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Messages - jluu

3
Project logs / Poor man's tektronix 2445 U800 mending
Tektronix 24xx series are notorious for their U800 horiz. amplifier failing, gess what: it happened to me.
I got mine 20 years ago from a compagny going out of business, using it from time to time. Display began to go away (sidewise ...)
I was being able to restore it by hitting the beam finder button with various level of success until 2 month ago when it would not
come back no matter what.
http://https://www.flickr.com/photos/22784046@N06/15886816828/in/set-72157647544996263/
Armed with the service manual and another scope, I quickly discovered the faulty U800 as well as this hybrid chip was well known for its failures.
Luckily, it was only half faulty, the output to one deviation plate (test point 61) was good, whereas testpoint 62 was a steady 80v.
http://https://www.flickr.com/photos/22784046@N06/15888486177/in/set-72157647544996263

I restored the second amplifier (U800:pin4) output by inverting the good one using a single transistor fixup, after having simulated it with ltspice:
http://https://www.flickr.com/photos/22784046@N06/15888568117/in/set-72157647544996263/
Here is how it looks in real life, OK I do need a real U800:
http://https://www.flickr.com/photos/22784046@N06/16074587805/

The screw on top of the U800 is at -5v
http://https://www.flickr.com/photos/22784046@N06/15888822497/in/set-72157647544996263/
Results are good when horiz scan is faster than 2us/cm, I suspect this is because of the low resistor values (5.6K), I used low values in an attempt to favor the higher speed settings by lowering the impedance faster loading the deviation plate capacitance.
At 2us/cm:
http://https://www.flickr.com/photos/22784046@N06/16048695126/in/set-72157647544996263
At 20us/cm
http://https://www.flickr.com/photos/22784046@N06/16048695986/in/set-72157647544996263/
at 100us/cm it becomes very messy
http://https://www.flickr.com/photos/22784046@N06/16048696646/in/set-72157647544996263/

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