a new BBS...

[Tony Duell]

> 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 

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

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 doubt you'll find valve rectifiers and the 'shunt stabiliser triode' in 
any computer monitor, though ;-)

> 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!.

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. 

> > 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. 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.

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). 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. 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).

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. 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. You get the dynamic convergence right by 
tilting/wedging the yoke. I've read plenty of service manuals that 
confirm this...

-tony