Author Topic: Remove botched screw from tuning dial knob  (Read 2354 times)

0 Members and 1 Guest are viewing this topic.

Offline michalism

  • Regular Contributor
  • *
  • Posts: 61
  • Country: gr
Remove botched screw from tuning dial knob
« on: February 11, 2019, 01:42:51 pm »
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 botched  :( I 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
 

Offline TheWelly888

  • Frequent Contributor
  • **
  • Posts: 340
  • Country: gb
Re: Remove botched screw from tuning dial knob
« Reply #1 on: February 11, 2019, 01:47:30 pm »
Only way I can think of is to very carefully drill out the screw.
You can do anything with the right attitude and a hammer.
 
The following users thanked this post: michalism

Offline michalism

  • Regular Contributor
  • *
  • Posts: 61
  • Country: gr
Re: Remove botched screw from tuning dial knob
« Reply #2 on: February 11, 2019, 02:12:21 pm »
Only way I can think of is to very carefully drill out the screw.
yeah...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...
 

Offline frozenfrogz

  • Frequent Contributor
  • **
  • Posts: 907
  • Country: de
  • Having fun with Arduino and Raspberry Pi
Re: Remove botched screw from tuning dial knob
« Reply #3 on: February 11, 2019, 02:15:54 pm »
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.
He’s like a trained ape. Without the training.
 
The following users thanked this post: michalism

Offline SilverSolder

  • Super Contributor
  • ***
  • Posts: 1126
  • Country: 00
Re: Remove botched screw from tuning dial knob
« Reply #4 on: February 11, 2019, 08:50:21 pm »

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!
 
The following users thanked this post: SeanB, michalism

Online Gregg

  • Frequent Contributor
  • **
  • Posts: 695
  • Country: us
Re: Remove botched screw from tuning dial knob
« Reply #5 on: February 12, 2019, 06:15:13 am »
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.
 
The following users thanked this post: michalism

Offline vealmike

  • Regular Contributor
  • *
  • Posts: 192
  • Country: gb
Re: Remove botched screw from tuning dial knob
« Reply #6 on: February 12, 2019, 07:06:08 am »
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.
 

Offline michalism

  • Regular Contributor
  • *
  • Posts: 61
  • Country: gr
Re: Remove botched screw from tuning dial knob
« Reply #7 on: February 12, 2019, 08:06:08 am »
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.


Great! Thanks, I will give this a try...

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.

I 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?
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 8181
Re: Remove botched screw from tuning dial knob
« Reply #8 on: February 12, 2019, 10:33:15 am »
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'.
« Last Edit: February 12, 2019, 10:48:13 am by Ian.M »
 
The following users thanked this post: SeanB, michalism

Offline frozenfrogz

  • Frequent Contributor
  • **
  • Posts: 907
  • Country: de
  • Having fun with Arduino and Raspberry Pi
Re: Remove botched screw from tuning dial knob
« Reply #9 on: February 12, 2019, 11:49:23 am »
If the setscrew is hardened you are in for a whole world of grief unless you have (or can build) an EDM 'tap burner'.

xD You may very well just buy a bunch of fully restored DCX-2000 for the price of that EDM approach.
He’s like a trained ape. Without the training.
 

Offline vealmike

  • Regular Contributor
  • *
  • Posts: 192
  • Country: gb
Re: Remove botched screw from tuning dial knob
« Reply #10 on: February 12, 2019, 12:15:07 pm »
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.

I 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?

Destroy 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.
 
The following users thanked this post: michalism

Offline amyk

  • Super Contributor
  • ***
  • Posts: 6616
Re: Remove botched screw from tuning dial knob
« Reply #11 on: February 12, 2019, 12:49:30 pm »
A bit of heat may help loosen the screw, or what remains of it.
 
The following users thanked this post: michalism

Offline tggzzz

  • Super Contributor
  • ***
  • Posts: 10690
  • Country: gb
    • Having fun doing more, with less
Re: Remove botched screw from tuning dial knob
« Reply #12 on: February 12, 2019, 12:51:32 pm »
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.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
The following users thanked this post: michalism

Online Bud

  • Super Contributor
  • ***
  • Posts: 3818
  • Country: ca
Re: Remove botched screw from tuning dial knob
« Reply #13 on: February 12, 2019, 02:22:28 pm »
Is the screw too small to try with a impact screwdriver? (after soaking in penetrating oil for a while)
Facebook-free life and Rigol-free shack.
 
The following users thanked this post: michalism

Offline michalism

  • Regular Contributor
  • *
  • Posts: 61
  • Country: gr
Re: Remove botched screw from tuning dial knob
« Reply #14 on: February 12, 2019, 02:34:10 pm »
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.

LOL!!! I will place it on the floor and start dancing around it... :-DD

Is the screw too small to try with a impact screwdriver? (after soaking in penetrating oil for a while)

Yes 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...
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 8181
Re: Remove botched screw from tuning dial knob
« Reply #15 on: February 12, 2019, 03:11:36 pm »
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.
« Last Edit: February 12, 2019, 03:15:13 pm by Ian.M »
 
The following users thanked this post: michalism

Offline alanb

  • Frequent Contributor
  • **
  • Posts: 340
  • Country: gb
Re: Remove botched screw from tuning dial knob
« Reply #16 on: February 12, 2019, 04:57:22 pm »
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
 
The following users thanked this post: michalism

Offline Bashstreet

  • Frequent Contributor
  • **
  • Posts: 298
  • Country: gb
Re: Remove botched screw from tuning dial knob
« Reply #17 on: February 12, 2019, 05:48:03 pm »
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.  :o
 
The following users thanked this post: michalism

Offline SilverSolder

  • Super Contributor
  • ***
  • Posts: 1126
  • Country: 00
Re: Remove botched screw from tuning dial knob
« Reply #18 on: February 12, 2019, 10:03:05 pm »
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.

That 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?

 

Offline SilverSolder

  • Super Contributor
  • ***
  • Posts: 1126
  • Country: 00
Re: Remove botched screw from tuning dial knob
« Reply #19 on: February 12, 2019, 10:06:24 pm »
Depending of the way the screw is used you might be able to just pull the knob (no pun intended)
...
Deepest apologies how this all sounded.  :o

 
The following users thanked this post: michalism

Offline Doctorandus_P

  • Frequent Contributor
  • **
  • Posts: 592
  • Country: nl
Re: Remove botched screw from tuning dial knob
« Reply #20 on: February 13, 2019, 12:39:07 am »
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.
 
The following users thanked this post: michalism

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 8181
Re: Remove botched screw from tuning dial knob
« Reply #21 on: February 13, 2019, 12:46:40 am »
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.
« Last Edit: February 13, 2019, 12:59:57 am by Ian.M »
 
The following users thanked this post: michalism

Offline michalism

  • Regular Contributor
  • *
  • Posts: 61
  • Country: gr
Re: Remove botched screw from tuning dial knob
« Reply #22 on: February 13, 2019, 08:36:28 am »
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 clueless :) :P Thanks 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.
 

Offline TerraHertz

  • Super Contributor
  • ***
  • Posts: 3640
  • Country: au
  • Why shouldn't we question everything?
    • It's not really a Blog
Re: Remove botched screw from tuning dial knob
« Reply #23 on: February 13, 2019, 10:06:18 am »
Sheding some light in the hole, revealed what I was afraid of.

?? 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.
Collecting old scopes, logic analyzers, and unfinished projects. http://everist.org
 
The following users thanked this post: michalism

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 8181
Re: Remove botched screw from tuning dial knob
« Reply #24 on: February 13, 2019, 10:16:47 am »
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.
 
The following users thanked this post: michalism


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf