Author Topic: Knob repair -Tektronix/HP  (Read 2903 times)

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Offline CatalinaWOW

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Knob repair -Tektronix/HP
« on: April 04, 2017, 05:46:16 am »
Searching the forum didn't show any prior posts on this subject so I thought I would share my method for repair/replacement of the clear plastic skirts used on many older instruments for range selection.  The particular example here is the vertical gain knobs on a Tektronix 2236, but I will be using this technique on several other veterans I have.

The first picture shows the knobs in place on the scope.  The skirts had obviously been missing for quite a while.  Someone used indelible ink to write values on the face of the scope and put a small indicator dot on the knob.  This also shows the benefits of a little dish soap and a toothbrush in removing the grime of years of use.  The Channel 1 knob is uncleaned, the Channel 2 knob has just a couple of minutes of cleaning.

I didn't get many pictures of the fabrication process, but that is ok.  They would just be pictures of the computer screen anyway.  The first step is to find what the skirt looked like.  I was fortunate to have another scope of the series, which has the same style knob.  Take a picture of this.  As you see by the pictures I am adding to this I am not a great photographer.  It isn't required.  The only critical thing in this picture is to have the camera centered over the reference knob as well as you can.  This will be the template for the new skirt.  See the second picture.

You now use the graphics program of your choice to lay out the knob.  I used Powerpoint because years in the corporate world made me proficient, and it or one of the free clones is widely available.   Paste the photograph of your reference knob in the back (or on a background layer).  Stretch it to be as large as your working screen will allow.   Working at this size gives you more effective resolution in font size.   Select a font that matches the lettering on your dial.  Arial comes remarkably close for this knob, with minor differences in the serif on the one.  Line up all of the sizes and orientations.  Set up a reference circle at the outside diameter of the skirt.   When you are happy with the placement of everything, remove the photograph from the background.

We now proceed to scaling the image to the right size.  The next step may not be necessary on all programs, but several of those I have tried do not scale fonts well, so the layout must be converted to a bitmap.  Working in Windows the easiest way I know is using the Snip Tool found in the Windows Accessories folder.  If you are good with this tool you can exactly capture your outside reference circle, otherwise use your favorite picture editor to trim off the excess.  Now the result can be put back in your graphics program for final scaling.  The physical dimension of the outside of the skirt on this instrument is 1.1 inches.  Scale the image to that size.  Then add a reference circle for the diameter of the ledge the skirt rests on.  In this case 0.560 inches.  (Inches used because that is what the instrument maker was using when this was built.)

Next print this using a laser printer on a transparency sheet (the kind that used to be used for overhead projectors).  I still have large stocks of this.  It should still be available from many sources.  The result is too thin and floppy so laminate it in one of those plastic pouches used to make important documents permanent.  If you don't have this capability in your shop, many office supply stores will perform the service.  The third photo shows one of the laminate printouts (along with the hole where a brother was cut out.)

The skirt is now trimmed out using scissors and/or a sharp exacto knife.  In this instrument there is an alignment key.  After determining where the key needs to be cut a knotch with the exacto knife.  On this instrument the notch lines up with the 5 millivolt scale marking.

Bond the skirt to the knob using an appropriate cement.  I used Formula #560 Canopy Glue, a specialty material from the model airplane world design to bond plastic canopies to the stretchy plastic covering used on those models.  Any flexible cement that dries clear and bonds to plastics would be suitable.  It takes only a tiny amount.  Put a drop of glue on a disposable surface and then use a toothpick or pin to pick up a little and spread it on the knob ledge.  Then install the skirt, being sure to get it right side up.  The third photo shows the end of this step.

Unless your hand is much surer than mine the edge of your skirt will be slightly out of round.  By chucking it in a lathe or electric drill and gently applying a file or sandpaper these imperfections can be removed and a slightly rounded edge added to the skirt.

The fourth photo shows the end result.  Fully functional and looking pretty close to the original.
 
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Offline SingedFingers

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Re: Knob repair -Tektronix/HP
« Reply #1 on: April 04, 2017, 08:16:21 am »
That's rather neat. Nice one.

I've actually bought an entire scope before just to recover a couple of knobs off with skirts.
 

Offline grumpydoc

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Re: Knob repair -Tektronix/HP
« Reply #2 on: April 04, 2017, 08:25:20 am »
The plastic skirts have a habit of splitting because they are just slightly too tight on the knob.

The markings also rub off even the skirts don't split

I have had a little success restoring the markings with a technique similar to toner transfer using the waxy paper used to back self adhesive labels on A4 sheets as the transfer medium.

There was a guy selling replacements on ebay a while back that he had had made up - they were not especially inexpensive though (maybe OK for a one off but a pain if you have more than one 'scope to do).
 

Offline tautech

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Offline SingedFingers

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Re: Knob repair -Tektronix/HP
« Reply #4 on: April 04, 2017, 08:52:13 am »
Last time I used Sphere I got shafted for $40 priority air mail and import duty.

Bought an HV multiplier for a 465B. Cost me as much as an entire 465B in the end.
 

Offline tautech

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Re: Knob repair -Tektronix/HP
« Reply #5 on: April 04, 2017, 09:06:29 am »
Last time I used Sphere I got shafted for $40 priority air mail and import duty.

Bought an HV multiplier for a 465B. Cost me as much as an entire 465B in the end.
Then try these guys then, found with a Google search for the knob part #:
http://www.tek-parts.com/shopdisplayproducts.asp?page=7&id=34
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Offline SingedFingers

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Re: Knob repair -Tektronix/HP
« Reply #6 on: April 04, 2017, 09:40:41 am »
I've used them. They never have what I want annoyingly.
 

Offline CatalinaWOW

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Re: Knob repair -Tektronix/HP
« Reply #7 on: April 04, 2017, 02:03:50 pm »
The plastic skirts have a habit of splitting because they are just slightly too tight on the knob.

The markings also rub off even the skirts don't split

I have had a little success restoring the markings with a technique similar to toner transfer using the waxy paper used to back self adhesive labels on A4 sheets as the transfer medium.

There was a guy selling replacements on ebay a while back that he had had made up - they were not especially inexpensive though (maybe OK for a one off but a pain if you have more than one 'scope to do).

The markings won't rub off on this repair because they are inside the lamination.  They could become unreadable as the surface gets scuffed.
 

Online tggzzz

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Re: Knob repair -Tektronix/HP
« Reply #8 on: April 04, 2017, 02:46:20 pm »
I did something similar for my 475's vertical sensitivity dial. I can't remember whether I used an inkjet of laser printer, but the font was "ubuntu condensed, 7pt, bold"
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Offline DmitryL

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Re: Knob repair -Tektronix/HP
« Reply #9 on: April 04, 2017, 04:05:40 pm »
I have had a little success restoring the markings with a technique similar to toner transfer using the waxy paper used to back self adhesive labels on A4 sheets as the transfer medium.

I have successfully restored markings on scopes knob skirt. It is nearly impossible to see the difference with the original one.
1. design this dial in any sort of a program you like.
2. print it normally (not mirrored) on a transparent plastic sheet (I used laser printer)
3. remove all old marking from knob skirt, AFAIR I used something slightly abrasive, like tooth paste.
4. cut that dial from transparent plastic , it will be glued to the exististing skirt at the side that faces scope front panel, toner side goes TO the plastic skirt.
5. aply UV-curable transparent "glass glue" to the skirt, stick new dial there, remove air bubbles and cure.
 

Offline CatalinaWOW

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Re: Knob repair -Tektronix/HP
« Reply #10 on: April 04, 2017, 04:13:45 pm »
Good approach DimitryL
 

Online tggzzz

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Re: Knob repair -Tektronix/HP
« Reply #11 on: April 04, 2017, 04:43:30 pm »
3. remove all old marking from knob skirt, AFAIR I used something slightly abrasive, like tooth paste.

ISTR just using IPA, and the lettering came off far too easily!

As for sticking the new label on, I just used a small dab of UHU (rubber solution) glue in the middle so that it would be hidden by the opaque grey knob. That means the flexible plastic is free to flap against the rigid plastic, but in practice that doesn't matter.

Guess which of the two is original, and which is homebrewed :)
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
 

Offline PaulAm

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Re: Knob repair -Tektronix/HP
« Reply #12 on: October 11, 2019, 04:06:49 pm »
I just used this technique to fix the sweep knob on my 8754A VNA.  Worked great.

For an adhesive, I used Loctite glass glue, a cyanoacrylate.  It worked great although a trial piece showed excess glue would smear the toner.  A light smear on the knob before applying the legend worked fine.
 

Offline floobydust

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Re: Knob repair -Tektronix/HP
« Reply #13 on: October 11, 2019, 07:17:16 pm »
I made new knob dial skirts for my 2235's. Anything to end my tears and screams of agony trying to remember V/DIV setting.

Redrew the original using AutoCAD, because of the multi-angles and text justification etc. it's bit challenging.
Custom laser-cut cast-acrylic with reverse-engraved text. Tek thickness is oddball at 1.8-1.0mm at bevel, I used 1.5mm

There was a problem with the laser cutter kerf being out a few thou, so the press-fit was too tight and I had to sand the new dial's inside hole. The old Tek plastic shrinks with age, which is why the dial cracks. Never use IPA on acrylic as it cracks.

I had to fill the text black with paint which was my patience limit - I was going to do a Rev. B version with dye etched text but never got around to it. The bevel is missing but I don't notice it.

edit: Tektronix 2235 Attenuator Knob 366-2148-01, note the skirt (knob dial) is part of it.
« Last Edit: October 11, 2019, 07:45:46 pm by floobydust »
 

Online SilverSolder

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Re: Knob repair -Tektronix/HP
« Reply #14 on: October 11, 2019, 09:52:15 pm »

Wonder if it isn't possible to do some kind of toner transfer job onto acrylic?  I sense an experiment coming up...
 

Offline james_s

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Re: Knob repair -Tektronix/HP
« Reply #15 on: October 11, 2019, 11:42:53 pm »
That looks pretty good. I once washed a set of knobs for a Tek 2215 and was horrified to find that they came out squeaky clean, not a trace of printing left on the skirt!  :o
 


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