[Boatanchors] [Hallicrafters] got the HT-20

[Robert Nickels]

On 6/21/2010 11:52 PM, Robert Kerr wrote:
> Perhaps the warping is from thermal stress in the casting, possibly from
> cooling it too fast when the part was made, or not annealing (heat
> treating) it to relieve the stress after fabricating.
>    
I think that's exactly right, Bob.   I'm no casting expert but I'm aware 
of the general principles and did a bit of research when I was restoring 
my SX-88.   The newly-cast part needs to cool at a certain rate to 
relieve internal stresses, and commonly this is done by leaving it in 
the mold to cool, or by heat treating, which usually means putting parts 
in a "cooling furnace" that creates the desired temperature profile 
although other forms of quenching are also used (see reference below).

Considering that these bezels were cosmetic parts and cost-sensitive, it 
seems pretty likely that the supplier shortcut this part of the process, 
with the results we see.  And since may have taken years for the warpage 
to become really noticable, Hallicrafters may not have  considered this 
much of a problem, especially as long as a box full of replacement 
bezels was sitting on a warehouse shelf for anyone who complained.

73, Bob W9RAN

Ref: *http://tinyurl.com/3a968hv*