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Tektronix Scope-Mobile Cart Repair Part 1


Posted on January 6th, by Shaun Merrigan in Electronics, Oscilloscopes. Comments Off on Tektronix Scope-Mobile Cart Repair Part 1

Tektronix Scope-Mobile Cart Repair Part 1

I was lucky enough to obtain a Tektronix 202-2 Scope-Mobile Cart.

Tektronix 202-2 Scope-Mobile scanned overview image

 

Upon examining it, I found that the scope tray was loose and the tilt mechanism did not function properly.  The cause for this was broken trunnion mounts on both sides of the cart as follows:

Tektronix 202-2 Mobile Cart Chassis Parts

 

Trunnion Mount and Broken Bosses Overview

 

Trunnion Mount and Broken Bosses Overview

Three of the four bosses were fractured with the fracture surface indicating tensile overload.  I suspect that the bolts were overtightened and the bosses fractured. Overtightening loaded the bosses in tension by trying to compress the box frame member.  Unfortunately this type of polymer (some type of hard thermoplastic) is not good in tension.

Other mobile carts I have seen use cast aluminum for these trunnion mounts, which is a better choice for strength and durability.  The cart I have for my 556, a 205-2 (triple wide), uses cast aluminum trunnion mounts.

Broken Bosses Closeup. The fracture surface shows brittle tensile overload.

 

Broken Bosses Closeup. The fracture surface shows brittle tensile overload.

 

Broken Bosses Closeup. The fracture surface shows brittle tensile overload.

 

Broken Bosses Closeup. The fracture surface shows brittle tensile overload.

I did try to glue the bosses back into place using cyanoacrylate (super glue).  However I believe this polymer is a low surface energy material, so the super glue did not hold under low load testing.  Thinking about the problem, I decided that similar repeat fractures would be likely if the bosses were loaded in tension or subjected to sudden high impact loads (like placing a scope on the tray or bumping a loaded cart).  An additional problem in a cold climate and my semi-heated lab is the glass transition temperature of this unknown hard polymer.  It may be high enough that the trunnion mounts become very brittle at cool winter ambient temperatures. Depending upon the polymer it may already be in the glassy state.  Thus,  I concluded that the bosses needed to be loaded in compression in order to make the trunnion mounts reliable.

I will continue this repair in part 2.

Shaun M





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