a new BBS...

[Roy J. Tellason]

On Monday 22 September 2008 16:03, Tony Duell wrote:
> 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.

Basically it comes down to "don't whack the neck of the tube".  I had one 
situation where a guy was bringing me a TV to look at and he'd taken the back 
off of it to get it in the trunk of his car,  for some reason.  We got it up 
the stairs just fine,  and through the little foyer just fine,  but when it 
came to getting it into the room I was using for an office at the time,  we 
had bad luck.  He wasn't looking at the back of the set and cut the tube 
right off,  against the door frame.  I told him I'd test the rest of it out 
and let him know if it was worth fixing,  which as it turned out it wasn't,  
two-thirds of the small tubes in the set being bad...

> 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.

I remember really old sets that had to have a small HV "filter" capacitor 
because they didn't have that 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).

In real early color stuff there was a separate focus rectifier,  but the focus 
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.

> 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.

One experience I had with that stuff was in a customer's apartment,  where I 
was testing tubes,  and I had at that point taken to ignoring all those 
little paper stickers they plastered all over the inside of the set figuring 
that I'd already seen them all.  Only I hadn't seen the one in that 
particular model of set that said that you should tilt the HV box out before 
trying to reach behind it,  and I reached behind it without doing so.  Which 
put me in contact with the base of the HV rectifier tube,  and the wire 
coming out of it to the 2nd anode.  My wrist impacted the metal edge of the 
cabinet twice on the way out,  creating nice deep gouges,  and I bounced 
around the room for a bit until things calmed down...

> I have an EHT voltmeter (Heathkit) which is simply a 50uA meter and an 800M
> resistor in series. 

I have one too,  brand name is Polaris.  Only I don't think it's a 50uA meter,  
as there are also a couple of terminals on the sides of the meter portion of 
it to allow for it to be used to measure cathode current,  a common 
requirement on those old tube sets,  keeping that current down.

> > 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 
capacitor of the tube makes a wonderful demonstration of what they 
call "dielectric absorption",  which I found out the hard way with my "test 
jig" -- basically a dud tube (weak green) that I picked up for under $10 back 
when,  and which had a yoke on it and a bunch of adapters I rigged up as 
needed.  I'd short that to ground to discharge it as needed,  and then a 
while later it'd give me a little bite,  to let me know that it still had 
some charge in there...

> 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!), 

I know exactly the sentiment you're expressing 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.

That approach was taken with some very early TVs as well,  but the flyback 
transformer approach was much cheaper to produce.

The key point though,  is the path that current takes.  If it just hits your 
arm that's one thing,  if it goes through your heart then you are likely to 
have serious problems.

> 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!).

One old mono monitor that I just scrapped out (bad screen burn among other 
things or I wouldn't have) had eight little magnets attached to the outer 
edges of the yoke.  Glued on,  but if you knock one of those loose the 
display could end up being a little distorted...

> 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 
adjustment to move the yoke toward the screen and away (for purity) and an 
assembly with three magnets for static convergence,  plus a "blue lateral 
magnet".  That set things up for the center of the screen,  and then dynamic 
convergence adjustments on a small board (that could typically be 
repositioned to be accessible from the front of the set) dealt with the 
edges.  This was 12 - 16 different adjustments,  all of which interacted with 
each other to some extent.

> 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.

Yup,  getting _those_ shocks was no fun at all when I was first learning this 
stuff,  as a kid.  :-)

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
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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Information is more dangerous than cannon to a society ruled by lies. --James 
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