Also the U800 DO gets hot, should I put for peace of mind some radiator on it ? Please advise, or the lucky X-mas scope will be closed and it will take a place on the bench together with the AGILENT and Rigol.
I concur with med6753 on this: leave it alone. The consensus is Tektronix knew what they where doing, so no need for any meddling with U800. So far the U800 in your scope has been doing its thing for a very long time, and has survived fine.
Also, notice that while the scope is idle, U800 may only get mildly warm to the touch, which some people take as a relief but it really doesn't mean anything. Under normal operating condition the IC mostly only gets hot under heavy load while displaying high repetition, or high frequency waveforms, etc, but proper airflow as-is with the cover on the scope, and a properly functioning fan, should provide ample cooling to keep the IC temperature within spec.
On the ever popular issue of trying to make improvements on the U800 heat dissipation department, I wanted to share this TekIO group message bellow from a very experienced member I think is the best explanation that so far I have found:
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START
https://groups.io/g/TekScopes/message/152372===============================================
Everybody please listen!
U800 is not heat sensitive, and I have found that many of
the "tricks" folks have gotten from the internet on how
to "protect" their U800 chip actually harm the chip due
to over-tightening the nuts on the studs that support the
chip. I have also found that glued/taped on heat sinks
have raised the temperature of the chip by impeding the air
circulation around the chip.
I have also found glued/taped heat sinks floating freely
inside of scopes that have been sent to me for service!
Tektronix botched the mounting of U800. That, in my
experience, is why they fail.
U800 was designed to have its pins pass through a 2-3mm
thick aluminum heat sink mounted underneath the chip.
The belly of the chip is metal to engage this heat sink.
DIP style IC's have pins that are made with a wide part to
prevent the belly of the IC from bottoming onto the PCB.
This allows air circulation under the chip. U800 is no
exception.
The engineer that designed U800 was excessively cautious,
and overestimated the heat that would be generated by his
new IC. When it came back from fab, he found that it did
*NOT* need any heat sinking to meet the full +50C design
temperature rating of the brand new 2465 scopes. So, the
heat sink was left off of the board.
(Thought experiment: when was the last time you operated
your 2465 at 50C?)
The problem is the designer needed -5V to bias U800's
substrate, and he used the metal heat sink frame and tab
to make that connection.
He found that U800's stud could not clamp tightly to
the PCB (REMEMBER: shoulders on pins...) without putting
excessive pressure on U800's pins, so he put a pair of star
lock washers onto the mounting studs before mounting U800.
The washers served to make the electrical connection, and
to provide the needed spacing to protect the chip....
He hoped.
Everything is fine if nobody ever, ever, over-tightens the
nuts that hold the U800 to the board. If they do, the
extra pressure will collapse the star lock washers and
embed them into the PCB and the bottom of one end of the
epoxy body. When this happens, the lead frame of U800 will
crack at some of the pins
If you come along later, having listened to all the bloviation
about U800 fixes, and happen to tighten up the nuts a little
bit more: crack! You will have damaged the chip while trying
to protect the chip.
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https://groups.io/g/TekScopes/message/152372===============================================
So if at this point you still want to put some form of additional heat dissipation on U800, make sure to verify that heat is actually being conducted away from the IC body into the added heatsink and you have not inadvertently created a heat barrier instead. And in the case of clipped-on or epoxied-on heatsinks, make sure that it can definitively not at some point fall off, and land right into a live circuit and possibly destroy other "unobtainium" stuff. And above all, do not to over tighten the mounting nuts. Or even better, try to avoid having to mess with the U800 mounting nuts altogether.
Happy New Year to all!
EDIT: small typo corrections.