On 2020-Apr-15, at 5:23 PM, Chris Zach via cctalk wrote:
> Wonderful: A few weeks ago I forgot to turn off my VT52 and left it running
> for a day or two. Now the screen is filled with snow and it looks like the
> text is all over the place horozontally.
>
> Any tips or thoughts on where to start looking to fix? The keyboard seems to
> be working as does the RS232 input (the snow on the screen changes when the
> pdp11 talks to it)
Try turning the contrast or brightness up till the black level starts becoming
illuminated. Is the raster (the screen character field / framing rectangle)
stable?
If no rectangle or stable display field becomes apparent then as Jon suggests
it may be loss of sync, probably horizontal, and likely to be in the monitor
circuitry.
(Any affect from twiddling the H or V sync controls?)
If the raster is stable then it might be something in the character generation
pipeline (the character generator or the row pixel serializer), or in the
monitor video amp.
("snow" as a description leaves for a range of possible interpretations.)
There's always checking power supply levels to start with. In addition to the
+5, there's probably a +/-12 or -5 for the character generator.
