Re: [midatlanticretro] Kaypro II retrace/blanking problem
Video can be either +ve or -ve depending on the system, I have no
knowledge of the Kaypro (is there a circuit somewhere ?), but the
following info from my basic video knowledge.
In a basic Black/white video signal (usually 1V p-p) 0% is the sync
pulse level, 33% is black, 100% is white. The Black level is usually
"clamped" in some way and the sync pulses both line and field are
imposed on on black as a pulse to 0%, the actual video is then left to
fill the remainder between 33% to 100%.
It would appear that your 100% "white" level is ok, but that the 33%
"black" level is changing so the flyback is seen on screen. Some systems
generate a "flyback blanking" signal from the video to turn the CRT beam
off during this period, if the Kaypro does it may have failed. Others
rely on the video to block the beam during flyback purely by the video
being below the 33% level.
As The Kaypro seems to get worse with time I suspect a component/s in
the video chain is drifting so that the 33% level drifts towards the
100% level, it could be either in the video generation or the display.
The basic structure of the video signal is good as the text is displayed
in the correct place and time, so the sync signals are correctly placed,
just that the "black" background level appears to drift up from black.
Basic video structure here - http://www.ni.com/white-paper/3020/en -
there are many examples on the web.
A circuit of the Kaypro video system would help, anybody ?.
On 17/11/2012 06:52, Douglas wrote:
> I have a Kaypro II that shortly after power on the display looks like this:
> Bad Video
> (stored in the DC Album of this group)
> For a few seconds, the background is normal; nice and dark.
> The text stays the same.
> The the abnormal retracing fades up.
> The video intensity knob in the back changes the text brightness but
> does not affect the
> brightness of the abnormality.
> Googling on Kaypro video subjects turned up no definitive trouble
> shooting/trouble spots.
> (some just say swap out the video board...)
> Need some pointers, thanks in advance for any shared wisdom!