video problems: debugging help

[Reti@RIVERSIDE.SCRC.SYMBOLICS.COM]

    Date: Mon, 11 Feb 91 09:52:39 MST
    From: drstrip@gnome.cs.sandia.gov (David R. Strip)

    My dying 3645 refuses to send video to the terminal (or so it seems).
    I have eliminated the following possibilities (usually by swapping parts
    with a 3640 next to it):
       monitor
       monitor cable
       FEP board
       I/O board
       paddle card (is this the right name for the small card in the back
		    that has the video output?)
Yes.

    The machine appears to be alive, since i get nice random patterns on the
    monitor that disappear for a moment when I blindly type reset video.
Do the patterns change when you type commands like "SET MON PHI" and "SET MON MON"
at the FEP?  If any of them are close to correct, perhaps your PE video is out of
adjustment (you can adjust it on the daughter video board in the console).  Since the
monitor works with the 3640, the adjustment would probably not be very much (i.e.
if this is the problem the monitor's adjustment doesn't overlap the output from both
machines).

If you blindly boot, does the machine come up?  If so, does <function>C have any
effect on the video pattern?

How long a cable are you using?  If it is a long cable, try it with a shorter cable.

Another common problem is with the video driver/receiver chips; since you
swapped I/O paddles, the driver is probably OK (actually, I now notice you
didn't specify which paddle you swapped).  Oftentimes one side of the dual
polarity driver or receiver blows out and things still work until either 
noise or cable length increases.

    What else can I pull/swap to try to isolate the problem?
    Thanks
    strip
    drstrip@cs.sandia.gov  (don't trust the return address in the header.)