Topics

Knob repair tips


Dave Voorhis
 

I just acquired a Telequipment (British subsidiary of Tek in the 60’s/70’s) CT71 curve tracer in excellent working condition and good cosmetic shape, but for a cracked knob. Like the usual Tek knobs, it’s a plastic shell around a linear-knurled metal core. The plastic appears to have shrunk slightly, causing it to crack and no longer grip the core.

If I recall correctly, someone posted tips on this forum on how to permanently repair such shrink-cracked knobs, but my search-fu must be broken today because I can’t find the relevant posts.

Can anyone can give me a quick recap and/or point me to the posts on groups.io <http://groups.io/>?


 

I don't know any posts references but these two seem most appropriate and have worked for me:

1. If the parts still (almost) close around the knurled Al hub, fit the plastic parts and apply very little very-low viscosity superglue. Use epoxy in case of gaps between plastic pieces.

2. Chuck Harris came up with this brilliant suggestion a while ago: Pre-fit the plastic pieces around the Al core. Carefully heat the core to just soften the plastic and press plastic parts further in (use either fingers or an elastic band or such thing). If deemed necessary, apply a small amount of very-low viscosity superglue afterward, between core and plastic.

Raymond


Chuck Harris
 

Because the plastic has shrunk, you need to increase
the space for the center hub. The easiest way I have
found is to assemble the parts as well as you can, and
then wrap some rubber bands around the knob to provide
tension.

Then take a soldering iron, and heat the center hub.

The instant the plastic relaxes, remove the heat, and
cool the hub by blowing on it.

After all is done, a drop of acrylic solvent glue will,
seal the deal, so to speak.

-Chuck Harris

Dave Voorhis wrote:

I just acquired a Telequipment (British subsidiary of Tek in the 60’s/70’s) CT71 curve tracer in excellent working condition and good cosmetic shape, but for a cracked knob. Like the usual Tek knobs, it’s a plastic shell around a linear-knurled metal core. The plastic appears to have shrunk slightly, causing it to crack and no longer grip the core.

If I recall correctly, someone posted tips on this forum on how to permanently repair such shrink-cracked knobs, but my search-fu must be broken today because I can’t find the relevant posts.

Can anyone can give me a quick recap and/or point me to the posts on groups.io <http://groups.io/>?




Tim Phillips
 

from Tim P (UK)
................ or use an adjustable hose-clamp ('Jubilee clip') to hold
the bits onto the hub ?

Tim

On Thu, 7 Nov 2019 at 12:36, Chuck Harris <cfharris@...> wrote:

Because the plastic has shrunk, you need to increase
the space for the center hub. The easiest way I have
found is to assemble the parts as well as you can, and
then wrap some rubber bands around the knob to provide
tension.

Then take a soldering iron, and heat the center hub.

The instant the plastic relaxes, remove the heat, and
cool the hub by blowing on it.

After all is done, a drop of acrylic solvent glue will,
seal the deal, so to speak.

-Chuck Harris

Dave Voorhis wrote:
I just acquired a Telequipment (British subsidiary of Tek in the
60’s/70’s) CT71 curve tracer in excellent working condition and good
cosmetic shape, but for a cracked knob. Like the usual Tek knobs, it’s a
plastic shell around a linear-knurled metal core. The plastic appears to
have shrunk slightly, causing it to crack and no longer grip the core.

If I recall correctly, someone posted tips on this forum on how to
permanently repair such shrink-cracked knobs, but my search-fu must be
broken today because I can’t find the relevant posts.

Can anyone can give me a quick recap and/or point me to the posts on
groups.io <http://groups.io/>?






Chuck Harris
 

I wouldn't, as you really have to be gentle here.

Even the elastic bands, if too tight, will leave
a depression when the knob is in the molten state.

You want the bands stretched a bit, but not even
close to tight.

You should have some compressed air to cool the center
on hand, as the hub can retain heat a lot longer than
you might think. A water dunk would be ok, if the
set screw is removed afterwards, and the knob let to
dry thoroughly.

-Chuck Harris

Tim Phillips wrote:

from Tim P (UK)
................ or use an adjustable hose-clamp ('Jubilee clip') to hold
the bits onto the hub ?

Tim




On Thu, 7 Nov 2019 at 12:36, Chuck Harris <cfharris@...> wrote:

Because the plastic has shrunk, you need to increase
the space for the center hub. The easiest way I have
found is to assemble the parts as well as you can, and
then wrap some rubber bands around the knob to provide
tension.

Then take a soldering iron, and heat the center hub.

The instant the plastic relaxes, remove the heat, and
cool the hub by blowing on it.

After all is done, a drop of acrylic solvent glue will,
seal the deal, so to speak.

-Chuck Harris


Dave Voorhis
 

Excellent, thanks! That’s what I'd vaguely remembered.

On 7 Nov 2019, at 12:57, Chuck Harris <cfharris@...> wrote:

I wouldn't, as you really have to be gentle here.

Even the elastic bands, if too tight, will leave
a depression when the knob is in the molten state.

You want the bands stretched a bit, but not even
close to tight.

You should have some compressed air to cool the center
on hand, as the hub can retain heat a lot longer than
you might think. A water dunk would be ok, if the
set screw is removed afterwards, and the knob let to
dry thoroughly.

-Chuck Harris

Tim Phillips wrote:
from Tim P (UK)
................ or use an adjustable hose-clamp ('Jubilee clip') to hold
the bits onto the hub ?

Tim




On Thu, 7 Nov 2019 at 12:36, Chuck Harris <cfharris@...> wrote:

Because the plastic has shrunk, you need to increase
the space for the center hub. The easiest way I have
found is to assemble the parts as well as you can, and
then wrap some rubber bands around the knob to provide
tension.

Then take a soldering iron, and heat the center hub.

The instant the plastic relaxes, remove the heat, and
cool the hub by blowing on it.

After all is done, a drop of acrylic solvent glue will,
seal the deal, so to speak.

-Chuck Harris


Dave Brown
 

I turn the diameter of the metal hub down just enough so it fits within the remnants of the knob. They I use epoxy and glue it all together. I only have examples of gears on the vintageTEK website but it works equally well with knobs.
https://vintagetek.org/repairing-knobs/

Dave


n4buq
 

I've done similar by glueing the plast parts together and boring the hole slightly larger. I really like Chuck's method, though, as it locks the knurled hub very nicely (at least I presume it would - haven't tried it yet).

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Dave Brown" <davebr@...>
To: TekScopes@groups.io
Sent: Thursday, November 7, 2019 1:26:47 PM
Subject: Re: [TekScopes] Knob repair tips

I turn the diameter of the metal hub down just enough so it fits within the
remnants of the knob. They I use epoxy and glue it all together. I only have
examples of gears on the vintageTEK website but it works equally well with
knobs.
https://vintagetek.org/repairing-knobs/

Dave




John Griessen
 

On 11/7/19 1:36 PM, n4buq wrote:
I really like Chuck's method, though, as it locks the knurled hub very nicely
Sure the hub is made for that hot plastic... and if you cool it quick and hold
with rubber, and it is with its original plastic, just cracked, it is likely to stay aligned.

If you turn down the hub and leave clearance, how to keep it aligned?


n4buq
 

If the knob is bored so that the insert is a snug, slightly friction fit when dry, the alignment is natural and the epoxy holds it tightly.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "John Griessen" <@jgriessen>
To: TekScopes@groups.io
Sent: Thursday, November 7, 2019 2:11:09 PM
Subject: Re: [TekScopes] Knob repair tips

On 11/7/19 1:36 PM, n4buq wrote:
I really like Chuck's method, though, as it locks the knurled hub very
nicely
Sure the hub is made for that hot plastic... and if you cool it quick and
hold
with rubber, and it is with its original plastic, just cracked, it is likely
to stay aligned.

If you turn down the hub and leave clearance, how to keep it aligned?