Preventative maintenance time!

I finally ran out of working spare WG6100 subassemblies, so I had to put
some time in fixing a WG6100 HV cage tonight so I can work on a couple
vector things (a small handful of AVG replacements and those display
correctors).
 
After finding the problem on a "known dead" HV cage (blown BD208 and
incorrect repairs by whoever tried to fix it the first time) I got to
thinking-- I wonder how much of a difference it would make in component
temperature if I just re-did the heatsink on a part as a preventative
measure?
 
I decided to run a little experiment:
 
1) Install an old HV cage in a Tempest upright and power up, then take a
couple thermal images after five minutes. I'm pretty sure that the BU409 on
this cage had never been altered since it left the factory.
 
2) Remove the cage and simply remove the BU409, clean the old heatsink
grease off all parts, buff up the metal with a brass bristle brush, and put
everything back together with a new coat of heatsink compound on it.
 
3) Allow the cage to cool completely back to ambient temp (76F in the shop),
then reinstall and fire it up and take temperature readings after five
minutes again.
 
The results were pretty dramatic. (More so than I expected!) Just putting
new heatsink grease on both sides of the mica insulator resulted in about a
33F drop in temperature at the package-- a ~16% decrease!
 
Here's a few photos for your consideration.
 
http://picasaweb.google.com/lh/photo/5fwScQizJaF92SduCtSsCg?feat=directlink
 
http://picasaweb.google.com/lh/photo/YHJ6lVKA2lSiPjsQoc4lmA?feat=directlink
 
http://picasaweb.google.com/lh/photo/BrYvx3W2uZS8IRZNF6pYXQ?feat=directlink
 
The first one is the blended visible/thermal image so you can get an idea of
what the thermal-only images are actually showing. As you can see it's
basically just the back of the vector monitor shown with the back off the
Tempest, then I'm looking 'up and under' the wood to get a clear view of the
BU409 mounted on the outside of the HV cage.
 
The two all-thermal images use the same scale (from 74F to 212F) showing
"old dried out heatsink compound" with a peak temperature of 210F, then the
same part/same mounting hardware with new heatsink compound and a peak
temperature of only 176.5F. Kinda neat.
 
If you figure that each 10C increase in temperature cuts the service life of
a semiconductor in half, it might just be a good idea to go through your
vector monitors and redo all the heatsinks every few years or so to keep the
thermal performance up to snuff!
 
I'm curious now to try some other experiments someday... I see that
RadioShack sells a few different kinds of heatsink grease, so it'd be
interesting to see if they actually make any difference or not. Along
similar lines, I wonder how phase change material would work on deflection
transistors... (Eliminate the separate grease/insulator/grease stack.) Also
tempting to strip and black annodize the HV cage itself and see what that
does. (Yes, black annodizing results in better heat dissipation than bare
aluminum-- explaining that is beyond my physics background, but I guess it
kinda makes sense. I contracted on an LED lighting project a while back and
we dropped the temperature at the LED package by ~6C just by annodizing the
custom aluminum heatsinks black vs. leaving them silver... Go figure.)
 
Now it's possible that when operating over a long period of time and in the
enclosed environment of the cabinet that the temperature gap between 'old'
and 'new' heatsink compound would close up (if cooling in the cabinet is a
limiting factor), but still-- getting heat off the die on the transistors is
bound to be a good thing.
 
Anyway, I thought it was interesting enough to share my results. :-)
 
-Clay

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Received on Sat Sep 5 04:20:07 2009