a new BBS...

[Tony Duell]

> I didn't want to have to tear down the whole machine just to make it easy 
> to move a single pin. :)

Sometimes it can be quicker to take things right apart.

> 
> >
> >> especially while simultaneously trying to avoid touching the yoke on the
> >> CRT neck. :)
> >
> > I assume you were doing this with the machine turned off. In whcih case
> > what's the problem with the yoke? It's not a colour CRT so there's no
> > real alignment problem if you do slightly move it. There's no stored high
> > voltage on the yoke.
> >
> I'm paranoid enough that I didn't want to come in contact with any part of 
> the CRT at all.  I wasn't worried about moving the yoke, I was worried 
> about it biting me.  It's nice to know that with the power off there 
> wasn't any danger though.

It would be wrong to say that there is no danger associated with CRTs, 
but IMHO this danger is often massively overrated. 

I'll consider the small-ish CRTs used in raster-scan monitors and 
terminals. These are magneticially deflected by the yoke around the CRT neck.

Now, obviously a high-vacuum device can implode. But most such CRTs have 
a tension band (rimband) to prevent violent implosion. I say 'most' 
becuase I know for a fact aht HP used some small (5" or so) CRTs with no 
rimband. There was a plastic implosion screen between the CRT and the 
user, but that's not a lot of help for a repairer. Certianly the HP9826's 
CRT is not rimbanded, I don't think the HP85 is either.

It is still worth takign care when working near a CRT not to fracture it, 
if only because a CRT is difficult to find/expensive.

Now for the voltages. The electron gun electrodes are brought out to 
base pins. On a monochrome CRT, expect a maximum of 500-1000V on any pin. 
On a colour CRT there is normally a 5kV focus supply, but that is well 
insulected. These voltages discharge rapidly when the unit is turned off, 
there is no real risk from them when working on a powered-down unit.

The final anode is conencted to the rubber cap connecotr on the side of 
the flare. This will be about 12kV on a monochrome CRT, about 25kV on a 
colour one. This is connected to the anode, which is an aquadag coating 
on the inside of the flare. and many years ago somebody realiaseed that 
adding a similar coation on the outside would form a capacitor which 
could then be used to smooth this EHT supply. And just about all CRTs are 
made that way, hence the earthing contacts connected to the external coating.

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). But many units have no bleeder 
here. The stored charage is not dangerous in itself, but it may cause to 
you jerk, drop the CRT, break it, whatever. I have an EHT voltmeter 
(Heathkit) which is simply a 50uA meter and an 800M resistor in series. 
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.

As for dangers from the EHT when the unit is turned on, it is a high 
voltage, but the supply can't source much current. It's darn unpleasant 
to get zapped by it (I had the 30kV EHT in a colour monitor flash over 
to my fingers, and I used language I won't repeat here!), 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.

As for the yoke iteslf, the vertical side is not a problem, even when the 
unit is turned on. It's a slow ramp, a slow flyback, and typical voltages 
are <30V. The horizotnal coil does produce a high votlage spike at 
flyback -- about 1000V. It's painful if it arcs over to you, but it's 
only present when the unit is turned on. Turned off, there's no danger 
here. But try to avoid knocking the yoke. In a monochrome monitor, 
setting it up is quite easy (rotate the yoke to get the picture level!). 
In many colour monitors, the dynamic (edge/corner) convergnece is set by 
tilting the yoke, settign that up takes a long time.

As I said, I think the dangers are overrateed. I feel that mains (at 
least in Europe) is a lot more dangerous than the voltages on such CRTs.

-tony