Re: Beam modulation on 2465B


Chuck Harris
 

Your video is showing an intensity modulation, which
makes it a Z-axis phenomenon.

Adjust the input signal frequency to synchronize with
the modulation so it becomes a slow throbbing sort,
and I think you will start to get clues as to what is
going on.

If the frequency necessary to synchronize is an even
multiple of the EHT transformer's oscillation frequency,
you likely have a problem with one of the filter caps
on the EHT board.

And, please, please, please, stop messing with your U800
chip! We are running out of them!


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.

-Chuck Harris

BUR wrote:

Hello Siggi



Many thanks for this information. Unfortunately turning the intensity button
to 12 o'clock has no effect on this modulation even turned fully off makes
no difference.

Just if you need a video demo. You can view it here:
https://www.dropbox.com/s/u8h1sk25ojkth77/Modulated%20Beam.mp4?dl=0



I also have seen some guides to adjust the grid Bias, also on my scope no
effect. Interesting is that this phenomenon is only visible at precise 1,2,3
etc MHz a offset of 100Hz is enough to stop the modulation.



I have documented all my efforts to find a solution on my YouTube channel.

https://www.youtube.com/channel/UC4S_r_4BgHbxptqiRipygYw



Sorry it’s in German!

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