19 Nov
2010
19 Nov
'10
9:23 p.m.
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
moment - I'll go and check some of the other
voltages and see if they're
right. (I'm a bit stuck because my 'scope is in storage overseas, and
obviously there are various things that I can't check with the DMM)
I assume you don't have an EHT probe, so you can't check the final anode
voltage (around 25kV) or the focus voltage (around 5kV)
Even with full brightness, anything on R, G or B below a level of about 25%
comes out completely black, and around 40% on greyscale values.
What adjustments do you have?
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. Normally around 10% will do someting. I've
taken 6.3V heaters up to 10V
(!) for rejuvenating the CRT (not the same thing) but only for a few
minutes. Certainly running them at 7V or so should show some improvement
_if the emission is the real problem_.
OK, well I'll do some more tests; if I don't have some useful odds and
sods in
the junk box for making a supply then it'll be a week before any bits
get here
anyway...