With an internal fan I think the covers actually can make a great heat 
exchanger as well. Often an instrument that is overheating will have portions 
of the case still cool.  Perhaps the worst example of fan cooling is the 
Symetricom 512XA Phase Noise Test set. It is kind of out of place on such an 
otherwise exceptional produce. No offense meant to the Symmetricom engineers. 
It is one of the few product I have purchased new.

Thomas Knox



> Date: Sun, 2 Feb 2014 20:42:54 +0100
> From: mag...@rubidium.dyndns.org
> To: time-nuts@febo.com
> Subject: Re: [time-nuts] Replacement fan in SR620
> 
> On 02/02/14 20:28, Tom Knox wrote:
> >
> > A little off topic but It seems many instruments (the SR620 and 53132A 
> > included) would work best with an internal fan. (A closed system, not 
> > exchanging outside air). Possibly with some sort of internal/external heat 
> > sink if needed. Or in high power situations outside air would flow through  
> > a hollow heatsink (again not exchanging outside and internal air). If 
> > nothing else this would keep dust out. Since fan filters can really 
> > restrict air-flow and without a filter dust on circuit boards can act as on 
> > thermal insulator leading to overheating while conducting static.
> 
> Good point. A fan in a closed environment will survive longer, or 
> rather, won't degrade as fast as it possibly would do.
> 
> These days I would assume that heat-pipes would be used to move heat to 
> a large external heat-sink. It's fairly cheap these days.
> 
> Cheers,
> Magnus
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