Re: 494 and 2756 Spectrum Analyzer questions...

John Miles

Any suggestions about the best way to open it in
a non-destructive way? Put it on a hot boiler
plate and remove the cover when the tin melts
or use brute force making the cover unuseable?
I usually use a Dremel tool with a cutting wheel. The filter takes a lot of vibration, unfortunately, and a respirator is needed unless you want to breathe a lot of lead particles. But it does get the job done.

Some have used propane torches successfully, but transferring just the right amount of heat to melt the solder is harder than it sounds. I've done more than my share of damage with propane torches, operating under the usual rationale ("I know what I'm doing, this'll only take a few seconds.")

After some time, the rear end with the integratedheat sink gets to hot to
touch. The Bueler fan
runs at highest speed. Other 49x analyzers that
I have used over the years does not even get
close to this high temperature. Easy fix
would be to add extra vent intake(s). Did Tek
ever made any modifications to the cabinet?
Good question, not that I know of.

At least some departments at Tektronix used a straightforward watts-per-cubic-inch metric to determine whether something was going to get too hot. Fans were recommended for instruments consuming more than 50 mW/in^3, and the documentation I've seen (Common Design Parts Catalog volume 2 from 1983) indicates that the surface area of the enclosure is also supposed to be taken into consideration.

So the thinking may have been that the large surface area of the 275x rack mount enclosure was enough to avoid the need for additional ventilation or a bigger fan, when in reality the instrument might have ended up hotter than the benchtop version. Much of the box is empty, so they would have needed to consider how the heat was supposed to reach the enclosure boundary in the first place. Perhaps that process needed some more work.

Maybe someone on the list remembers more about how these decisions were made back in the day?

-- john, KE5FX

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