Tektronix 4051 repair.

Bob Rosenbloom bobalan at sbcglobal.net
Thu Sep 22 08:53:26 CDT 2016

On 9/22/2016 1:17 AM, Mattis Lind wrote:
> I cannot find anything mentioned about character blanking in the circuit
> description.
> I read this on the topic "Alphanumeric Display":
> "Refer to the display control block diagram (Fig 6-13). The display
> coordinate position is generated through the digital-to-analog converters
> (X DAC and Y DAC) and fed through the filter circuitry with the filtering
> disabled. A short time later, either SOT is activated to print a cursor
> refresh dot or ADOT is activated to print a character dot. The timers for
> cursor intensity and character intensity are adjustable by potentiometers
> accessible through the bottom of the assembled instrument."
> Maybe this is the "A short time later" that it is not working since seems
> to draw instantly. But as far as In understand that timer is generated in
> the digital parts.
> Note that this is not a computer or terminal. It is an instrument per
> Tektronix definition...
> Unfortunately the detailedness of the circuit description is not up to the
> DEC standards in my opinion. There are no scope diagrams describing the
> pulses going to the Z-axis as far as I can see which is unfortunate.
> /Mattis

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.


Vintage computers and electronics

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