Re: Preventative maintenance time!

Clay, great tip... however- warning!! For those looking to do this
you will want to avoid some of todays CPU heat sink grease such as
Artic Silver. It's semi conductive! Using a meter, you may not
initially read a short from transistor to chassis... but it can be a
different story once the monitor is fired up. A collector had sent
me a 4900 for repair that had this stuff between the HOT and chassis,
took a bit to track this one down....

In terms of amperage/voltage a BU409 isn't an 'apples to oranges'
comparison with a HOT, but I still wouldn't want to try using that
stuff.

James Hagen

On Sat, Sep 5, 2009 at 4:20 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 08:25:59 2009