Re: Preventative maintenance time!

Wow what a difference.. And anodizing the cage will most likely result in
more of a drop in temp as black will radiate the heat more effectively than
silver, thus drawing more heat away from the transistor. Basically the same
reason it absorbs more heat than lighter colors. That's why you see things
like condenser coils on AC units painted black.

Great pictures by the way. I wish I had a thermal camera, for now I'll have
to stick with my trusty infrared thermometer.

Kevin

On Sat, Sep 5, 2009 at 3:20 AM, Clay Cowgill <c.cowgill@comcast.net> wrote:

> 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 06:23:10 2009