22 Nov
2010
22 Nov
'10
5:55 p.m.
Tony Duell wrote:
It's a pty you didn't measure th voltages as soon as you got it, and then
again wuen it had got even worse....
Yes, hindsight and all that :-) I really should have, because it was obviously
unwell right from the start, although at first it was perfectly usable, just
with the controls set far higher that what I would have considered "normal".
After a while though it went downhill, until brightness set at 100% was still
'too dark'.
They're just easier on my eyes, I find. LCDs give me a headache after quite a
short while, but I can look at a (well-adjusted) CRT for much longer before it
bothers me. I'm not sure why - something to do with pixel aliasing, perhaps.
Ah, you have a schematic. Excellent... Does it give any voltages?
Yes, quite a few - all those that I can measure with a DMM seem to be healthy
though, and within 1% of target (125V, 75V, 5V and 6.3V to the CRT neck board,
12V that gets used for various IC supplies, 160V from which the B+ signal is
derived etc.)
What adjustments do you have?
Aha, well I found a 1k PCB-mount pot that might do it (it's in the feedback
path between the flyback's 30V tap and the Bin input on the display's TDA4856
IC). It's completely gooped with epoxy unfortunately, but I do have a spare 1k
pot that I could set to the same value and then swap in.
Not sure if I'll get to that today though as it means separating the CRT's
main PCB from the frame (which looks to be quite the job). I'll let you know
if I try it, though...
cheers
Jules
before you go any further, have you checked all the
CRT voltages? I guess
you didn't do what I do, and measure/record them when the unit is new, so
as to have somethign to compare against. But you might be able to spot
any that are way out...
Unfortunately this was a freecycle acquisition, so not one
I'd had from
anything like new - the picture was already pretty dim when I got it, but
seemed to get worse after just a few hours of use. Nothing lost really if it dies
completely, other than it being extremely difficult to get CRT monitors
around here (and the weight makes shipping them difficult, too)
Any particualr reason you prefer CRT monuitors? OK, I prefer them, but
only (a) becasue they were the original displays for many of my classic
computers nad (b) because they're easier to fix than the LCD type. Is there any colour cast? The reason I ask is that
it's unusual for all 3
cathodes to loose emision together, so normally one primary colour will
fade first. if the colours are still fine, I would not try to boost the
heater.
No, they're all even... I'm just poring over a really bad
schematic at the Is this monitor new enoguh to have automatic grey
scale tracking?
Hmm, yes it apparently does... (via a TDA4886 IC)
I'll see if I can check some things there, but I suspect I'm limited as to
what I can do without a 'scope (which might mean I have to find a different
monitor to use until I get my 'scope here next year)
I really would check the a1/g2 voltage on the CRT.
I suspect my DMM might load it too much to get an accurate reading, but I'll
give it a try.
Have you tried a cautious tweak of the g2/screen
control (often on the
flyback transformer)?
Unfortunately there isn't one on this display. Two
focus controls on the
flyback, but no screen.