16 Nov
2014
16 Nov
'14
2:49 p.m.
On 10/26/2014 8:09 PM, Josh Dersch wrote:
On 10/25/2014 10:17 PM, Don North wrote:
Just a a quick follow-up here: I recapped the BLIT this weekend.
Recapping the monitor's power supply had no effect but after doing the
main PCB it seems to be humming along nicely. I also redid the logic
power supply while I was at it.
Now to fix the mouse...
- Josh
On 10/25/2014 8:14 PM, Josh Dersch wrote:
Thanks (to both you and JWS) for the tips. I'll see if I can get a
scope hooked up to the sync signals sometime this week, though the way
the behavior progresses (it gets steadily worse and the amplitude of
the oscillations gets greater) makes me think it's probably not a
problem with the signal coming from the display logic -- a digital
failure I'd expect to stay approximately the same, something that
increases in magnitude over time seems more like an analog thing, but
maybe I'm jumping to conclusions :).
At any rate, it doesn't appear to be a thermal issue -- I powered it
on this afternoon and it's now immediately showing the issue, much
worse than before; to the extent that I can hear a not-very-pleasant
whining from the flyback that makes me nervous to run it for any
significant length of time as I'm afraid something (like the flyback)
might get damaged.
I may attempt a recap at some point (when I find time -- I don't know
why I keep getting into new projects when I have no time for them...)
since it's likely due anyway. There are a lot of capacitors to
enumerate and stuffed into a relatively tight space. Did I mention
that I hate working on CRTs? :)
Thanks again,
- Josh
I picked up a Teletype DMD 5620 (aka BLIT)
terminal a couple of
weeks back and I finally got the keyboard working this week; in the
meantime a problem with the display has cropped up.
See here for a video:
http://yahozna.dyndns.org/scratch/blit/WP_20141023_001.mp4
(apologies for the quality and that it's rotated 90 degrees... you
get the general idea).
For those without video-playback capability, essentially after
warming up for awhile (generally after a couple of minutes) the
screen starts shaking horizontally; like the horizontal position is
oscillating rapidly. This causes "fringes" on the horizontal edges
of the picture, as this oscillation occurs multiple times during the
course of a single rescan. Generally it gets worse and worse the
longer it's been on, though sometimes it will stabilize for a little
while.
I thought it might be a bad solder joint and I started prodding
around with a dowel when the set was "cold" but I was unable to make
the screen jump at all while doing so. I also cleaned the
horizontal position and horizontal oscillator pots with some contact
cleaner to no real effect.
I am not particularly good with CRTs, working on them is not my
favorite thing, something about the high voltages I guess :). Anyone
have any suggestions?
Thanks,
Josh
It appears the the screen is jumping in some integral number of
pixels horizontally, like 32 or 64. I don't see any vertical scan
line displacement.
I would check that the digital horizontal sync (and blanking if
available) are rock solid, and that the digital pixel stream is gives
a stable eye, and
it solidly locked to the horizontal sync signal.
Based on the short little movie I would suspect more of a digital
issue than a monitor analog issue, at least for starters.
Don