17 Apr
2020
17 Apr
'20
6:21 a.m.
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.