23 Sep
2008
23 Sep
'08
9:06 p.m.
On Tuesday 23 September 2008 15:50, Tony Duell wrote:
I remember those but it's been a *really* long time since I've seen one...
I was specifically talking about the sort of monitors that are likely to
be used with computers.
Ok.
Yes. I tend to rely on those grounds that are attached to the outside of the
tube for stuff like that. Unless we're talking *really* old stuff with a
metal chassis, OTOH... :-)
Mains-derived EHT was used in the UK in some _very_ old monochrome TVs.
Yes.
I remember
really old sets that had to have a small HV "filter" capacitor
because they didn't have that coating.
Some old TVs in the UK had a metal-flared CRT, a metal cone bonded to the
glass faceplate(screen) and neck. This flare fomred the final anode, of
course. Downright unpleant, that flare was at EHT votlage when the set
was on, and of coruse there are a separate filter capacitor (which if it
heldf its charge meant that the CRT flare would belt you even when the
set was off). On well-desinged units, there is some kind of bleeder
resistor to
discharge this when you turn the unit off (this may be a potential
divider network either to provide the focus electrode voltage or as the
sense circuit of a voltage regulator).
In real early color stuff there was a separate focus rectifier, but the
focus It is not unherad-of for a colour monitor, at least
not in the UK, to
have a flyback transofmrer producing 8kV or so and a separate
diode/capacitor votlage tripler module. The latter generally provides
the focus supply. Some Microvitec monitors (commonly used with the BBC
micro) and the Acorn Cambride Workstation's internal monitor (a
Microvitec chassis, of course) were like this.
I was of the impression that most of the current crop of stuff used those
multipliers.
I doubt you'll find valve rectifiers and the
'shunt stabiliser triode' in
any computer monitor, though ;-)
Not likely, no. :-)
voltage is
typically 20% of the HV, so a divider came to make a whole
lot more sense later on. In some sets they were a separate little
component (Zenith stuff I worked on in the early 1970s comes to mind and
in fact I just scrapped one of those not too long ago) but anything more
recent and it's built into the flyback transformer.
Well, provided you pick that 'ground' carefully. Clipping a probe on a
handy 0V point and using the end to short the anode connector to ground
mau well result in currents flowing where you least expect them!. > Not only does it cheack the EHT voltage, it
also discharges it. I
> would advise against shorting the contact to chassis ground (even
> though this is recomended in some service manuals), there's a very
> real risk of casuing damage to semiconductors.
Not a problem if you have that 2nd anode wire disconnected from the tube,
and you short it to ground with a wire. However! The glass and coating I still dislike doing it. I rememebr -- well -- a TV
where even the spark
of connecting an EHT meter to the anode cap (earth side of the meter to
the CRT ground contact) would blow a few transistors.
Making a loud noise in the process to let you know it wasn't pleased with what
you'd done.
but I did live to tell the tale. Be warned, though,
that some _vector_
monitors, the DEC VR14 being one such, get the EHT from a step-up
trnasformer straight from the mains, and that can supply a much higher
current. Getting connected across that is very likely to be fatal.
That approach was taken with some very early TVs as well, but the
flyback transformer approach was much cheaper to produce. A few, particulalry back-projection sets
(White-Ibbotson???) had a semarate
oscillator 9around 50kHz I think) driving a transformer/voltage
multiplier circuit -- that one is high enough impedance to be no more
dangerous than a flyback-derivied EHT supply.
But direct mains EHT -- a 50Hz (or 60Hz) transfoemr and rectifier -- is
low enough impedance that you want to keep well away from it.
You'll not get any argument from me on that.
I think every raster-scan monitor you're likely to
come across will have
flyback EHT. It's cheap, and it makes used of otherwise wasted energy
stored in the defleciton field. If you work on vector monitors, you
can't do this because there's no regualr scan. You might have a mains EHT
supply, or a separate osicallator/transformer circuit (the Vectrex gmaes
unit does the latter, the transofrmer is essentially the same as a
monochrome TV flyback transoformer, it just has nothing to do with the
deflection circuit).
I disagree.
On delta-gun CRTs (unlikely to be found in colour computer monitors, but
there are a few 1970's ones), the purity is set by ring magnets on the
back of the youke (they look like the cnetring magnets on a monochrome
CRT).
Right. That was a part of the process. The other part of it was the
forward-rear adjustment of the yoke. BTDT... :-)
In many
colour monitors, the dynamic (edge/corner) convergnece is set
by tilting the yoke, settign that up takes a long time.
No, that's static convergence and color purity. Typical early TVs had
an The xtatic convergence is normally set by 3 permanent
magnets on
the 'convergence yoke' -- a Y-shaped thing bechind the deflection yoke on
the CRT nexk with a separate 'blue lateral' unit behind that.
Yup. Sometimes the convergence yoke was a separate assembly, sometimes it
was a combined assembly with the deflection yoke.
Dynamic convergence is set by carefully controlled
waveforms on the coils on
the latter 2 yokes, there will be a dozen or more presets to adjust to set
them up (and you have to do it if you move the monitor, turn it round, or
anything that would affect the external magneic field as seen by the CRT).
Supposedly, yeah.
On in-line gun CRTs -- almost all computer monitors --
there are
typcially 6 or more ring magnets on the back of the defleciton yoke.
These set the purity and the static convergence.
Excepting one Trinitron TV that I brought with me when leaving NYC (in early
1978, and it's still working fine although a little dim), most of what I
worked on was earlier stuff, delta-gun arrangement. The inline-gun stuff
didn't start to take off really until after I got out of the business. I do
know that they sure took a whole lot less fiddling with than the earlier
stuff did.
Dynamic convergence is set on older units (well by,
again, careuflly
controlled currents through coils in the yoke -- I've never seen this in a
computer monitor though. In later models (anything after the late 70's,
basically), there's no convergence _circuitry_ at all, the field produced by
the defleciton yoke does the job on its own.
Which is as it should be. :-)
--
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ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
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