Tektronix 4051 repair.

Mattis Lind mattislind at gmail.com
Thu Sep 22 14:04:12 CDT 2016

>> Snip..
> The beam is required to stay at one location for a certain time and
> intensity to be stored in the CRT. If it's moved, or if the intensity is
> too low, it will not be stored.
> The cursor is set to a lower intensity so it can be seen on the CRT,
> flashing, but will not store. Characters are brighter and left on long
> enough to be stored. Only
> the cursor is refreshed, the characters are stored once, then the beam is
> moved to the next position.
> It looks like something is moving the beam around when it shouldn't,
> drawing on the screen.
> Bob
Thanks Bob for this information. I am starting to get some understanding of
the machine now. I was looking in the calibration section to check the
voltages on the display board. It is extremely vague on voltages there
should be.

"Remove J-50, which will disable the high voltage generator. Place a
voltage probe to J58-3. Power up, and adjust R359 for a collimation
electrode voltage of about 85V, or as specified on the crt shield"

There is a label:


I am not 100% sure I understand what it means. CE might stand for
"Collimation Electrode" and then 75 (V) and is "storage level" the same as
backplate voltage? Anyway it is said to be 164V. The calibration section
says about 200V...

The voltages I measured was nothing near 200V nor 164V. It was 149V on op
level/backplate and 68V for collimation electrode


Anyway. When tuning the voltages there were sudden pop and a small 0.25W
resistor failed spectacularly with a smelly cloud. It was the R339 47 ohm
resistor connecting to the 320V supply voltage. It should not fail unless
something else has failed and yes a TIP50 transistor generating the "op
level" / "backplate voltage" had also failed. Short circuit emitter / base.

So I need to find a new suitable transistor before continuing this.
Meanwhile it would be interesting if there are any other 4051 owners out
there that could maybe share what the reading of the label is on their
machines. Even better if it could be measurements on an operating machine.

Before the smoke escaped I recorded this short videoclip to show the
behavior when typing on the machine and running simple PRINT/GOTO program.



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> Vintage computers and electronics
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