a new BBS...

[Roy J. Tellason]

On Tuesday 23 September 2008 15:50, Tony Duell wrote:
> > 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).

I remember those but it's been a *really* long time since I've seen one...

> > > 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
>
> I was specifically talking about the sort of monitors that are likely to
> be used with computers.

Ok.

> 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.
> >
> > > > 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
>
> 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!.

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...    :-)

> 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.
>
> Mains-derived EHT was used in the UK in some _very_ old monochrome TVs.

Yes.

> 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).
>
> > > 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
>
> 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...   :-)

> 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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