"Wiebe Cazemier" wrote in message
b.home.nl...
On Saturday 03 May 2008 01:33, Wiebe Cazemier wrote:
Just to confirm something: the CRT repair FAQ says this about K-G1
shorts:
Cathode to control grid (K-G1). Since the G1 electrodes for all the guns
are
connected together, this will affect not only the color of the guilty
cathode
but the others as well. The result may be a very bright overloaded
*negative*
picture with little, none, or messed up colors.
But [1] says that cathode to G1 can produce similar symptoms as
heater-cathode shorts. I ask, because I noticed that G1 is connected to
ground on the CRT socket board. I can imagine that one cathode can very
well
short to G1 without the others being affected. Is that a right assement?
Can I disconnect G1 to test if it's a cathode-G1 short? There are two
pins on
the CRT to which G1 connects (which I don't understand BTW).
And, what is the use of G1 being at ground potential...?
BTW, [1] also states a 25% success rate of blowing out cathode-heater
shorts
with the flyback...
[1] http://www.thegleam.com/ke5fx/crt/sencrt.pdf.
I just did a test. In off state, I can't measure any short. But when I
heat up
the heater with an external power supply, I can measure 1K resistance
between
G1 and any of the cathodes (R, G and B). As the heater heats up, the
resistance decreases. The strange thing is, even tough they all read 1K,
the
continuity meter only read complete short on red (that is, "0". The others
read "0.2"). I still couldn't measure anything between heater and
cathodes.
So, as far as I can see, it's a cathode-G1 short after all (that is, that
1K
resistance should be there, right?). The only thing that stands in the way
of
this conclusion, is the comment from the repair FAQ above.
Any comments on this are appreciated.
k to g1 is by far the most common leakage path as far as I recall - and I
have to say it's been a while since I have been into CRT based repairs at
this level, so what John said above about h-k shorts may well be true with
'modern' tubes, although I'm sure that I remember being able to remove h-k's
in some circumstances with my B&K 465. As far as what you are measuring,
there should be no reading between g1 and any cathode. If the grid
connection is indeed grounded, then the statement about all guns being
affected equally, is not valid in this case, and individual gun conditions
could be affected by individual shorts. What you have to remember, is that
it is irrelevant what *actual* potentials the individual electrodes are
connected to, as long as the correct *differentials* are maintained. Hence,
if the gun requires the grid to be say -40v to achieve cut-off, it doesn't
matter whether the cathode is at +40v and the grid at ground, or the grid at
+20v and the cathode at +60v, the differential of -40v is maintained, and
the bias conditions remain the same.
You questioned earlier what constituted a "firm rap". I really can't tell
you in terms of anything that you could take as 'definitive instructions'.
It's just one of those things that's a 'feel' that you gain over years of
doing it. When I was directly involved with CRT equipment repairs, it was
something you 'just did' on a daily basis, and you never actually broke a
neck as a result ... Let's say that if you rap your fingertips on the bench
just hard enough to get that sort of 'buzz' in them immediately after,
that's probably about the most force that you want to be hitting the neck
with. I would normally be using an old box spanner that I have, which has a
wooden handle, and is a bit 'kinder' to the glass. As far as putting the
monitor on its side or upside down when you do it, it's just a case of
propping or holding it in position whilst you do it. It sometimes allows
particles to dislodge under the effects of gravity coming at them from a
different direction.
Other than this, use the capacitor discharge method to try and vapourise the
particles. You might want to try this with the heater on, as you say that
the resistance of the short goes down as the heater warms up. This is
probably because the particle(s) causing the short get 'squeezed' tighter in
the k-g1 gap, as the cathode cylinder expands. A better contact between the
two electrodes, via the contamination, is likely to result in a higher
discharge current from the cap, and a better chance of vapourising the
material, rather than just dislodging it.
Arfa