a short piece of pencil lead, in contact with the head of the setscrew. Insulate the wire/rod with a non-overlapping spiral wrap of narrow kapton tape, or with a length of glassfibre roving wrapped on and glued with waterglass so it doesn't short to the sides of the grubscrew hole. Clamp the return conductor onto the shaft if possible, otherwise onto the knob.Although heating up the screw rather than the knob jams it tighter while its hot, its expansion will deform the aluminum, loosening it when it cools. Quench the screw with spray penetrating oil for best results.

author: alanb
author_rank: Frequent Contributor
timestamp: February 12, 2019, 04:57:22 pm
content: use your nearest size fitting screwdriver and put a small amount of epoxy resin on it then leave in position in the screw head until it sets. You may need to find something to hold everything in position whilst the epoxy sets. Don't be too generous with the epoxy or you may make things worse.Once the epoxy has set add some penetrating oil and unscrew.I've used this technique successfully a couple of times on damaged recessed grub screws.Hope this helps

author: Bashstreet
author_rank: Frequent Contributor
timestamp: February 12, 2019, 05:48:03 pm
content: Depending of the way the screw is used you might be able to just pull the knob (no pun intended)If the screw attaches into a corresponding hole your in no luck if it purely relies in friction to keep it on your all set.Use lot of lubricating oil and try rocking the knob gently while pulling it might move and come off.If you have any other knobs with screw you can check the attachment method.Deepest apologies how this all sounded.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 12, 2019, 10:03:05 pm
content: Quote from: alanb on February 12, 2019, 04:57:22 pmThat sounds like a pretty cool trick. You could get a much better grip on a damaged kerf... This might even work on a rounded Allen/hex head?

author: SilverSolder
author_rank: Super Contributor
timestamp: February 12, 2019, 10:06:24 pm
content: Quote from: Bashstreet on February 12, 2019, 05:48:03 pm

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 13, 2019, 12:39:07 am
content: You may have luck if you have a round rod (old screwdriver?) and then ground a flat part on one side, so it just fits next to the screw.The round part of the rod will get guidance from the side of the hole, so you can exert more force on the side of the screw.Apart from that.For a vintage piece of equipment the knob may be more valuable (difficult to replace) then the potentiometer.

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2019, 12:46:40 am
content: Its the tuning knob. Replacing the tuning drive mechanism is likely to be more difficult than replacing a pot, unless you've got a home machine shop.We are still waiting on the O.P. to post a photo of what that shaft drives behind the panel, (promised in reply #7), and until we can see that, we have no idea if it could be as simple as removing the shaft from a pulley or coupler.@O.P. Web photos of your receiver indicate its almost certainly got a cord drive. *DO* *NOT* attempt any interior dismantling without detailed closeup photos of the exact drive arrangement and cord routing and if you haven't re-strung a lot of cord drives before stop and ask. If you disturb the cord then restring it even slightly wrong, it will jam or if there are any plastic parts the excess force when it jams will probably break something and totally FUBAR it.

author: michalism
author_rank: Regular Contributor
timestamp: February 13, 2019, 08:36:28 am
content: Thank you all for your responses. This is really great, a lot of ideas and a lot to try!I apologise for not sending photos, I do have a full-time job and these days are really hectic, leaving no time for my hobby... I will post the photos as soon as possible.@Ian.M: I'm new to the sport, but I'n not THAT cluelessThanks for the interest though, much appreciated.I need to check again, but i am pretty sure the tunning knob is indeed a cord drive, so I am not planning to do any dismandling from the back. I would rather loose the knob and then try to find a replacement of some kind rather than hsaving to rebuild the cord routing, which is something I have never done in the past!Thanks again to everybody, will try to return with the photos as soon as possible.

author: TerraHertz
author_rank: Super Contributor
timestamp: February 13, 2019, 10:06:18 am
content: Quote from: michalism on February 11, 2019, 01:42:51 pm?? But it looks like just a plain slotted grub screw, intact. Or is one side of the slot broken off? It's hard to tell from the pic.Is it possible your flat bladed screwdriver was just too fat to fit in the slot?If so, sacrifice some cheap (but tough) screwdriver by grinding it down till it just fits. Don't let the metal get hot while grinding. Veeery gently, lots of water for cooling.Anyway, it also looks like the knob can be rested on a bit of soft wood on the opposite side to the screw, while you...Get a small pin punch that fits in the hole. Put it against the screw top. With a small tack hammer, strike the punch firmly. The idea is to do two things: shock-break any oxides/corrosion/metal contact welds between the screw and the knob, and also drive the screw point into the shaft a little. Thus leaving it a bit loose.Then try the screwdriver again.I have a similar but worse problem atm. A toothed-belt drive cog, with a small hex head grub screw. Except the hex has been well rounded off. The shaft has a mating hole, so I can't just forcibly drag the cog off with a puller.Fortunately I don't have to preserve the cog, so as a last resort can just angle grind it down to near the shaft, then split it with a chisel.

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2019, 10:16:47 am
content: Few except service trade old-timers and antique radio restorers know that cord drives are notorious for re-stringing problems if you don't have the diagram from the service manual or an exact record of which way round, how many turns and whether it was wound top to bottom or bottom to top on each drum or spindle it went round. e.g. if you get the top to bottom or visa-versa winding direction wrong on one spindle or drum even though the clockwise/anticlockwise winding direction is correct and all the rest of the cord route is correct, the cord will typically pile up on itself and jam.Any broken cord's exact placement and route should be documented before removal to aid correct re-stringing. Back in the days of valve radios, if the owner had removed or disturbed the remains of a simple broken cord, it would typically add hours to the repair if you didn't have the official diagram from the service manual + a lot of re-stringing experience. However at least those valve radios had a metal chassis and robust mechanicals in the drive system, so permanent damage not resulting from violent frustration was unlikely.Newer radios with cord drives and too much ageing plastic are an even worse problem. When the little lug the end of the tensioner spring goes over snaps off a plastic drum, and manufacturer spares support for the model is long gone, or the post on the plastic frame supporting a pulley at one end of the tuning scale cracks, days of heroic bodgery and hand-crafted parts are your only hope.

author: langwadt
author_rank: Super Contributor
timestamp: February 13, 2019, 10:18:28 am
content: Quote from: TerraHertz on February 13, 2019, 10:06:18 amif the screw is steel and the cog something else like aluminium you could try etching it out with alumhttps://hackaday.com/2015/08/03/dissolve-steel-drill-bits-in-alum-from-the-grocery/

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: February 13, 2019, 10:23:48 am
content: Slightly OT:This thread is driving me nuts! The tension is killing me!Will he succeed? Will he be beaten by a set screw?Why does it take for forever for the OP to get it done?Am I impatient? Is it much more complicated then usual?How can we help this guy to just get it done already?Oh the horrors of being damned to merely watch and wait for someone to just do it. xD

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2019, 10:30:16 am
content: Quote from: langwadt on February 13, 2019, 10:18:28 amYes, but I'd hesitate to try that if the shaft is also steel.Maybe in the O.P's situation I'd consider it as a last resort short of cutting the knob away. I do have a few ounces of alum sitting on the shelf . . . Also, if you do use alum, pack behind the knob with well greased string to keep the alum out of its bearings, or you'll probably FUBAR it.

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: February 13, 2019, 10:52:49 am
content: Quote from: TerraHertz on February 13, 2019, 10:06:18 amTry force-feeding it a slightly oversized TX or slotted head driver.This has helped me out several times.The driver/bit should be decent quality though!

author: 6PTsocket
author_rank: Regular Contributor
timestamp: February 13, 2019, 12:43:56 pm
content: There are bits By Alden calked Grabit. One end drills just a dimple and the other end grabs the screw. With a hex or torx drive setscrew you probabably wouldn't even have to drill. Use a hex bit extension to get the drill away from the front panel. The pilot dimple (I wouldn't even call it a hole) and the extraction are both done with the drill in reverse. They come in small sets. It was hard to tell in the picture but I hope the setscrew is not too deep in the knob.Sent from my SM-G900V using Tapatalk

author: michalism
author_rank: Regular Contributor
timestamp: February 14, 2019, 07:47:28 am
content: Hello!thank you all for your comments and advice...this is a little bit overwelming with so many options to try :pQuote from: frozenfrogz on February 13, 2019, 10:23:48 amLOL!!!Sorry for creating so much stress to you! Not in my intentions at all!As said, I am working full time and these days I finish late at nights. I also have a 3y.o. son, that is taking a lot of my attentionI hope that I will have some results during the weekend for you!Thanks again!

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: February 14, 2019, 09:01:24 am
content: No worries!I might be over-exaggerating a bit.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 14, 2019, 05:15:14 pm
content: I guess we all feel your pain... stuck screws, and 3 year old kids, are not always quickly dealt with!

author: michalism
author_rank: Regular Contributor
timestamp: February 15, 2019, 07:39:35 am
content: OK then!!Up for the great Finale!The screw is out!Much easier than expected, although this thing is sitting on my shelf for more than two months now, and I had several previously failed attempts.What I did now: I lightly heated the knob, and then use a light hammer to tap it. A couple of days ago I had sprayed the hole with penetrating oil. After it colled down, I used 4-5 different flat headed screw-drivers trying to "feel" if the screw was moving. Then as I was getting ready to go to the epoxy-drop solution, the screw come out as if it was never stuck!As you can see from the pictures, this is not a botched screw, it's just a piece of screw that a technician or ex-owner put in place to keep the knob on the shaft, because he had lost the original piece. That's my assumption.Now that I see the extracted part, I consider myself extremely lucky that this turned out like it did...I am also including pictures from the inside, as promised. I would be terrible to mess that up...Anyway, thanks for all your help, and sharing my agony!!!I promise to come back with more questions

author: tggzzz
author_rank: Super Contributor
timestamp: February 15, 2019, 08:45:01 am
content: Quote from: michalism on February 15, 2019, 07:39:35 amIt is good to see:you've successfully extracted the culpritsomeone that listens and thinks

author: Chris56000
author_rank: Frequent Contributor
timestamp: February 15, 2019, 02:03:17 pm
content: Hi!Well d;ne!You should ne able to locate a replacement no problem –an M1.5 x 8 or M2 x 8 grub–screw should do to replace it and you can get these in packs of 10 from fleabay!Chris Williams

author: Ian.M
author_rank: Super Contributor
timestamp: February 15, 2019, 03:13:24 pm
content: I wouldn't trust a fleabay grubscrew. If it isn't properly hardened it wont bite on a steel shaft and if it even slightly over-hardened, it will be too brittle and if it jams again the head is likely to break. You already know its a PITA to remove so unless you are planning to flip the item for cash as a private seller as soon as you've fixed it, cheapening out on small parts is a false economy.Incidentally, if you ever have to fabricate a grubscrew from a bolt, (Not recommended. Least evil is if you shape it to a point to match a drilled dimple in the shaft) don't cut the slot too deep. Otherwise the leverage from the screwdriveer will break it

author: michalism
author_rank: Regular Contributor
timestamp: February 15, 2019, 07:54:16 pm
content: I already have a small set of such screws, which I bought from a reputable local hardware store.These are so cheap, it makes no sense to get them from China, IMHO.

author: TerraHertz
author_rank: Super Contributor
timestamp: February 15, 2019, 10:46:27 pm
content: Mine is out too. Turned out the reason the grub screw hex socket rounded off, was that the grub screw was not hardened at all. And some form of thread-locking compound had been used.As a last despairing attempt before destroying the pulley with an angle grinder, I finally tried drilling the screw. Expecting to get just a blunted drill. Surprisingly the drill cut in and went down the center of the screw easily. After going right through it cut off enough of the screw's end to free the pulley from the shaft.The hollow remnant of grub screw was _still_ solidly glued in the pulley. Using a larger drill bit, it finally came free and screwed inwards, into the empty shaft hole. And was short enough to come completely free and drop out in the center.Edit to add: speaking of grub screw sets, they are very cheap from Aliexpress. Pic: a couple I bought in 2017.Store name: Son Day Day Up Store200pcs Stainless Steel Hex Socket Set Screw Grub Screws Cup Point Assortment Kit M3-M8 With Plastic Box$ 4.99 per box.

author: MathWizard
author_rank: Super Contributor
timestamp: February 16, 2023, 03:58:15 pm
content: Quote from: Ian.M on February 13, 2019, 10:16:47 amSo how did they calculate it, or what principles should let someone figure out the best way, for a given system ?I have a rare radio that I took apart, and by accident took apart the dial cord assembly. It has a main wheel, w/ a string, a tuning shaft, and 2 bobbins with a larger upper, and smaller lower section. IDK if it even used both halves.So what book would describe how to design such a pulley system ? I could come up with some stuff, but measuring or including string friction and stretching, which is essential, well forget it.I'm sure it's really some partial differential equations, and some Lagrangian/Hamiltonian system would be what describes it best.