Topics

494 and 2756 Spectrum Analyzer questions...

ulf_r_k
 

1st question.

I have brought a 494 analyzer back to life.
Except for the Xtal filter box on top of the
CRT, the instrument performs OK.
The filter box does not show any temperaturerise when in operation with cover off.

Looking in KE5FX service notes, the box
must be opened in order to replace the transistor.

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?


2nd question.
I have also repaired a 2756P with various problems
and the instrument does now work.

I am just a bit puzzled by the fact that there is only a very small vent intake on the left side of the cabinet.

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?
Cheers
Ulf KylenfallSM6GXV

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

Ken Eckert
 

I have no knowledge of the particular component, I use a chemistry hot
plate that I have roughly calibrated for pre heating, just dandy for
certain surface boards where one leg of the component is tied to a big
ground plane. 125 - 175 C is the recommendation usually, your mileage and
experience may be different.....

On Tuesday, August 20, 2019, John Miles <john@...> wrote:

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