Re: Kaypro II retrace/blanking problem
- --- In firstname.lastname@example.org, joshbensadon <no_reply@...> wrote:
>Thanks very much! Cold Spray, oh yeah! I remember that!
> --- In email@example.com, "Douglas" <touchetek@> wrote:
> > For a few seconds, the background is normal; nice and dark.
> > The text stays the same.
> > The the abnormal retracing fades up.
> > (some just say swap out the video board...)
> Wow, that's great advice, I would have said swap out the computer ;)
> Seriously, here's what I would do to start breaking down the problem.
> Two roads.
> 1. the problem occurs much after the capacitors charge, the picture tube warms up, so I would guess some part is getting excessively hot. Get some cold spray and gently spray some area of the video board. Spray with the straw 2 or 3" from board. Cover the whole board a few times, do this within 30 seconds of the problem appearing. If there are no changes, power down, repeat but be more agressive with cold spray, say 1". If you do encounter a change, wait for the problem and find the component(s) that are causing it. When you find the component, you'll decide then what to do about it? replace? supplement a heat sink? trace the heat problem to some other source ie voltage too high?
> 2. Scope the pins to the CRT. In general, there could be 3 grid voltages I think they were Bias, Focus and Video? Then there's the cathode and the heater. I think most of these voltages are low, perhaps the bias voltage might be high? but nothing a scope can't handle. Stay away from the high voltage line that is a single wire to the side of the tube, here you can find 15,000 volts (up to 35K on large colour TV's). While scoping, look for which line (if any) shifts it's voltage. If none of the lines shift voltage, then the problem could be with the high voltage or the picture tube? But I've never heard of a picture tube causing this problem and a change in high voltage typically results in a loss of focus.
> PS. I have a Kaypro in my repair queue too, but it will be a while before I get to it.
> Good luck!
Still have some so that's something I can do.
Your instructions are a great start at this. Though I know video from a signaling point of view, I don't know much about the tube and its control. Soon I may...
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!