15 May
2008
15 May
'08
4:48 p.m.
Tony Duell wrote:
Well, the video I linked to is about the best description I could think
of...
When the screen is showing static information, and the disk(s) are not
running, then the display kind of looks like it's got a h-synch problem. Or,
try this: It looks like there's a strong right-to-left motion blur. That's
still not 100% accurate, but I'm no artist & trying to describe it is beyond
my ken.
When the machine is booting from disk, the screen flashes and flickers like
a mad thing: But as I say, it seems to be *digital* noise rather than
analogue. But with some analogue effects thrown in for good measure...
Maybe I should monitor the PSU voltage on the 'scope while the machine's
booting - I hadn't thought of that one. Could be interesting - the bulk of
the computer is currently upside down on the bench to make it easy to access
the bits of the PSU I need to probe... if the 5v line is flapping about,
this could cause the digital effects. Mind you, the fact the machine boots
properly indicates that the logic side of the machine is working fine; and
I'd have thought the disk drive would be more sensitive to 12v fluctuations
than the monitor?
I've not counted the pins, but one thing - where's the brightness control
go? The machine has external (to the monitor) controls for both contrast &
bright; as well as trimpots for both on the monitor PCB itself.
Shouldn't be a problem; the screen is lit up, so I guess it's discharging.
Hmm; I
don't think it is wholly an analogue problem: I would expect
the screen to break into horizontal slopes, as you say -
and I've seen
that on badly adjusted monitors in the past (I
used to have an old
Philips monitor which could be provoked into this with the
front panel
adjusters).
What's happening is a corruption of the display, rather than an
adjustment
A more accurate fault description would be helpful... Does this machine use a 6845 or similar for hre video
timing
chain?
Dunno, will look tomorrow (err, later today).
If so, mis-programming that will cause apparent sync
problems. So the fault could bever well be proscessor, ROM,
or bus related.
Bearing in mind the machine boots, and works behind its corrupted video
output, would that necessarily eliminate any of those parts from enquiries?
That's why I'd start by looking at the
horizotnal sync pulses
when you have the lines and when you have a good display
(even if the machine has crashed). If they're the same
timing, then the fault is monotor (or PSU?) releated. If not,
then the video timing circuitry is palying up _or being
mis-configured by software_
Gotcha - will look at that tomorrow.
I'll look
for it... the monitor circuit board is quite well
labeled;
but I'm not sure if the connectors themselves
have individually
labelled connections.
Sony were sometimes kind enough to label connector pins,
supply lines, CRT pinouts, etc in the slikscreen, few other
manufacturers were.
If it is a standard pinout, then the connections are
1 Ground
2 One end of the contrast control
3 t'other end of the contrast control
4 Slider of the contrast control
5 ground
6 Hsync
7 +12V power input
8 video input
9 Vsync
10 ground Question: How
long should I leave the tube to "cool down"
(unplugged)
before I can rummage around it without fear of
major electrocution?
Depends on what you're working on, and the design of the
monitor. The PCB-mounted capacitors will normally discharge
fairly fast (minute at most), so there'll be no masty
votlages on PCB tracks or CRT _pins_ after that. The EHT,
stored in the 'capacitor' of the CRT glass can stay for days,
but (a) it'll be discharrged by the beam current in any
normal monitor (so this is really only a concern if the
screen is totally blank), () the only way you'll get in
contact with it is if you remove the anonde connector and (c)
good monitors have a bleeder resistor, often inside the
flyback trasnformer.
No plans to remove any connectors inside the monitor....
If I am going to be removing the EHT connector, I check the
residual voltage with an EHT voltmeter (which also happens to
discharge it :-)).
Otherwise, I don't worry. So far I've never had a shock doing that.
OK, that's good to know. Many thanks again Tony - I'll try to report back
the status of the h-sync tomorrow.
Cheers,
Ade.
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16:44