18 Nov
2010
18 Nov
'10
8:35 p.m.
Hi all,
I suspect some folk here have done this before.... :-)
My desktop CRT (over 10 years, but not what I'd consider vintage!) has been
ailing for some time, with the picture getting darker and darker despite
having the brightness at 100%. It's still good for high-contrast stuff such as
black text on white background, but forget trying to pull details out of most
photos, for instance.
Heater voltage seems good at 6.4VDC / 350mA (it's derived from the PSU in this
monitor rather than the flyback section), but I'm considering boosting it a
little and see if it improves things, obviously shortening the life of the
tube in the process.
Question is, what's a sensible amount to over-run things by? Say I aimed for
around 10%, is that too much and going to kill the heaters in next to no time,
or so little that unlikely to really make any useful difference?
cheers
Jules
18 Nov
18 Nov
8:40 p.m.
From: "Jules Richardson" <jules.richardson99 at gmail.com>
Sent: Thursday, November 18, 2010 9:35 PM
To: <cctalk at classiccmp.org>
Subject: boosting CRT heater voltage
Hi all,
I suspect some folk here have done this before.... :-)
My desktop CRT (over 10 years, but not what I'd consider vintage!) has
been ailing for some time, with the picture getting darker and darker
despite having the brightness at 100%. It's still good for high-contrast
stuff such as black text on white background, but forget trying to pull
details out of most photos, for instance.
Heater voltage seems good at 6.4VDC / 350mA (it's derived from the PSU in
this monitor rather than the flyback section), but I'm considering
boosting it a little and see if it improves things, obviously shortening
the life of the tube in the process.
Question is, what's a sensible amount to over-run things by? Say I aimed
for around 10%, is that too much and going to kill the heaters in next to
no time, or so little that unlikely to really make any useful difference?
cheers
Jules
Hmmm, I guess the specified heater voltage would be 6.3V (common value).
I do not know how much you can increase that and how much it will shorten
the life of the CRT. May be some "regeneration" of the tube will help, but I
have no idea how to do that. There are/were gizmos that did just that.
When the brightness went down because of loss of emission of the cathode,
I do not know how much increasing the heater voltage will help anyway.
- Henk.
8:44 p.m.
Back in the 70's I worked as a Radio/TV repairman and it was a known
thing to add a boost transformer into old TV's to get more life out of
the CRT. iirc there were 10,15, 20% taps, cost was not much better
than crt replacement iirc.
But for an out of production crt was the only way. I did my parents TV
and it carried on like that for many years occasionally I put another
turn on as the emission dropped with time, and as it happened it was
left on 100% of the time while the rest of the TV was off.
Regeneration was to over run even more for a short period and also
play with the voltages to knock a layer of oxide off. You need one of
the regenerator tools for that though.
Dave Caroline
On Thu, Nov 18, 2010 at 8:35 PM, Jules Richardson
<jules.richardson99 at gmail.com> wrote:
Hi all,
I suspect some folk here have done this before.... :-)
My desktop CRT (over 10 years, but not what I'd consider vintage!) has been
ailing for some time, with the picture getting darker and darker despite
having the brightness at 100%. It's still good for high-contrast stuff such
as black text on white background, but forget trying to pull details out of
most photos, for instance.
Heater voltage seems good at 6.4VDC / 350mA (it's derived from the PSU in
this monitor rather than the flyback section), but I'm considering boosting
it a little and see if it improves things, obviously shortening the life of
the tube in the process.
Question is, what's a sensible amount to over-run things by? Say I aimed for
around 10%, is that too much and going to kill the heaters in next to no
time, or so little that unlikely to really make any useful difference?
cheers
Jules
8:57 p.m.
Dave Caroline wrote:
Regeneration was to over run even more for a short
period and also
play with the voltages to knock a layer of oxide off. You need one of
the regenerator tools for that though.
There are some hints and DIY suggestions in the RepairFAQ at
http://www.repairfaq.org/sam/crtfaq.htm#crtdeg (degradation)
http://www.repairfaq.org/sam/crtfaq.htm#crtcrj (rejuvenation)
--
Pete Peter Turnbull
Network Manager
University of York
9:22 p.m.
Back in the 70's I worked as a Radio/TV repairman and it was a known
thing to add a boost transformer into old TV's to get more life out of
the CRT. iirc there were 10,15, 20% taps, cost was not much better
than crt replacement iirc.
Another trick is to wind a couple of turns of insulated wire around the
flyback transfoirmer core and connect it in series with the CRT heater.
If you want to measure the resulting heater voltage, you need a goodRMS
meter that will correctly measure the very 'peaky' waveform that results.
Regeneration was to over run even more for a short
period and also
play with the voltages to knock a layer of oxide off. You need one of
the regenerator tools for that though.
There have been many designs for these published over the years. A lot
depend on transformers that were reasonably common when the device was
designed, but hard to find now...
Basically, you over-run the heater (try 8V or 10V for a 6.3V heater) and
then apply a high-sih DC voltage between the cathode (-ve) and first grid
(+ve). The idea is to get a high emission that will strip th etop layer
off the cathode, exposing new emissive oxide.
You need a few hundred volts. A 'butchers trick' was to use half-wave
rectified mains witha 15W pygmy bubl in series (!). I won't say any more,
if you can't work it out, you don't know how to do it safely, if you can,
then you probably will realise why it's not that good an idea :-)
The problem comes if you strip off too much oxide and leave a totally
non-emissive cathode. New CRT time :-(. Also modern CRTs have a much
thinner emissive coating than the ones of 40 or so yeara ago, and tend to
tkae much less rjuvenation.
-tony
9:14 p.m.
Hi all,
I suspect some folk here have done this before.... :-)
Indeed. But be warned modern CRTs (and 10 years old is 'modern') don't
necessariy respond to this.
My desktop CRT (over 10 years, but not what I'd consider vintage!) has been
ailing for some time, with the picture getting darker and darker despite
having the brightness at 100%. It's still good for high-contrast stuff such as
black text on white background, but forget trying to pull details out of most
photos, for instance.
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...
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.
Have you tried a cautious tweak of the g2/screen control (often on the
flyback transformer)?
Heater voltage seems good at 6.4VDC / 350mA (it's derived from the PSU in this
monitor rather than the flyback section), but I'm considering boosting it a
little and see if it improves things, obviously shortening the life of the
tube in the process.
Question is, what's a sensible amount to over-run things by? Say I aimed for
around 10%, is that too much and going to kill the heaters in next to no time,
or so little that unlikely to really make any useful difference?
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_.
-tony
9:57 p.m.
Tony Duell wrote:
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)
My desktop CRT
(over 10 years, but not what I'd consider vintage!) has been
ailing for some time, with the picture getting darker and darker despite
having the brightness at 100%. It's still good for high-contrast stuff such as
black text on white background, but forget trying to pull details out of most
photos, for instance.
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... 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)
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.
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...
cheers!
Jules
10:14 p.m.
Another fault causing all colours to be dim is EHT current limit fault
(detection of cathode current).
also look for "blooming" picture size/focus changing with brightness a
classic sign of low emission in the old days
Dave Caroline
19 Nov
19 Nov
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...
22 Nov
22 Nov
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. 
7:07 p.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
--- On Mon, 11/22/10, Jules Richardson <jules.richardson99 at gmail.com> wrote:
You know, I have the same problem. I don't know what it is about LCD monitors, but
they do tend to induce headaches. CRT's never seem to bother me. I can stay up many
hours into the night staring into a VT100 in a dark room with no problems - but an hour of
using a laptop and I have a headache.
Glad that it isn't just me.
-Ian
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.
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. 
7:51 p.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
On 11/22/10 2:07 PM, Mr Ian Primus wrote:
You know, I have the same problem. I don't know what it is about LCD
monitors, but they do tend to induce headaches. CRT's never seem to
bother me. I can stay up many hours into the night staring into a
VT100 in a dark room with no problems - but an hour of using a laptop
and I have a headache.
Glad that it isn't just me.
Hmm, I wonder what the difference is. One possibility is the much
more well-defined pixels of an LCD; that could be tested by placing some
sort of a diffuser (but not TOO much of a diffuser of course) in front
of an LCD to see if that makes any difference.
-Dave
--
Dave McGuire
Port Charlotte, FL
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.
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.
8:22 p.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
Dave McGuire wrote:
Hmm, I wonder what the difference is. One
possibility is the much
more well-defined pixels of an LCD; that could be tested by placing some
sort of a diffuser (but not TOO much of a diffuser of course) in front
of an LCD to see if that makes any difference.
Hmm, good thinking. That's always been my main suspicion, because even good
CRTs aren't quite as crisp as an LCD, but maybe there's something else at work
too. Most LCDs do seem quite low-contrast compared to a well-adjusted CRT, so
that's another possibility - or maybe something to do with some form of scan
frequency (like some people have issues with lighting at 50 or 60Hz, not that
that's ever bothered me)
I'm moderately short-sighted, incidentally, but have never needed glasses for
anything close-up, and I don't wear mine when I'm sitting at a computer.
It's a hard thing to measure in terms of how many people it affects, too,
because so many these days have never used anything other than a CRT, and of
those that have a good proportion have only seen poor quality examples.
cheers
Jules
9:22 p.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
You know, I have the same problem. I don't know
what it is about LCD
monitors, but they do tend to induce headaches. CRT's never seem to
bother me. I can stay up many hours into the night staring into a VT100
Badly adjusted CRTs, partcilaurly poor convergence, drive me insane.
in a dark room with no problems - but an hour of using
a laptop and I
have a headache.
However, LCDs don't seem to botyehr me, although I don't own an LCD
monitor so I've not used one for long periods.
I wonder if the problem isn't the LCD itself, but some characteristic of
the backlight,
-tony
9:39 p.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
----- Original Message -----
From: "Tony Duell" <ard at p850ug1.demon.co.uk>
To: <cctalk at classiccmp.org>
Sent: Monday, November 22, 2010 3:22 PM
Subject: Re: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
I wonder if the problem isn't the LCD itself, but
some characteristic of
the backlight,
Some people are sensitive to flicker, and most LCD monitors, from what I
understand, are backlit by fluorescent light, which does have a flicker. In
particular, people who are prone to migraine headaches often find that
flickering can induce or worsen headaches. (Fluorescent lights don't bother
me, but rapidly flashing lights as seen in occasional movies can drive me
nuts, and even looking at paint test patterns on a highway as I drive by can
be very bothersome. I don't get headaches from them, but it is very
uncomfortable to look at these things.)
Of course, the refresh rate of CRT monitors can be an issue too, but if it's
high enough, it might no longer be detectable to a person.
Jim
10:03 p.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
On 22 Nov 2010 at 15:39, Jim MacKenzie wrote:
Some people are sensitive to flicker, and most LCD
monitors, from what
I understand, are backlit by fluorescent light, which does have a
flicker. In particular, people who are prone to migraine headaches
often find that flickering can induce or worsen headaches.
(Fluorescent lights don't bother me, but rapidly flashing lights as
seen in occasional movies can drive me nuts, and even looking at paint
test patterns on a highway as I drive by can be very bothersome. I
don't get headaches from them, but it is very uncomfortable to look at
these things.)
Of course, the refresh rate of CRT monitors can be an issue too, but
if it's high enough, it might no longer be detectable to a person.
Ever pulled an LCD monitor apart to see what was inside? The
backlight is driven from a high-frequency inverter (tens of KHz),
while a CRC vertical retrace is 80-90 Hz at best.
I'm very sensive to flicker--if I drive by a house with a
conventional CRT TV and one with an LCD TV, I can tell the difference
even with the curtains drawn because of the amount of flicker
displayed by the CRT. Similarly, I can immediately tell, from blocks
away, who's using LED Christmas tree lights by the flicker.
I used to be able to tell LED traffic signals by the amount of
flicker visible, but that's changed in the past few years, so that
said signals appear to be nearly flicker-free.
While the color rendition appears to be better on a CRT than on an
LCD screen, the CRT displays flicker and a certain amount of
fuzziness.
--Chuck
23 Nov
23 Nov
6:28 a.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
On Mon, Nov 22, 2010 at 02:03:22PM -0800, Chuck Guzis wrote:
While the color rendition appears to be better on a
CRT than on an
LCD screen, the CRT displays flicker and a certain amount of
fuzziness.
Have you guys tried an IPS panel? It's definitely a step forward from TN
panels. I have one at home and I'm not going back. Now I just have to
convince my employer to get me one :)
/P
8:27 a.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
While the
color rendition appears to be better on a CRT than on an
LCD screen, the CRT displays flicker and a certain amount of
fuzziness.
Have you guys tried an IPS panel? It's definitely a step forward from TN
panels. I have one at home and I'm not going back. Now I just have to
convince my employer to get me one :)
7:02 p.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
On 23 Nov 2010 at 0:27, Cameron Kaiser wrote:
I went from a CRT to a Samsung TN panel to this NEC
IPS panel. The CRT
had great colour, but the flicker was noticible. The Samsung had a
beautiful display that had great saturation but was no good for
graphics work, and suffered from the typical limited TN viewing angles
to the point where I couldn't reliably tell how dark my images were.
The NEC I have now is just about perfect except for an inability to
handle 1080i (it's 1080p or bust). But its colour gamut and clarity
are fabulous.
I can't say that I'm much of a connoisseur when it comes to buying
LCD monitors--just about every single one of mine were obtained at no
cost (being thrown out mostly for PSU capacitor failure; some are
less than a year old). The only one I paid for ($40) is the one I'm
sitting in front of--a NEC 2010 Multisync 20" unit. I was attracted
to it because video input is via 5 BNC connectors and it will sync
down to 24KHz. It also has the most pleasing image overall. MSRP in
2000 was something like $4995.
I'm still waiting for an affordable 20" AMOLED display myself... :)
--Chuck
6:35 p.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
Some people
are sensitive to flicker, and most LCD monitors, from what
I understand, are backlit by fluorescent light, which does have a
flicker. In particular, people who are prone to migraine headaches
often find that flickering can induce or worsen headaches.
(Fluorescent lights don't bother me, but rapidly flashing lights as
seen in occasional movies can drive me nuts, and even looking at paint
test patterns on a highway as I drive by can be very bothersome. I
don't get headaches from them, but it is very uncomfortable to look at
these things.)
Of course, the refresh rate of CRT monitors can be an issue too, but
if it's high enough, it might no longer be detectable to a person.
Ever pulled an LCD monitor apart to see what was inside? The backlight is driven from a high-frequency inverter
(tens of KHz),
while a CRC vertical retrace is 80-90 Hz at best.
I've seen this sort of comment before in relation ot th flickr from CFL
bulbs. And I am not sure it's necessarily correct.
Yes, the inverter iotself runs at 10s of kHz. But it gets it supply from
the normal 60Hz mains. I could well beleive that if the smoothing of the
input DC was not adequate, you could get a 60Hz component in the output.
I have certainly seen SMPSUs with mains (or twice mains) frequency ripple
on their outputs, due to input side smoothing problems. Yes, the
regulation loop should take care of it, but it may not always do so. And
I wonder how much regulation there is in the inerter of a cheap CFL, or
indeed the backplight of an LCD monitor.
Alas the only LCD TV service manual I have does not contain PSU
schematics (it's not regarded as field-repairable, the signal PCB is, for
all it contains BGA-packaged devices, go figure...), so I can't check.
-tony
7:28 p.m.
New subject: CRT's vs. LCD - (Was: Re: boosting CRT heater voltage)
On 23 Nov 2010 at 18:35, Tony Duell wrote:
Yes, the inverter iotself runs at 10s of kHz. But it
gets it supply
from the normal 60Hz mains. I could well beleive that if the smoothing
of the input DC was not adequate, you could get a 60Hz component in
the output. I have certainly seen SMPSUs with mains (or twice mains)
frequency ripple on their outputs, due to input side smoothing
problems. Yes, the regulation loop should take care of it, but it may
not always do so. And I wonder how much regulation there is in the
inerter of a cheap CFL, or indeed the backplight of an LCD monitor.
I'm not convinced here--it would seem to me that excessive line-
frequency ripple would also introduce some artifacts into the
display. I see no evidence of that in the displays that I own.
--Chuck
22 Nov
22 Nov
9:19 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
I know... FWIW, there are plenty of times I've not realaised I should
have measured something or whatever until it's too late...
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 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'.
What is a 'while'? I am suprised a CRT would loose emision as quickly as
you imply, and wonder if there's some other fault.
Is this
monitor new enoguh to have automatic grey scale tracking?
Hmm, yes it apparently does... (via a TDA4886 IC)
10:49 p.m.
Jules Richardson wrote:
Just FYI, I got 700VDC at the flyback for G2, and 400VDC where it leaves the
main PCB (and joins via a lead to the CRT's neck board).
cheers
Jules
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.
23 Nov
23 Nov
6:42 p.m.
Jules Richardson wrote:
Just FYI, I got 700VDC at the flyback for G2, and 400VDC where it leaves the
main PCB (and joins via a lead to the CRT's neck board).
That could be about right. What is between the flyback and the main PCB
output?
I've looked at the video chip data sheet, it appears to contain DACs for
gain and black level for each of the 3 channels, loaded via an i2C bus.
How does this monitor measure the beam current for the auto grey scale?
Could the problem be there (it thinks the beam current is higher than it
really is)?
-tony
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.
18 Nov
18 Nov
9:14 p.m.
--- On Thu, 11/18/10, Jules Richardson <jules.richardson99 at gmail.com> wrote:
My desktop CRT (over 10 years, but not what I'd
consider
vintage!) has been ailing for some time, with the picture
getting darker and darker despite having the brightness at
100%. It's still good for high-contrast stuff such as black
text on white background, but forget trying to pull details
out of most photos, for instance.
A couple of things can cause this, but yes, most common is a weak picture tube. The tube
in my monitor here at work is pretty worn out too - to the point where most photos lack
all detail, appearing as blackish patches. It's still fine for text though, and since
that's all I really ever work with, it doesn't bother me.
Heater voltage seems good at 6.4VDC / 350mA (it's
derived
from the PSU in this monitor rather than the flyback
section), but I'm considering boosting it a little and see
if it improves things, obviously shortening the life of the
tube in the process.
Yes. That is a common method if improving the picture. Boosting the voltage causes the
filament to heat up the cathode more, thus emitting more electrons. A standalone picture
tube rejuvinator can also usually "zap" the cathode to help improve emissions as
well, by stripping off the coating. If you have access to a rejuvinator, try that first.
Another common trick that helps in some cases is to turn up the SCREEN control on the
flyback. This increases the voltage to the screen grid on the tube, and will produce a
brighter picture. Sometimes this is enough, but if the tube is real weak, it'll just
wash out the picture.
Question is, what's a sensible amount to over-run
things
by? Say I aimed for around 10%, is that too much and going
to kill the heaters in next to no time, or so little that
unlikely to really make any useful difference?
It'll help a lot, usually. Start with maybe 10% increase. You can create a higher
heater voltage by making your own turns around the flyback core - a couple of turns of
insulated wire should be enough. Filament voltage does not have to be DC. Another thing to
check before boosting the filament would be the monitor's main B+ voltage. If it's
sagged too much due to worn out electrolytics, you'll have a dim, shrunken picture.
Similarly, faulty electrolytics in the signal path will cause smearing and dim colors. So,
start by checking voltages - the main B+ and the supply voltage to the video amplifiers.
One thing I have seen is a monitor where the video signal from the computer was dim and
faded, even with the brightness cranked up - but the OSD menus were really bright and
clear! The problem was related to the video amplifier for the signal input.
If that all looks good, then crank up the heater voltage a bit. Worse case, you burn out
the tube, but hey, it was already unusable when you started, you haven't much to lose.
You should be able to get several more years out of it though.
-Ian
11:42 p.m.
Mr Ian Primus wrote:
Yeah, as I mentioned to Tony, there's no screen adjustment on this one, which
is a shame.
Hmm, I think I'd rather not mess with that though, purely because I don't have
access to my 'scope and I don't think my DMM will give an accurate reading for
such a signal (and it'd be wise to check the voltage before feeding it to the
heaters :-) I think my best bet is to generate a DC voltage somehow, and
probably with an external supply just while I'm testing. Maybe I should go
with some form of adjustable regulator IC...
--- On Thu, 11/18/10, Jules Richardson
<jules.richardson99 at gmail.com>
wrote:
The funny thing is, I've dealt with lots of CRTs and seen then die in various
ways, but this is the first one I've ever had with the poor brightness issue -
yet googling seems to suggest that it's pretty common. I must have been rather
lucky... :-)
My desktop CRT (over 10 years, but not what
I'd consider vintage!) has
been ailing for some time, with the picture getting darker and darker
despite having the brightness at 100%. It's still good for high-contrast
stuff such as black text on white background, but forget trying to pull
details out of most photos, for instance.
A couple of things can cause this, but yes, most common is a weak picture
tube. Heater voltage
seems good at 6.4VDC / 350mA (it's derived from the PSU in
this monitor rather than the flyback section), but I'm considering
boosting it a little and see if it improves things, obviously shortening
the life of the tube in the process.
Yes. That is a common method if improving the picture. Boosting the voltage
causes the filament to heat up the cathode more, thus emitting more
electrons. A standalone picture tube rejuvinator can also usually "zap" the
cathode to help improve emissions as well, by stripping off the coating. If
you have access to a rejuvinator, try that first. Another common trick that
helps in some cases is to turn up the SCREEN control on the flyback. This
increases the voltage to the screen grid on the tube, and will produce a
brighter picture. Sometimes this is enough, but if the tube is real weak,
it'll just wash out the picture. Question is,
what's a sensible amount to over-run things by? Say I aimed
for around 10%, is that too much and going to kill the heaters in next to
no time, or so little that unlikely to really make any useful difference?
It'll help a lot, usually. Start with maybe 10% increase. You can create a
higher heater voltage by making your own turns around the flyback core - a
couple of turns of insulated wire should be enough. Another thing to check before boosting the filament
would be the monitor's main B+ voltage. If it's sagged too much due to worn
out electrolytics, you'll have a dim, shrunken picture. Similarly, faulty
electrolytics in the signal path will cause smearing and dim colors. So,
start by checking voltages - the main B+ and the supply voltage to the
video amplifiers.
Well, all the main voltages seem good - all within a 1% of design. I wish I
had my ESR meter with me, but that's stuck overseas too :-(
One thing I have seen is a monitor where the video
signal
from the computer was dim and faded, even with the brightness cranked up -
but the OSD menus were really bright and clear! The problem was related to
the video amplifier for the signal input.
Interesting! The OSD for this one is on the neck board, and it seems dark and
low-contrast to me - but of course I've got nothing to compare it against.
(One other observation is a simple eyeball test: the heaters *look* rather
dark, with a dull orange glow. Again though it's hard to be sure without a
direct comparison against something with an identical tube)
cheers
Jules
19 Nov
19 Nov
9:27 p.m.
A couple of
things can cause this, but yes, most common is a weak picture
tube.
The funny thing is, I've dealt with lots of CRTs and seen then die in various
ways, but this is the first one I've ever had with the poor brightness issue -
yet googling seems to suggest that it's pretty common. I must have been rather
lucky... :-) (One other observation is a simple eyeball test: the
heaters *look* rather
dark, with a dull orange glow. Again though it's hard to be sure without a
direct comparison against something with an identical tube)
Wher dfid you measure the heater voltage? If not at the CRT pins, could
it be a bad connection somehwere between the CRT base and the PSU? Heck
could uit even be a bad connection between the CRT pins and the base
connector?
-tony
9:14 p.m.
--- On Thu, 11/18/10, Jules Richardson <jules.richardson99 at gmail.com> wrote:
Is it? I am not convinced. I think in all my classic computer repairs
I've had one low-emission CRT, and that was in a Mac+. The picture coul;d
be made bright enough, but there was a curious shaddowing to the right of
objects on the screen. There as nothing wrong with the video amplifier
circuit, and a new CRT did cure it.
But many times I've had CRT electrode voltages drift, upsetting
brightness or focus.
My desktop CRT (over 10 years, but not what
I'd consider
vintage!) has been ailing for some time, with the picture
getting darker and darker despite having the brightness at
100%. It's still good for high-contrast stuff such as black
text on white background, but forget trying to pull details
out of most photos, for instance.
A couple of things can cause this, but yes, most common is a weak
picture tube. The tube in my monitor here at work is pretty worn out too It'll help a lot, usually. Start with maybe 10%
increase. You can
create a higher heater voltage by making your own turns around the
flyback core - a couple of turns of insulated wire should be enough.
Filament voltage does not have to be DC. Another thing to check before
boosting the filament would be the monitor's main B+ voltage. If it's
sagged too much due to worn out electrolytics, you'll have a dim,
shrunken picture. Similarly, faulty electrolytics in the signal path
will cause smearing and dim colors. So, start by checking voltages - the
main B+ and the supply voltage to the video amplifiers. One thing I have
seen is a monitor where the video signal from the computer was dim and
faded, even with the brightness cranked up - but the OSD menus were
really bright and clear! The problem was related to the video amplifier
for the signal input.
The other thing to distinguish betwene is low brightness and low
contrast. They are not the same. Contrast problems are almost always due
to video amplifier problems.
Oh, and clean the screen :-). Seriously, I once spent a day tracking
down a nonexistant video amplifier fault in a VT100 with a dim picture
which turned out to b caused by a filthy CRT screen...
If that all looks good, then crank up the heater voltage a bit. Worse
case, you burn out the tube, but hey, it was already unusable when you
started, you haven't much to lose. You should be able to get several
Hmm.. Sure, if the CRT is defective, there's no harm in trying to boost
it a bit. But if the fault is elsewhere, then boosting the heater voltage
will not help, and may damage the CRT. Then you have both the original
fault to find and cure, and a CRT to replace.
-tony
10:39 p.m.
--- On Fri, 11/19/10, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
Is it? I am not convinced. I think in all my classic
computer repairs
I've had one low-emission CRT, and that was in a Mac+. The
picture coul;d
be made bright enough, but there was a curious shaddowing
to the right of
objects on the screen. There as nothing wrong with the
video amplifier
circuit, and a new CRT did cure it.
I've run across several - with low emissions confirmed by my picture tube tester.
Primarily in terminals and coin operated video game monitors. In game monitors especially,
since they were typically left on nearly 24/7, I fairly often see weak tubes that are
pretty consistently weak across all three guns. Also, in television sets, Zenith had a
particularly bad run of tubes in the early 90's - some of which were also in WG arcade
monitors of the era. These will go low emissions across all three guns. Zapping them with
the rejuvinator brings them back... for a little while.
One thing I find in computer monitors is a tube with good emissions but with a burned
face. This seems to be pretty common on Sun monitors used on servers. There will be a
burned, dark rectangle covering the entire screen except for the very edges. No doubt a
result of the white background/black text firmware console display.
Oh, and clean the screen :-). Seriously, I once spent
a day
tracking
down a nonexistant video amplifier fault in a VT100 with a
dim picture
which turned out to b caused by a filthy CRT screen...
Hehe. Reminds me of a television I was given many years ago, it was going to be thrown
away because "the color was no good". Took it home and after scrubbing the
picture tube with glass cleaner, the picture was fine. Shows you what a nasty habit
smoking is...
-Ian
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